Primers • NumPy
 Introduction
 Arrays
 Indexing
 Datatypes
 Array math
 Broadcasting
 Data centering
 Selected methods
 Asarray
 Initialization
ndarray.item()
: Convert Single Value Tensor to Scalarndarray.tolist()
: Convert Multi Value Tensor to Scalar Random choice
 Nonzero
 Arange
 Linspace
 Reshape
 Transpose
 Reverse
 Flatten
 Squeeze
 Copy
 Elementwise sign
 Sum
 Average/Mean
 Product
 Len
 Dot Product
 Outer Product
 Matrix Product
 Max
 Elementwise max
 Argmax
 Argmin
 Where
 Pad
 Ravel
 Unravel Index
 Exponential
 Unique
 Bincount
 Elementwise square
 Elementwise square root
 Split
 Swapaxes
 Moveaxis
 Horizontal split
 Vertical split
 Append
 Stack
 Vertical stack
 Horizontal stack
 Column Stack
 Concatenate
 Concatenate vs. hstack/vstack vs. append vs. column_stack
 New axis
 Expand dims
 Inverse
 Solve
 Test any
 Test all
 Convert NumPy Array to Python List
 References and credits
 Citation
Introduction
 NumPy is the core library for scientific computing in Python. It is informally known as the swiss army knife of the data scientist.
 It provides a highperformance multidimensional array object
numpy.ndarray
, and tools for operating on these arrays.  If you’re already familiar with numerical processing in a different language like MATLAB and R, here are some recommended references:
 Related primers: Matplotlib and SciPy.
Arrays
 A NumPy array is a grid of values, all of the same type, and is indexed by a tuple of nonnegative integers.
 The rank of an array is the number of dimensions it contains.
 The shape of an array is a tuple of integers giving the size of the array along each dimension.
 The size of an array is the number of elements it contains (which is equivalent to
np.prod(<ndarray>.shape)
, i.e., the product of the array’s dimensions).  We can initialize NumPy arrays from (nested) lists and tuples, and access elements using square brackets as array subscripts (similar to lists in Python).
import numpy as np
a = np.array([1, 2, 3]) # Define a rank 1 array using a list
print(type(a)) # Prints <class 'numpy.ndarray'>
print(a.shape) # Prints (3,)
print(a.ndim) # Prints 1 (the rank of the array); equivalent to "len(a.shape)"
print(a.size) # Prints 3; equivalent to "np.prod(a.shape)"
print(a[0], a[1], a[2]) # Prints (1, 2, 3)
a[0] = 5 # Change an element of the array
print(a) # Prints [5 2 3]
b = np.array([[1, 2, 3]]) # Define a rank 2 array (vector) using a nested list
print(b.shape) # Prints (1, 3)
print(b.size) # Prints 3
c = np.array([[1, 2, 3], [4, 5, 6]]) # Define a rank 2 array (matrix) using a nested list
print(c.shape) # Prints (2, 3)
print(c.size) # Prints 6
print(c[0, 0], c[0, 1], c[1, 0]) # Prints (1, 2, 4)
d = np.array((1, 2, 3)) # Define a rank 1 array using a tuple
print(d) # Prints [1 2 3]
print(d.shape) # Prints (3,)
e = np.array(((1, 2, 3), (4, 5, 6))) # Define a rank 2 array using a nested tuple
print(e) # Prints [[1, 2, 3]
# [4, 5, 6]]
f = np.array([[1, 2, 3], [4, 5]]) # Define a rank 2 array using
print(f) # Prints [list([1, 2, 3]) list([4, 5])]
# NumPy arrays can be initialized using other NumPy arrays or lists
# but note that the resulting matrix is always of type NumPy ndarray
l = [1, 2, 3] # Define a python list
g = np.array([l, l, l]) # Matrix initialized with lists
a = np.array([1, 2, 3]) # Define a NumPy array by passing in a list
h = np.array([a, a, a]) # Matrix initialized with NumPy arrays
i = np.array([a, [1, 2, 3], g]) # Matrix initialized with both types
# All the below statements print [[1 2 3]
# [1 2 3]
# [1 2 3]]
print(g)
print(h)
print(i)
 Note the difference between a Python list and a NumPy array. NumPy arrays are designed for numerical (vector/matrix) operations, while lists are for more general purposes.
import numpy as np
l = [1, 2, 3] # Define a python list
a = np.array([1, 2, 3]) # Define a numpy array by passing in a list
print(l) # Prints [1 2 3]
print(a) # Prints [1 2 3]
print(type(l)) # Prints <class 'list'>
print(type(a)) # Prints <class 'numpy.ndarray'>
 Note that when defining an array, be sure that all the rows contain the same number of columns/elements. Otherwise, algebraic operations on malformed matrices could lead to unexpected results:
import numpy as np
a = np.array([[1, 2], [3, 4]]) # Define a 2x2 matrix
# Print a scaled version of 'a', more on this in the section on "scaling and translating arrays" below
print(a * 2) # Prints [[2 4]
# [6 8]]
# Define a malformed matrix. Note the third row contains 3 elements, while other rows contain 2 elements
b = np.array([[1, 2], [3, 4], [5, 6, 7]])
# Print the malformed matrix
print(b) # Prints [list([1, 2]) list([3, 4]) list([5, 6, 7])]
# Supposed to scale the whole matrix but does *not*
print(b * 2) # Prints [list([1, 2, 1, 2]) list([3, 4, 3, 4]) list([5, 6, 7, 5, 6, 7])]
 NumPy also provides many functions to create arrays:
import numpy as np
a = np.array([]) # Define an empty array
print(a) # Prints array([], dtype=float64)
print(a.shape) # Prints (0,)
b = np.zeros((2, 2)) # Define an array of all zeros
print(b) # Prints [[ 0. 0.]
# [ 0. 0.]]
c = np.ones((1, 2)) # Define an array of all ones
print(c) # Prints [[ 1. 1.]]
d = np.full((2, 2), 7) # Define a constant array
print(d) # Prints [[ 7. 7.]
# [ 7. 7.]]
e = np.eye(2) # Define a 2x2 identity matrix
print(e) # Prints [[ 1. 0.]
# [ 0. 1.]]
f = np.empty((2, 2)) # Define a float array without initializing entries
print(f) # Prints [[1.13224202e+277 1.94241498e109]
# [4.94065646e323 0.00000000e+000]]
g = np.empty((2, 2), dtype=int) # Define an int array without initializing entries
print(g) # Prints [[8751743591039004782 2980593642150976296]
# [ 10 0]]
h = np.random.random((2, 2)) # Define a 2x2 matrix from the uniform distribution [0, 1)
print(h) # Prints a 2x2 matrix of random values
i = 5 * np.random.random_sample((2, 2))  5 # Sample 2x2 matrix from Unif[5, 0)
# Sample from Unif[a, b), b > a: (b  a) * random_sample() + a
print(i) # Prints a 2x2 matrix of random values
j = np.random.randn(2, 2) # Sample a 2x2 matrix from the standard normal distribution
print(j) # Prints a 2x2 matrix of random values
k = 2.5 * np.random.randn(2, 2) + 3 # Sample 2x2 matrix from N(mean=3, var=6.25)
# General form: stddev * np.random.randn(...) + mean
print(k) # Prints a 2x2 matrix of random values

Note that with
np.random.randn()
, the length of each dimension of the output array is an individual argument. On the other hand,np.random.random()
accepts its shape argument as a single tuple containing all dimensions. More on this in the section on standard normal. 
To create a new array with the same shape and type as a given array, NumPy offers the following methods:
a = ([1, 2, 3], [4, 5, 6]) # Python list
print(a.shape)
b = np.empty_like(a)
# Uninitialized array
# array([[1073741821, 1073741821, 3],
# [ 0, 0, 1073741821]])
print(b.shape) # Prints (2, 3)
c = np.array([[1., 2., 3.], [4., 5., 6.]])
d = np.empty_like(c)
# Uninitialized array
# array([[ 2.00000715e+000, 1.48219694e323, 2.00000572e+000], # uninitialized
# [ 4.38791518e305, 2.00000715e+000, 4.17269252e309]])
print(d.shape) # Prints (2, 3)
# Note the difference between np.ones() and np.ones_like() below.
# np.ones(): Return a new array of given shape and type, filled with ones.
# np.ones_like(): Return a new array with the same shape and type as a given array, filled with ones.
e = np.ones((1, 2, 3))
f = np.ones_like(e)
# array([[[ 1., 1., 1.],
# [ 1., 1., 1.]]])
print(e.shape) # Prints (1, 2, 3)
print(f.shape) # Prints (1, 2, 3)
 You can read about other methods of array creation in the NumPy documentation.
Indexing
 NumPy arrays can be indexed by integers, a tuple of nonnegative integers, by booleans or by another array.
Integer indexing
 To “select” a particular row or column in an array, NumPy offers similar functionality as Python lists:
import numpy as np
a = np.array([[1, 2], [3, 4]])
# Select a row
a[0] # Prints [1 2]
a[1] # Prints [3 4]
# Select a column
a[:, 0] # Prints [1 3]
a[:, 1] # Prints [2 4]
 Note that
:
implies that the entire dimension is selected (as opposed to a particular element or a range of elements within a dimension). Also, if:
is the trailing/last element in the index subscript, it can be skipped.
import numpy as np
a = np.ones((1, 2, 3))
# Select the first dimension
a[0].shape # Prints (2, 3)
a[0,].shape # Prints (2, 3)
a[0, :].shape # Prints (2, 3)
# Select the second dimension
a[0, 1].shape # Prints (3,)
a[0, 1, :].shape # Prints (3,)
Slicing
 Similar to Python lists, NumPy arrays can be sliced.
 Since arrays may be multidimensional, you must specify a slice for each dimension of the array:
import numpy as np
# Define the following rank 2 array with shape (3, 4)
# [[ 1 2 3 4]
# [ 5 6 7 8]
# [ 9 10 11 12]]
a = np.array([[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]])
# Use slicing to pull out the subarray consisting of the first 2 rows
# and columns 1 and 2; b is the following array of shape (2, 2):
# [[2 3]
# [6 7]]
b = a[:2, 1:3]
# A slice of an array is a "view" into the same data, so modifying it
# will modify the original array
print(a[0, 1]) # Prints 2
print(a[(0, 1)]) # Also prints 2
b[0, 0] = 77 # b[0, 0] is the same piece of data as a[0, 1]
print(a[0, 1]) # Prints 77
 You can also mix integer indexing with slice indexing. However, doing so will yield an array of lower rank than the original array.
 Note that this is quite different from the way that MATLAB handles array slicing:
import numpy as np
# Define the following rank 2 array with shape (3, 4)
# [[ 1 2 3 4]
# [ 5 6 7 8]
# [ 9 10 11 12]]
a = np.array([[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]])
# Basic slicing
a[0:3] # Select rows 0, 1 and 2, all columns
a[0:2, 1] # Select rows 0 and 1, column 1
a[:1] # Select row 0, all columns (same as a[0:1, :])
a[1:2, :] # Select row 1, all columns
# Two ways of accessing the data in the middle row of the array.
# Mixing integer indexing with slices yields an array of lower rank,
# while using only slices yields an array of the same rank as the
# original array
row_r1 = a[1, :] # Rank 1 view of the second row of a
row_r2 = a[1:2, :] # Rank 2 view of the second row of a
print(row_r1, row_r1.shape) # Prints [5 6 7 8] (4,)
print(row_r2, row_r2.shape) # Prints [[5 6 7 8]] (1, 4)
# We can make the same distinction when accessing columns of an array
col_r1 = a[:, 1]
col_r2 = a[:, 1:2]
print(col_r1, col_r1.shape) # Prints [ 2 6 10] (3,)
print(col_r2, col_r2.shape) # Prints [[ 2]
# [ 6]
# [10]] (3, 1)
# Mix row and column slicing to print the first 2 rows and alternate
# columns
arr_row_col = a[:2, ::2]
print(arr_row_col, arr_row_col.shape) # Prints [[1, 3],
# [5, 7]] (2, 2)
Integer array indexing
 When you index into NumPy arrays using slicing, the resulting array view will always be a subarray of the original array.
 In contrast, integer array indexing allows you to construct arbitrary arrays using the data from another array.
 Here is an example:
import numpy as np
a = np.array([[1, 2], [3, 4], [5, 6]])
# An example of integer array indexing
# The returned array has shape (3,)
rows = [0, 1, 2]
cols = [0, 1, 0]
print(a[rows, cols]) # Prints [1 4 5]
# or using direct indexing
print(a[[0, 1, 2], [0, 1, 0]]) # Prints [1 4 5]
# The above example of integer array indexing is equivalent to
# the following:
print(np.array([a[0, 0], a[1, 1], a[2, 0]])) # Prints [1 4 5]
# Note that this doesn't work and results in
# IndexError: too many indices for array: array is 2dimensional,
# but 3 were indexed
print(a[(0, 0), (1, 1), (2, 0)])
# When using integer array indexing, you can reuse the same
# element from the source array
print(a[[0, 0], [1, 1]]) # Prints [2 2]
# Equivalent to the previous integer array indexing example
print(np.array([a[0, 1], a[0, 1]])) # Prints [2 2]
 You can use
np.arange()
to select the rows/columns of an array. For more details onnp.arange()
, refer to the section on arange below.
import numpy as np
a = np.array([[1, 2], [3, 4]])
# Return the entire array
a[np.arange(2), :] # Prints [[1 2]
# [3 4]]
# Return the first row
a[np.arange(1), :] # Prints [[1 2]]
 Along with
np.arange()
, you can use an “index array” that contains indices of rows or columns to index into another array. This is a very common usecase in NumPybased projects.
import numpy as np
a = np.array([[1, 2], [3, 4]])
# Selecting columns using an index array
b = [0, 0] # Select the first column for both rows (see below)
a[np.arange(1), b] # Prints [1 1] (same as a[0, [0, 0]])
a[np.arange(2), b] # Prints [1 3] (same as a[[0, 1], [0, 0]])
a[:, b] # Prints [[1 1]
# [3 3]]
# Selecting rows using an index array
b = [0, 0] # Select the first row for both columns (see below)
a[b, np.arange(1)] # Prints [1 1] (same as a[[0, 0], 0])
a[b, np.arange(2)] # Prints [1 2] (same as a[[0, 0], [0, 1]])
a[b, :] # Prints [[1 2]
# [1 2]]
 One useful trick with
np.arange()
and index array is selecting or mutating one element from each row of a matrix:
import numpy as np
# Define a new array from which we will select elements
a = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]])
print(a) # Prints [[ 1 2 3]
# [ 4 5 6]
# [ 7 8 9]
# [10 11 12]]
# Define an array of indices
b = np.array([0, 2, 0, 1])
# Select one element from each row of a using the indices in b
print(a[np.arange(4), b]) # Prints [ 1 6 7 11]
# Mutate one element from each row of a using the indices in b
a[np.arange(4), b] += 10
print(a) # Prints [[11 2 3]
# [ 4 5 16]
# [17 8 9]
# [10 21 12]]
Boolean array indexing
 Boolean array indexing lets you pick out arbitrary elements of an array.
 Frequently this type of indexing is used to select the elements of an array that satisfy some condition.
 Here is an example:
import numpy as np
a = np.array([[1, 2], [3, 4]])
print(a[True]) # Same as print(a), interpreted as a "True" mask
# on each of a's elements
# Prints [[[1 2]
# [3 4]]]
bool_idx = (a > 2) # Find the elements of a that are bigger than 2;
# this returns a NumPy array of Booleans of the same
# shape as a, where each slot of bool_idx tells
# whether that element of a is > 2
print(bool_idx) # Prints [[False False]
# [ True True]]
# We use boolean array indexing to construct a rank 1 array
# consisting of the elements of a corresponding to the True values
# of bool_idx
print(a[bool_idx]) # Prints [3 4]
# We can do all of the above in a single concise statement:
print(a[a > 2]) # Prints [3 4]
 As an extension of the above concept, to select elements on an array based on the elements of another array:
import numpy as np
a = np.array([1, 2, 3, 4, 5, 6])
b = np.array(['f','o','o','b','a','r'])
# Note that & is the bitwise AND operator, and && is not supported in NumPy,
# so np.logical_and() is used in the below example
print(b[np.logical_and((a > 1), (a < 5))]) # Prints ['o' 'o' 'b']
# Another way to accomplish this is using np.all(),
# which is explained in the section on "all" below.
print(b[np.all([a > 1, a < 5], axis=0)]) # Prints ['o' 'o' 'b']
 For brevity, we have left out a lot of details about NumPy array indexing; if you want to know more you should read the NumPy documentation.
Datatypes
 Every NumPy array is a grid of elements of the same type.
 NumPy provides a large set of numeric datatypes that you can use to construct arrays.
 NumPy tries to guess a datatype when you create an array, but functions that construct arrays usually also include an optional argument to explicitly specify the datatype.
 As an example:
import numpy as np
x = np.array([1, 2]) # Let NumPy choose the datatype
print(x.dtype) # Prints int64
x = np.array([1.0, 2.0]) # Let NumPy choose the datatype
print(x.dtype) # Prints float64
x = np.array([1, 2], dtype=np.int64) # Force a particular datatype
print(x.dtype) # Prints int64
 Note that with NumPy, the default float datatype is
float64
(double precision), while that with PyTorch isfloat32
(single precision). However, the default int datatype for both NumPy and PyTorch isint64
.  You can read all about datatypes in the NumPy documentation.
Changing datatypes
 The
astype()
method ofnp.ndarray
can change the datatype.
import numpy as np
a = np.array([1, 2, 3])
print(a) # Prints [1 2 3]
print(a.dtype) # Prints int64
a_float = a.astype(np.float32)
print(a_float) # Prints [1. 2. 3.]
print(a_float.dtype) # Prints float32
# 'a' remains unchanged.
print(a) # Prints [1 2 3]
print(a.dtype) # Prints int64
a_str = a.astype('str')
print(a_str) # Prints ['1' '2' '3']
print(a_str.dtype) # Prints <U21
Array math
 NumPy provides many functions for manipulating arrays; you can find an exhaustive list in the NumPy documentation. Some common ones are listed below.
Elementwise operations
 Algebraic operations such as
+
,
,*
,/
etc. are available both as operator overloads and as functions in the NumPy module. These operators carry out elementwise operations on NumPy arrays:
import numpy as np
x = np.array([[1, 2], [3, 4]], dtype=np.float64)
y = np.array([[5, 6], [7, 8]], dtype=np.float64)
# Elementwise sum; both produce the array
# [[ 6.0 8.0]
# [10.0 12.0]]
print(x + y)
print(np.add(x, y))
# Elementwise difference; both produce the array
# [[4.0 4.0]
# [4.0 4.0]]
print(x  y)
print(np.subtract(x, y))
# Elementwise product; both produce the array
# [[ 5.0 12.0]
# [21.0 32.0]]
print(x * y)
print(np.multiply(x, y))
# Elementwise division; both produce the array
# [[ 0.2 0.33333333]
# [ 0.42857143 0.5 ]]
print(x / y)
print(np.divide(x, y))
# Elementwise square root; produces the array
# [[ 1. 1.41421356]
# [ 1.73205081 2. ]]
print(np.sqrt(x))
NumPy arrays vs. Python lists
 A common mistake is to mix up the concepts of NumPy arrays and Python lists.
 Note that the
+
operator in the context of NumPy arrays performs an elementwise addition, while the same operation on Python lists results in a list extension.
import numpy as np
l = [1, 2, 3] # Define a python list
a = np.array([1, 2, 3]) # Define a numpy array by passing in a list
print(a + a) # Prints [2 4 6]
print(l + l) # Prints [1 2 3 1 2 3]
 With NumPy arrays, we can scale the vector with
*
by performing elementwise multiplication, while the same operation on Python lists results in a list concatenation (and in MATLAB, results in matrix multiplication). We instead use the
dot
function to compute inner products of vectors, to multiply a vector by a matrix, and to multiply matrices. We discuss this in detail in the section on dot product below.
 We instead use the
import numpy as np
l = [1, 2, 3] # Define a python list
a = np.array([1, 2, 3]) # Define a numpy array by passing in a list
print(a * 3) # Prints [3 6 9]
print(l * 3) # Prints [1 2 3 1 2 3 1 2 3]
 Make note of this while coding since a function can utilize both Python lists and NumPy arrays. Knowing this can save many headaches!
Scaling and translating arrays
 Using regular algebraic operators like
+
and
that we saw in the prior section on elementwise operations, we can scale and translate/shift NumPy arrays.  Operations can be performed between:
 Two NumPy arrays or,
 NumPy arrays and scalars.
import numpy as np
a = np.array([[1, 2], [3, 4]]) # Define a 2x2 matrix
# Scale by 2 units and translate by 1 unit
result = a * 2 + 1 # Multiply each element in the matrix by 2 and add 1
print(result) # Prints [[3 5]
# [7 9]]
Norm
 NumPy offers a set of algebraic functions in the class linalg, which includes the norm function:
import numpy as np
a = np.array([1, 2, 3, 4]) # Define an array
norm1 = np.linalg.norm(a)
a = np.array([[1, 2], [3, 4]]) # Define a 2x2 matrix
norm2 = np.linalg.norm(a)
# Both print 5.477225575051661
print(norm1)
print(norm2)
 By default,
np.linalg.norm()
calculates the Frobenius norm for matrices and the 2norm for vectors, unless theord
argument is overridden. Recall from linear algebra that the norm (or magnitude) of an ndimensional vector \(v\) is the square root of the sum of its elements squared:
 Also, the Frobenius norm is the generalization of the vector norm for matrices, and is defined for a matrix \(A\) as:
 The following code snippet compares a manual implementation of the norm vs. the
np.linalg.norm()
function. Both yield the same result.
import numpy as np
a = np.array([[1, 2], [3, 4]]) # Define a 2x2 matrix
norm = np.sqrt(np.sum(np.square(a)))
# Both should yield the same result and print:
# 5.477225575051661 == 5.477225575051661
print(norm, '== ', np.linalg.norm(a))
a = np.array([1, 2, 3, 4]) # Define an array
norm = np.sqrt(np.sum(np.square(a)))
# Both should yield the same result and print:
# 5.477225575051661 == 5.477225575051661
print(norm, '== ', np.linalg.norm(a))
 Note that if the
axis
argument tonp.linalg.norm()
is not explicitly specified, the function defaults to treating the matrix as a “flat” array of numbers implying that it takes every element of the matrix into account. However, it is possible to get the a rowwise or columnwise norm using theaxis
parameter:axis=0
operates across all rows, i.e., gets the norm of each column.axis=1
operates across all columns, i.e., gets the norm of each row.
import numpy as np
a = np.array([[1, 1], [2, 2], [3, 3]]) # Define a 3x2 matrix
normByCols = np.linalg.norm(a, axis=0) # Get the norm for each column; returns 2 elements
normByRows = np.linalg.norm(a, axis=1) # get the norm for each row; returns 3 elements
print(normByCols) # [3.74165739 3.74165739]
print(normByRows) # [1.41421356 2.82842712 4.24264069]
Dot product
 Dot product is available both as a function in the NumPy module
np.dot()
and as an instance method of array objects<ndarray>.dot()
:
import numpy as np
x = np.array([[1, 2], [3, 4]])
y = np.array([[5, 6], [7, 8]])
v = np.array([9, 10])
w = np.array([11, 12])
# Dot/scalar/inner product of vectors; both produce a scalar: 219
print(v.dot(w))
print(np.dot(v, w))
# Matrix/vector product; both produce the rank 1 array [29 67]
print(x.dot(v))
print(np.dot(x, v))
# Matrix/matrix product; both produce the rank 2 array
# [[19 22]
# [43 50]]
print(x.dot(y))
print(np.dot(x, y))
 Some alternative ways to obtain the dot product:
import numpy as np
print(np.sum(v * w)) # Using the definition of dot product
print(v @ w) # numpy>=1.10 overloads the new Pythonic operator "@" for dot product
# Inefficient, unrolled, nonvectorized implementation
dotProduct = 0
for a, b in zip(v, w):
dotProduct += a * b
print(dotProduct)
 While
a @ b
andnp.sum()
accept only NumPy arrays,np.dot()
accepts both NumPy arrays and Python lists:
import numpy as np
n1 = np.dot(np.array([1, 2]), np.array([3, 4])) # Dot product on NumPy arrays
n2 = np.dot([1, 2], [3, 4]) # Dot product on python lists
# Both print 11
print(n1)
print(n2)

Note that while
np.dot()
accepts 2D arrays (and carries out matrix multiplication), usingnp.matmul()
ora @ b
for NumPy arrays is preferred. The section on Matrix Product talks about hownp.matmul()
differs fromnp.dot()
. 
The following rules dictate the behavior of
np.dot()
depending on the arguments: If both
a
andb
are 1D arrays, it is the inner product ofa
andb
.  If both
a
andb
are 2D arrays, it is matrix multiplication, but usingnp.matmul(a, b)
ora @ b
is preferred.  If either argument is 0D (scalar), it is scalar multiplication and using
np.multiply(a, b)
ora * b
is preferred.  If
a
is an Ndimensional array andb
is a 1dimensional array, it is a sum product over the last axis of a and b. 
If
a
is an Ndimensional array andb
is an Mdimensional array (where \(M \geq 2\)), it is a sum product over the last axis of a and the secondtolast axis of b:dot(a, b)[i, j, k, m] = sum(a[i, j, :] * b[k, :, m])
 As an example,
import numpy as np a = np.ones([9, 5, 7, 4]) c = np.ones([9, 5, 4, 3]) print(np.dot(a, c).shape) # Prints (9, 5, 7, 9, 5, 3)
 If both
Broadcasting
 Broadcasting is a powerful mechanism that allows NumPy to work with arrays of different shapes when performing arithmetic operations. Frequently, we have a smaller array and a larger array, and we want to use the smaller array multiple times to perform some operation on the larger array.
 Broadcasting typically makes your code more concise and faster, so you should strive to use it where possible.
 The simplest broadcasting example occurs when an array and a scalar value are combined in an operation:
import numpy as np
a = np.array([1, 2, 3])
b = 2
print(a * b) # Prints [2 4 6]
# Note that the result is equivalent to the next example where b is an array!
a = np.array([1, 2, 3])
b = np.array([2, 2, 2])
print(a * b) # Prints [2 4 6]
 Consider another example where we’d like to add a constant vector to each row of a matrix. We could do it like this:
import numpy as np
# We will add the vector v to each row of the matrix x,
# storing the result in the matrix y
x = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]])
v = np.array([1, 0, 1])
y = np.empty_like(x) # Define an empty matrix with the same shape as x
# Add the vector v to each row of the matrix x with an explicit loop
for i in range(4):
y[i, :] = x[i, :] + v
# Now y is the following
# [[ 2 2 4]
# [ 5 5 7]
# [ 8 8 10]
# [11 11 13]]
print(y)
 This works; however when the matrix
x
is very large, computing an explicit loop in Python could be slow.  Note that adding the vector
v
to each row of the matrixx
is equivalent to forming a matrixvv
by stacking multiple copies ofv
vertically, then performing elementwise summation ofx
andvv
. We could implement this approach like this:
import numpy as np
# We will add the vector v to each row of the matrix x,
# storing the result in the matrix y
x = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]])
v = np.array([1, 0, 1])
vv = np.tile(v, (4, 1)) # Stack 4 copies of v on top of each other
print(vv) # Prints [[1 0 1]
# [1 0 1]
# [1 0 1]
# [1 0 1]]
y = x + vv # Add x and vv elementwise
print(y) # Prints [[ 2 2 4
# [ 5 5 7]
# [ 8 8 10]
# [11 11 13]]
 NumPy broadcasting allows us to perform this computation without actually creating multiple copies of
v
. Consider this version, using broadcasting:
import numpy as np
# We will add the vector v to each row of the matrix x,
# storing the result in the matrix y
x = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]])
v = np.array([1, 0, 1])
y = x + v # Add v to each row of x using broadcasting
print(y) # Prints [[ 2 2 4]
# [ 5 5 7]
# [ 8 8 10]
# [11 11 13]]
 The line
y = x + v
works even thoughx
has shape(4, 3)
andv
has shape(3,)
due to broadcasting; this line works as ifv
actually had shape(4, 3)
, where each row was a copy ofv
, and the sum was performed elementwise.
Rules
 Broadcasting two arrays together follows these rules:
 If the arrays do not have the same rank, prepend the shape of the lower rank array with ones until both shapes have the same length.
 This implies that the arrays do not need to have the same number of dimensions.
 If the arrays have the same rank, NumPy compares their shapes elementwise. It starts with the trailing dimensions and works its way forward, checking if the arrays are compatible in every dimension, as follows:
 The two arrays are said to be compatible in a dimension if:
 They have the same size in the dimension or,
 One of the arrays has size 1 in that dimension.
 The arrays can only be broadcast together if they are compatible in all dimensions.
 When either of the dimensions belonging to both arrays being compared is 1, the other is used. In other words, dimensions with size 1 are stretched or “copied” to match the other. This is the primary broadcasting step.
 After broadcasting, each array behaves as if it its shape is now equal to the elementwise maximum of shapes of the two input arrays.
 The two arrays are said to be compatible in a dimension if:
 If the arrays do not have the same rank, prepend the shape of the lower rank array with ones until both shapes have the same length.
 Functions that support broadcasting are known as universal functions. You can find an exhaustive list of universal functions in the NumPy documentation.
Examples

Examples from NumPy’s documentation on Broadcasting:
 Either array has axes with length 1: In the following example, array \(B\) has an axis with length 1.
 To tackle an axis with length 1 during broadcasting, we simply expand that dimension to match the other array’s.
A (3d array): 15 x 3 x 5 B (3d array): 15 x 1 x 5 Result (3d array): 15 x 3 x 5
 Both arrays have axes with length 1: In the following example, arrays \(A\) and \(B\) both have axes with length 1.
 Extrapolating the case above, to tackle an axis with length 1 in either array during broadcasting, we simply expand that dimension to match the other array’s.
A (4d array): 8 x 1 x 6 x 1 B (3d array): 7 x 1 x 5 Result (4d array): 8 x 7 x 6 x 5
 Rank mismatch: In the following example, arrays \(A\) and \(B\) do not have the same rank.
 Recall that to tackle rank mismatch during broadcasting, we prepend the shape of the lower rank array with ones until both shapes have the same length.
A (2d array): 5 x 4 B (1d array): 4 Result (2d array): 5 x 4
A (3d array): 15 x 3 x 5 B (2d array): 3 x 5 Result (3d array): 15 x 3 x 5
A (3d array): 256 x 256 x 3 B (1d array): 3 Result (3d array): 256 x 256 x 3
 Rank mismatch and axes with length 1: In the following example, arrays \(A\) and \(B\) do not have the same rank and \(B\) has an axis of length 1.
 Tackling rank mismatch was discussed in example 3. Also, handling an axis with length 1 was discussed in example 1.
A (2d array): 5 x 4 B (1d array): 1 Result (2d array): 5 x 4
A (3d array): 15 x 3 x 5 B (2d array): 3 x 1 Result (3d array): 15 x 3 x 5
 Either array has axes with length 1: In the following example, array \(B\) has an axis with length 1.

Some examples that do not broadcast:
 Dimension mismatch: In the following example, arrays \(A\) and \(B\) are not compatible in each dimension and thus, cannot be broadcasted.
 Recall that the two arrays need to be compatible in each dimension (starting from the trailing dimension) for broadcasting to be defined.
A (1d array): 3 B (1d array): 4 # Trailing dimensions do not match
A (2d array): 2 x 1 B (3d array): 8 x 4 x 3 # Second from last dimensions mismatched
 Dimension mismatch: In the following example, arrays \(A\) and \(B\) are not compatible in each dimension and thus, cannot be broadcasted.

Broadcasting examples in code:
import numpy as np
# Compute outer product of vectors
v = np.array([1, 2, 3]) # v has shape (3,)
w = np.array([4, 5]) # w has shape (2,)
# To compute an outer product, we first reshape v to be a column
# vector of shape (3, 1); we can then broadcast it against w to yield
# an output of shape (3, 2), which is the outer product of v and w:
# [[ 4 5]
# [ 8 10]
# [12 15]]
print(np.reshape(v, (3, 1)) * w)
# Note that np.reshape() is described in detail in the section on "reshape" below.
# Add a vector to each row of a matrix
x = np.array([[1, 2, 3], [4, 5, 6]])
# x has shape (2, 3) and v has shape (3,) so they broadcast to (2, 3),
# giving the following matrix:
# [[2 4 6]
# [5 7 9]]
print(x + v)
# Add a vector to each column of a matrix
# x has shape (2, 3) and w has shape (2,)
# If we transpose x then it has shape (3, 2) and can be broadcast
# against w to yield a result of shape (3, 2); transposing this result
# yields the final result of shape (2, 3) which is the matrix x with
# the vector w added to each column. Gives the following matrix:
# [[ 5 6 7]
# [ 9 10 11]]
print((x.T + w).T)
# Another solution is to reshape w to be a column vector of shape (2, 1);
# we can then broadcast it directly against x to produce the same
# output.
print(x + np.reshape(w, (2, 1)))
# Multiply a matrix by a constant:
# x has shape (2, 3). NumPy treats scalars as arrays of shape ();
# these can be broadcast together to shape (2, 3), producing the
# following array:
# [[ 2 4 6]
# [ 8 10 12]]
print(x * 2)
Data centering
 In some scenarios, centering the elements of a dataset is an important preprocessing step. Centering a dataset involves subtracting the mean from the data. The resultant data is called zerocentered, since the mean of the resultant dataset is 0.
Columncentering
 Columncentering a matrix involves subtracting the column mean from each element within a particular column. Note that the sum by columns of a centered matrix is always 0.
import numpy as np
a = np.array([[1, 1], [2, 2], [3, 3]]) # Define a 3x2 matrix.
centered = a  np.mean(a, axis=0) # Remove the mean for each column
# Columncentered matrix
print(centered) # Prints [[1. 1.]
# [ 0. 0.]
# [ 1. 1.]]
# New meanbycolumn should be 0
print(centered.mean(axis=0)) # Prints [0. 0.]
Rowcentering
 For row centering, transpose the matrix, center by columns, and then transpose back the result.
import numpy as np
a = np.array([[1, 3], [2, 4], [3, 5]]) # Define a 3x2 matrix.
centered = a.T  np.mean(a, axis=1) # Remove the mean for each row
centered = centered.T # Transpose back the result
# Rowcentered matrix
print(centered) # Prints [[1. 1.]
# [1. 1.]
# [1. 1.]]
# New meanbyrows should be 0
print(centered.mean(axis=1)) # Prints [0. 0. 0.]
Selected methods
 NumPy provides a host of useful functions for performing computations on arrays. Below, we’ve touched upon some of the most useful ones that you’ll encounter regularly in projects.
 You can find an exhaustive list of mathematical functions in the NumPy documentation.
 As discussed in the section on norm, most of these functions accept an
axis
parameter which when specified, performs the operation on a percolumn (axis=0
) or perrow basis (axis=1
) rather than on the entire array.
Asarray
np.asarray()
converts the input to an array.
# Convert a list into an array
a = [1, 2]
print(np.asarray(a)) # array([1, 2])
# Existing NumPy arrays are not copied
a = np.array([1, 2])
np.asarray(a) is a # Prints True
# If dtype is set, array is copied only if dtype does not match:
a = np.array([1, 2], dtype=np.float32)
np.asarray(a, dtype=np.float32) is a # Prints True
np.asarray(a, dtype=np.float64) is a # Prints False
 Note that the main difference between
np.asarray()
andnp.array()
is thatnp.array()
will make a copy of the object by default, whilenp.asarray()
will not unless the input and target are not compatible.
Initialization
Random seed
 Provide a seed to the generator. Used to ensure reproducibility of random numbers.
import numpy as np
np.random.seed(0) # Here, 0 is the seed.
a = np.random.rand()
print(a) # Print 0.5488135039273248
np.random.seed(0) # Set seed again for next random number to match the prior one.
b = np.random.rand()
print(b) # Print 0.5488135039273248
c = np.random.rand(3, 2) # Initialize a 3x2 matrix of random values with seed 0
print(c) # Prints a 3x2 matrix of random values
Standard normal
 Return samples from the standard normal (also called the standard Gaussian) distribution.
 Note that
np.random.randn()
is a convenience function for users porting code from MATLAB, and wrapsnp.random.standard_normal()
. As such,np.random.randn()
takes dimensions as individual integers. On the other hand,
np.random.standard_normal()
takes ashape
argument as a tuple to specify the size of the output, which is consistent with other NumPy functions likenp.zeros()
andnp.ones()
.
 On the other hand,
import numpy as np
a = np.random.randn(2, 2) # Sample a 2x2 matrix from the standard normal distribution
print(a) # Print a 2x2 matrix of random values
b = np.random.standard_normal((2, 2)) # Sample a 2x2 matrix from the standard normal distribution
print(b) # Print a 2x2 matrix of random values
 To sample the normal distribution \(a \sim \mathcal{N}(\mu, \sigma^2)\), multiply the output of
np.random.randn()
by \(\sigma\) and add \(\mu\), i.e.,stddev * np.random.randn(...) + mean}
:
import numpy as np
a = 2.5 * np.random.randn(2, 2) + 3 # Sample a 2x2 matrix from N(mean=3, var=6.25)
# General form: stddev * np.random.randn(...) + mean
print(a) # Print a 2x2 matrix of random values
Uniform
 Samples random floats from a continuous uniform distribution – the halfopen interval \([0.0, 1.0)\).
 Note that
np.random.rand()
is a convenience function for users porting code from MATLAB, and wrapsnp.random.random()
. As such,np.random.rand()
takes dimensions as individual ints. On the other hand,
np.random.random()
takes ashape
argument as a tuple to specify the size of the output, which is consistent with other NumPy functions likenp.zeros()
andnp.ones()
.
 On the other hand,
import numpy as np
a = np.random.rand(2, 2) # Define a 2x2 matrix from the uniform distribution [0, 1)
print(a) # Print a 2x2 matrix of random values
b = np.random.random((2, 2)) # Define a 2x2 matrix from the uniform distribution [0, 1)
print(b) # Print a 2x2 matrix of random values
 To sample the uniform distribution \(a \sim Unif[x, y)\), given \(y > x\), Note that
np.random.uniform()
offers this functionality (drawing samples from a uniform distribution within an arbitrary range) much more directly by accepting three arguments:low=0.0, high=1.0, size=None
. The following code snippet thus yields the same output as the one above.
import numpy as np
a = np.random.uniform(5, 0, size=(2,2)) # Sample 2x2 matrix from Unif[5, 0)
print(a) # Print a 2x2 matrix of random values
 Note that
np.random.random()
also offers this functionality (drawing samples from a uniform distribution within an arbitrary range) by multiplying its output by \((yx)\) and adding \(x\), i.e.,(y  x) * np.random.random(...) + x
import numpy as np
a = 5 * np.random.random((2, 2))  5 # Sample 2x2 matrix from Unif[5, 0)
# Sample from Unif[x, y), y > x: (y  x) * np.random.random() + x
print(a) # Print 2x2 matrix of random values
 Without explicit arguments, the functions
rand()
,random()
,uniform()
andrandom_sample()
are equivalent, producing a random float in the range \([0.0, 1.0)\).
import numpy as np
np.random.seed(0)
a = np.random.rand()
print(a) # Print 0.5488135039273248
b = np.random.random()
print(b) # Print 0.7151893663724195
c = np.random.uniform()
print(c) # Print 0.6027633760716439
d = np.random.random_sample() # numpy.random.random() is an alias for numpy.random.random_sample()
print(d) # Print 0.5448831829968969
ndarray.item()
: Convert Single Value Tensor to Scalar
 Returns the value of an
ndarray
as a Python int/float. This only works for arrays with one element. For other cases, see[tolist()](#ndarraytolistconvertmultivaluetensortoscalar)
.
import numpy as np
a = np.asarray([1.0])
a.item() # Prints 1.0
a.tolist() # Prints [1.0]
ndarray.tolist()
: Convert Multi Value Tensor to Scalar
 Returns the
ndarray
as a (nested) list. For scalars, a standard Python number is returned, just like with[item()](#ndarrayitemconvertsinglevaluetensortoscalar)
.
import numpy as np
a = np.random.random((2, 2))
a.tolist() # Prints [[0.012766935862600803, 0.5415473580360413],
# [0.08909505605697632, 0.7729271650314331]]
a[0,0].tolist() # Prints 0.012766935862600803
Random choice
 Generates a random sample from a given 1D array. The main arguments to
np.random.choice()
area
andsize
.a
can be anndarray
, in which case a random sample is returned from its elements. If it’s an int, the random sample is generated as ifa
werenp.arange(a)
.size
is an optional argument that can hold an int or a tuple of ints. If it is not overriden, a single value is returned by default.
import numpy as np
# Generate a uniform random sample from np.arange(5) of size 3
# This is equivalent to np.random.randint(0, 5, 3) or np.random.randint(0, 5, size=3)
print(np.random.choice(5, 3)) # Prints [0 3 4]
# Generate a nonuniform random sample from np.arange(5) of size 3
print(np.random.choice(5, 3, p=[0.1, 0, 0.3, 0.6, 0])) # Prints [3 3 0]
# Generate a uniform random sample from np.arange(5) of size 3 without replacement
print(np.random.choice(5, 3, replace=False)) # Prints [4 1 3]
Nonzero
 Return the indices of the elements that are nonzero using a tuple of arrays, one for each dimension of the input array, containing the indices of the nonzero elements in that dimension. The values in the input array are always tested and returned in rowmajor, Cstyle order.
import numpy as np
a = np.array([[3, 0, 0], [0, 4, 0], [5, 6, 0]])
print(np.nonzero(a)) # Prints (array([0, 1, 2, 2]), array([0, 1, 0, 1]))
print(a[np.nonzero(a)]) # Prints [3 4 5 6]
print(np.transpose(np.nonzero(a))) # Prints [[0, 0],
# [1, 1],
# [2, 0],
# [2, 1]])
 A common use for nonzero is to find the indices of an array, where a condition is true. Given an array
a
, the conditiona > 3
is a boolean array and sinceFalse
is interpreted as0
,np.nonzero(a > 3)
yields the indices of thea
where the condition is true.
a = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
a > 3 # Prints array([[False, False, False],
# [ True, True, True],
# [ True, True, True]])
np.nonzero(a > 3) # Prints (array([1, 1, 1, 2, 2, 2]), array([0, 1, 2, 0, 1, 2]))
 Using this result to index the array is equivalent to using the mask directly:
a[np.nonzero(a > 3)]
array([4, 5, 6, 7, 8, 9])
a[a > 3] # prefer this spelling
array([4, 5, 6, 7, 8, 9])
nonzero
can also be called as a method of the array.
(a > 3).nonzero()
(array([1, 1, 1, 2, 2, 2]), array([0, 1, 2, 0, 1, 2]))
Arange
 Return evenly spaced values within the halfopen interval \([start, stop)\) (in other words, the interval including start but excluding stop).
 For integer arguments the function is equivalent to the Python builtin
range
function, but returns anndarray
rather than a list.
import numpy as np
print(np.arange(8)) # Prints [0 1 2 3 4 5 6 7]
print(np.arange(3, 8)) # Prints [3 4 5 6 7]
print(np.arange(3, 8, 2)) # Prints [3 5 7]
# arange() works with floats too (but read the disclaimer below)
print(np.arange(0.1, 0.5, 0.1)) # Prints [0.1 0.2 0.3 0.4]
 When using a noninteger step, such as \(0.1\), the results can often be inconsistent. It is better to use
np.linspace()
for those cases as below.
Linspace
 Return evenly spaced numbers over a specified interval.
 Returns 50 evenly spaced samples (which can be overriden by
num
), calculated over the interval \([start, stop]\).
import numpy as np
print(np.linspace(2.0, 3.0, num=5)) # Prints [2. 2.25 2.5 2.75 3. ]
print(np.linspace(2.0, 3.0, num=5, endpoint=False)) # Prints [2. 2.2 2.4 2.6 2.8]
Reshape
 Gives a new shape to an array without changing its data.
 Note that
np.reshape()
returns a view of the array when the elements are contiguous in memory (just likenp.ravel()
). So modifying the result ofnp.reshape()
would also modify the original array. However,np.reshape()
returns a copy if, for e.g., the input array were made from slicing another array using a nonunit step size (e.g.a = x[::2]
).
import numpy as np
a = np.array([[1, 2, 3], [4, 5, 6]])
print(np.reshape(a, (3, 2)) # Prints [[1 2]
# [3 4]
# [5 6]]
print(np.reshape(a, 6)) # Prints [1 2 3 4 5 6]
1 in Reshape
 NumPy allows us to assign one (and only one) value of the shape argument to a function as 1. NumPy figures out the unknown dimension by looking at the length of the array and the remaining dimensions, while making sure it satisfies the criterion that the new shape should be compatible with the original shape.
import numpy as np
a = np.array([[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]])
print(a.shape) # Prints (3, 4)
# Reshaping with *just* (1) leads to array flattening.
# Yields new shape as (12,), a rank 1 array which is compatible with original shape (3, 4)
print(a.reshape(1)) # Prints [ 1 2 3 4 5 6 7 8 9 10 11 12]
# Note that np.reshape(1) returns the same result as np.flatten()
# with the exception that np.flatten() returns a copy while reshape() returns a view of the original array.
# For more on np.flatten(), refer the section on "flatten" below.
# Transform the input array to a row vector by reshaping with (1, 1), i.e., 1 row and 3*4 number of columns.
# Yields new shape as (1, 12), a rank 2 array which is compatible with original shape (3, 4)
print(a.reshape(1, 1)) # Prints [[ 1 2 3 4 5 6 7 8 9 10 11 12]]
# Transform the input array to a column vector by reshaping with (1, 1), i.e., 3*4 number of rows and 1 column.
# Yields new shape as (12, 1), a rank 2 array which is compatible with original shape (3, 4)
print(a.reshape(1, 1)) # Prints [[ 1]
# [ 2]
# [ 3]
# [ 4]
# [ 5]
# [ 6]
# [ 7]
# [ 8]
# [ 9]
# [10]
# [11]
# [12]]
Transpose
 To transpose a 2D array (matrix), i.e., swap rows and columns, simply use the
T
attribute of an array object. Note thatnp.transpose()
and<ndarray>.transpose()
also yield the same result.
import numpy as np
a = np.array([[1, 2], [3, 4]])
print(a) # Prints [[1 2]
# [3 4]]
print(a.T) # Prints [[1 3]
# [2 4]]
print(np.transpose(a)) # Prints [[1 3]
# [2 4]]
 Note that taking the transpose of a rank 1 (1D) array does nothing:
import numpy as np
a = np.array([1, 2, 3])
print(a) # Prints [1 2 3]
print(a.T) # Prints [1 2 3]
 Instead, to “transpose” a rank 1 array, use
[np.newaxis](#newaxis)
:
import numpy as np
a = np.array([1, 2, 3])
# Make "a" a row vector by inserting an axis along first dimension
print(a[np.newaxis, :]) # Prints [[1 2 3]]
# Make "a" a column vector by inserting an axis along second dimension
print(a[:, np.newaxis]) # Prints [[1]
# [2]
# [3]]
 You can also use
None
in place ofnp.newaxis
to “transpose” a rank 1 array; more on this in the section onNone
as a shortcut fornp.newaxis
.
import numpy as np
a = np.array([1, 2, 3])
# Here, "None" serves as a shortcut for np.newaxis and yields the same row vector
print(a[None, :]) # Prints [[1 2 3]]
# Here, "None" serves as a shortcut for np.newaxis and yields the same column vector
print(a[:, None]) # Prints [[1]
# [2]
# [3]]
 With
np.transpose()
, you can not only transpose a matrix but also rearrange/permute the axes of a multidimensional array in any order. This functionality is similar to what PyTorch offers usingtorch.permute()
(but note thattorch.transpose()
only swaps two dimensions).
import numpy as np
a = np.ones((1, 2, 3))
print(a.shape) # Prints (1, 2, 3)
print(np.transpose(a, (1, 0, 2)).shape) # Prints (2, 1, 3)
Internals
 The way NumPy internally accomplishes a transpose is by swapping the shape and stride information for each axis. This section details what happens underthehood when NumPy performs a transpose.

Let’s start with a \(2 \times 2 \times 4\) matrix and see how it’s transpose looks:
import numpy as np a = np.arange(24).reshape((2, 3, 4)) print(a) # Prints [[[ 0 1 2 3] # [ 4 5 6 7] # [ 8 9 10 11]] # # [[12 13 14 15] # [16 17 18 19] # [20 21 22 23]]] b = a.transpose((1, 0, 2)) print(b) # Prints [[[ 0 1 2 3] # [12 13 14 15]] # # [[ 4 5 6 7] # [16 17 18 19]] # # [[ 8 9 10 11] # [20 21 22 23]]]
 Now, let’s check the strides for the array before the transpose:
import numpy as np a.shape # Prints (2, 3, 4) a.strides # Prints (96, 32, 8)
 Now, let’s check the strides for the array after the transpose:
import numpy as np b.shape # Prints (3, 2, 4) b.strides # Prints (32, 96, 8)
 Note that the transpose operation swapped the lengths and strides for axis 0 and axis 1. No data needs to be copied for this to happen; NumPy can simply change how it looks at the underlying memory to construct the new array. Here’s how:
 The stride value represents the number of bytes that must be traversed in memory in order to reach the next value of an axis of an array.
 Now, our 3D array looks this (with labelled axes):
 This array is stored in a contiguous block of memory (i.e., to access the next value in the array, we just move to the next memory address); essentially it is onedimensional. To interpret it as a 3D object, NumPy must jump over a certain constant number of bytes in order to move along one of the three axes:
 Since each integer takes up 8 bytes of memory (we’re using the
int64
dtype), the stride value for each dimension is 8 times the number of values that we need to jump. For instance, to move along axis 1, four values (32 bytes) are jumped, and to move along axis 0, eight values (64 bytes) need to be jumped.  When we write
a.transpose(1, 0, 2)
, we are swapping axes 0 and 1. The transposed array looks like this:
 All that NumPy needs to do is to swap the stride information for axis 0 and axis 1 (axis 2 is unchanged). Now we must jump further to move along axis 1 than axis 0:
 This basic concept works for any permutation of an array’s axes. The actual code that handles the transpose is written in C and can be found here.
Reverse
 Reverse (or flip) the contents of an array along an axis.
 If
axis
is not specified, it is assumedNone
, which flips over all of the axes of the input array.  If axis is a tuple of ints, flipping is performed on all of the axes specified in the tuple.
import numpy as np
a = np.arange(8).reshape((2, 2, 2))
print(a) # Prints [[[0 1]
# [2 3]]
#
# [[4 5]
# [6 7]]]
print(np.flip(a, 0)) # Prints [[[4 5]
# [6 7]]
#
# [[0 1]
# [2 3]]]
print(np.flip(a, 1)) # Prints [[[2 3]
# [0 1]]
#
# [[6 7]
# [4 5]]]
print(np.flip(a)) # Prints [[[7 6]
# [5 4]]
#
# [[3 2]
# [1 0]]]
print(np.flip(a, (0, 2))) # Prints [[[5 4]
# [7 6]]
#
# [[1 0]
# [3 2]]]
Flatten
 Return a copy of the array collapsed into one dimension.
import numpy as np
a = np.array([[1, 2, 3], [4, 5, 6]])
print(a.flatten()) # Prints [1 2 3 4 5 6]
Squeeze
 Remove singledimensional axes from an input array.
 The “squeeze” nomenclature has been borrowed from MATLAB which removes axes of size one (singletons).
import numpy as np
a = np.array([[[0], [1], [2]]])
print(a.shape) # Prints (1, 3, 1)
print(np.squeeze(a)) # Prints [0 1 2]
print(np.squeeze(a).shape) # Prints (3,)
print(a) # Prints [[[0]
# [1]
# [2]]]
 Note that unlike PyTorch, NumPy does not have offer an unsqueeze function, but similar functionality can be achieved using
np.newaxis
.
Copy
 Return an array copy of the given object.
import numpy as np
a = np.array([[1, 2], [3, 4]])
b = a
c = np.copy(a)
a[0] = 10
print(a) # Prints [[10 10]
# [ 3 4]]
a[0] == b[0] # Prints True
a[0] == c[0] # Prints False
 Note that
np.copy()
performs a shallow copy and will not copy object elements within arrays. This is mainly important for arrays containing Python objects.
Elementwise sign
 Returns an elementwise sign of an array’s elements.
import numpy as np
a = np.array([[1, 2], [3, 4]])
print(np.sign(a)) # Prints [[ 1 1]
# [1 1]]
Sum
 Sum of an array’s elements.
import numpy as np
a = np.array([[1, 2], [3, 4]])
# Compute sum of each column
print(np.sum(a, axis=0)) # Prints [4 6]
# Compute sum of each row
print(np.sum(a, axis=1)) # Prints [3 7]
 The
axis
argument specifies the axis along which the sum is performed. If axis is a tuple of ints, a sum is performed on all of the axes specified in the tuple instead of a single axis or all the axes as before.  Note that
np.sum()
yields an array with the same shape as the original array with the specified axis removed (which implies that if you sum over the rows in a 2D matrix, you get as many entries in the output as the number of columns and vice versa).  In the above example, since
axis=0
refers to the rows of the input matrix,np.sum()
sums over all rows and in other words, computes the sum of each column.  As a visual description here’s how
np.sum()
works under the hood (the arrows indicate the flow of summation foraxis=0
oraxis=1
):
 If
axis
isn’t specified, it defaults toNone
, in which casenp.sum()
sums over all of the elements of the input array:
import numpy as np
a = np.array([[1, 2], [3, 4]])
# Compute sum of all elements
print(np.sum(a)) # Prints 10
 Some examples of
np.sum()
:
a = np.array([[1, 0], [0, 1]])
# Count number of 1s in the input array
print(np.sum(a)) # Prints 2
b = np.array([[1, 2], [2, 1]])
# Count number of 1s in the input array
print(np.sum(b == 1)) # Prints 2
Average/Mean
 Recall that the mean is the sum of the elements divided by the length of the vector.
import numpy as np
a = np.array([[1, 1], [2, 2], [3, 3]]) # Define a 3x2 matrix. Chosen to be a matrix with 0 mean.
# Get the mean for the whole matrix
print(np.mean(a)) # Prints 0.0
# Get the mean for each column. Returns 2 elements.
print(a, axis=0) # Prints [ 2. 2.]
# Get the mean for each row. Returns 3 elements.
print(a, axis=1) # Prints [0. 0. 0.]
Product
 Return the product of array elements over a given axis.
import numpy as np
a = np.array([[1, 2], [3, 4]])
print(np.prod(a)) # Prints 24
Len
len()
returns the size of the first dimension of the input tensor, similar to PyTorch.
import numpy as np
a = np.array([[1, 2], [3, 4]])
print(a) # Prints [[1, 2],
# [3, 4]]
len(a) # 2
b = np.array([1, 2, 3, 4])
print(b) # Prints [1 2 3 4]
len(b) # 4
Dot Product
 Refer to the earlier section on dot product to learn the usage of
np.dot()
. Note that the dot product (also called the inner/scalar product) yields a scalar.
Outer Product
 Return the outer product of two vectors. Note that the outer product yields a vector.
import numpy as np
a = np.ones((5,))
b = np.linspace(2, 2, 5)
print(np.outer(a, b))
# Prints:
# [[2. 1. 0. 1. 2.]
# [2. 1. 0. 1. 2.]
# [2. 1. 0. 1. 2.]
# [2. 1. 0. 1. 2.]
# [2. 1. 0. 1. 2.]]
Matrix Product
 Return Matrix product of two arrays.
 The following rules dictate the behavior of
np.matmul()
depending on the arguments: If both arguments are 2D, they are multiplied like conventional matrices.
 If either argument is Ndimensional (with N > 2), it is treated as a stack of matrices residing in the last two indexes and broadcast accordingly.
 If the first argument is 1D (while the second is Ndimensional, where N = \(\geq 2\)), it is promoted to a matrix by prepending a 1 to its dimensions. After matrix multiplication, the prepended 1 is removed.
 If the second argument is 1D (while the second is Ndimensional, where N = \(\geq 2\)), it is promoted to a matrix by appending a 1 to its dimensions. After matrix multiplication, the appended 1 is removed.
import numpy as np
# For 2D arrays, np.matmul() is the conventional matrix product.
a = np.array([[1, 0], [0, 1]])
b = np.array([[4, 1], [2, 2]])
print(np.matmul(a, b)) # Prints [[4 1]
# [2 2]]
# For 2D mixed with 1D, the result is the usual., np.matmul() is the conventional matrix product.
a = np.array([[1, 0], [0, 1]])
b = np.array([1, 2])
# The output matrices below are both 1D after the removal of the appended/prepended 1 dimension
print(np.matmul(a, b)) # Prints [1 2]
print(np.matmul(b, a)) # Prints [1 2]
np.matmul()
differs fromnp.dot()
in two important ways: Multiplication by scalars is not allowed, use
*
instead.  Stacks of matrices are broadcast together as if the matrices were elements, respecting the signature \((n, k) \times (k, m) \rightarrow (n, m)\):
 Multiplication by scalars is not allowed, use
import numpy as np
a = np.ones([9, 5, 7, 4])
c = np.ones([9, 5, 4, 3])
print(np.dot(a, c).shape) # Prints (9, 5, 7, 9, 5, 3)
print(np.matmul(a, c).shape) # Prints (9, 5, 7, 3) / n is 7, k is 4, m is 3 per the stackedmatrices rule
Max
 Return the maximum element of an array or maximum along an axis.
import numpy as np
a = np.array([[1, 2, 3], [4, 5, 6]])
print(np.max(a)) # Prints 6
Elementwise max
 Compare two compatible arrays and return their elementwise maximum. Here, ‘compatible’ means that one array can be broadcast to the other.
import numpy as np
a = np.array([[1, 2, 3], [4, 5, 6]])
b = np.array([[7, 8, 9], [2, 1, 0]])
print(np.maximum(a, b)) # Prints [[7 8 9]
# [4 5 6]]
Argmax
 Return the indices of the maximum values along an axis.
import numpy as np
a = np.array([[1, 2, 3], [4, 5, 6]])
print(np.argmax(a)) # Prints 5
print(np.argmax(a, axis=0)) # Prints [1 1 1]
print(np.argmax(a, axis=1)) # Prints [2 2]
 Note that in case of multiple occurrences of the maximum values, only the index corresponding to the first occurrence is returned.
Argmin
 Return the indices of the minimum values along an axis.
import numpy as np
a = np.array([[1, 2, 3], [4, 5, 6]])
print(np.argmin(a)) # Prints 0
print(np.argmin(a, axis=0)) # Prints [0 0 0]
print(np.argmin(a, axis=1)) # Prints [0 0]
 Note that in case of multiple occurrences of the minimum values, only the index corresponding to the first occurrence is returned.
Where
np.where()
accepts three arguments:(condition, x, y)
. If only thecondition
argument is supplied,np.where()
simply returns the indicies of the elements in the input array which satisfy the condition:
a = np.array([[1, 0], [0, 1]])
print(np.where(c == 1)) # Print (array([0, 1]), array([0, 1]))
 If all three arguments are supplied, it first evaluates the
condition
argument – if it returnsTrue
, it returnsx
, otherwise yieldsy
.
import numpy as np
a = np.array([0, 1, 2, 3, 4, 5])
print(np.where(a < 3, a, 10*a)) # Prints [ 0 1 2 30 40 50]
Pad
 Pad an array.
np.pad()
accepts three arguments:array, pad_width, mode='constant'
.pad_width
specifies the number of values padded to the edges of each axis. It accepts both an int and a tuple.(pad,)
or simply apad
(as an int) is a shortcut for before = after = pad width for all axes.((before_1, after_1), ... (before_N, after_N))
specifies the padding widths for each axis.((before, after),)
yields same before and after padding width for each axis.
 When
mode
is set toconstant
(which is the default), an optional parameterconstant_values
let’s you specify the padding values for each axis, which has a default of0
(i.e., zero padding). Rather than just an int,constant_values
accepts a tuple as well:(constant,)
or simply aconstant
(as an int) is a shortcut forbefore = after = constant
for all axes. Default is0
.((before_1, after_1), ... (before_N, after_N))
specifies the unique pad constants for each axis.((before, after),)
yields same before and after constants for each axis.
import numpy as np
a = np.array([1, 2, 3, 4, 5])
print(np.pad(a, 1, 'constant')) # Prints [0 1 2 3 4 5 0]
print(np.pad(a, (2, 3), 'constant', constant_values=(4, 6))) # Prints [4 4 1 2 3 4 5 6 6 6]
a = np.array([[1, 2], [3, 4]])
print(np.pad(a, 1, 'constant', constant_values=(4, 6))) # Prints [[4 4 4 6]
# [4 1 2 6]
# [4 3 4 6]
# [4 6 6 6]]
# With pad_width = [(0, 1), (0, 1)], each tuple item in the pad_width list
# corresponds to a dimension and the item therein represents the padding
# before (first element) and after (second element). Graphically,
# [(0, 1), (0, 1)]
# ^^^^^^ padding for second dimension
# ^^^^^^ padding for first dimension
#
# ^ no padding at the beginning of the first axis
# ^ pad with one "value" at the end of the first axis.
a = np.ones(2, 2)
print(np.pad(a, [(0, 1), (0, 1)], mode='constant')) # Prints [[1 2 0]
# [3 4 0]
# [0 0 0]]
# In this case the padding for the first and second axis are identical,
# so one could also just pass in the 2tuple.
print(np.pad(a, (0, 1), mode='constant')) # Prints [[1 2 0]
# [3 4 0]
# [0 0 0]]
# In case the padding before and after is identical, one could even omit
# the tuple (not applicable in this case though):
print(np.pad(a, 1, mode='constant')) # Prints [[0 0 0 0]
# [0 1 2 0]
# [0 3 4 0]
# [0 0 0 0]]
# Or if the padding before and after is identical but different for the axis, you could also omit the second argument in the inner tuples:
print(np.pad(a, [(1, ), (2, )], mode='constant')) # Prints [[0 0 0 0 0 0]
# [0 0 1 2 0 0]
# [0 0 3 4 0 0]
# [0 0 0 0 0 0]]
 However using the explicit
pad_width
specification should be preferred because it’s just to easy to make mistakes, as in the case below:
# Here NumPy thinks you wanted to pad all axis with 1 element before and
# 2 elements after each axis!
# Even if you intended it to pad with 1 element in axis 1 and 2 elements for axis 2.
print(np.pad(a, [1, 2], mode='constant')) # Prints [[0 0 0 0 0]
# [0 1 2 0 0]
# [0 3 4 0 0]
# [0 0 0 0 0]
# [0 0 0 0 0]]
Ravel
 Return a contiguous flattened array.
 Note that
np.ravel()
returns a view of the array when the elements are contiguous in memory (just likenp.reshape()
). So modifying the result ofnp.ravel()
would also modify the original array. However,np.ravel()
returns a copy if, say the input array were made from slicing another array using a nonunit step size (for e.g.,a = x[::2]
).
import numpy as np
a = np.array([[1, 2, 3], [4, 5, 6]])
print(np.ravel(a)) # Prints [1 2 3 4 5 6]
 Note that
np.ravel()
is equivalent tonp.reshape(1)
.
import numpy as np
a = np.array([[1, 2, 3], [4, 5, 6]])
print(np.reshape(a, 1)) # Prints [1 2 3 4 5 6])
Unravel Index
 Converts a “flat” index (i.e., an index into the flattened version of an array) into a tuple of coordinate arrays.
import numpy as np
# Passing an integer array whose elements are indices into the flattened version of an array
print(np.unravel_index([1, 2, 3], [4, 5])) # Prints (array([0, 0, 0]), array([1, 2, 3]))
# You're not limited to the 2dimensional XY coordinate space
print(np.unravel_index(7, (1, 1, 7))) # Prints (0, 0, 6)
Internals
 Computer memory is addressed linearly. Each memory cell corresponds to a number. A block of memory can be addressed in terms of a base, which is the memory address of its first element, and the item index. For example, assuming the base address is 10,000:
item index 0 1 2 3
memory address 10,000 10,001 10,002 10,003
 To store multidimensional blocks, their geometry must somehow be made to fit into linear memory. In C and NumPy, this is done rowbyrow (which is known as the rowmajor order). A 2D example would be:
 0 1 2 3
+
0  0 1 2 3
1  4 5 6 7
2  8 9 10 11
 So, for example, in this 3by4 block the 2D index \((1, 2)\) would correspond to the linear index 6 which is \(1 \times 4 + 2\). In general, the linear index is given by: \(\text{row index }\times\text{row length }+\text{ column index}\).
np.unravel_index()
does the inverse. Given a linear index, it computes the corresponding coordinates. Since this depends on the block dimensions, these also have to be passed. So, in our example, we can get the original 2D index \((1, 2)\) back from the linear index 6:np.unravel_index(6, (3, 4))) # Prints (1, 2)
Exponential
 Calculate the exponential of all elements in the input array.
import numpy as np
a = np.array([[1, 2, 3], [4, 5, 6]])
print(np.exp(a)) # Prints [[ 2.71828183 7.3890561 20.08553692]
# [ 54.59815003 148.4131591 403.42879349]]
Unique
 Find the unique elements of an array.
import numpy as np
a = np.array([[1, 1, 1], [2, 2, 2]])
print(np.unique(a)) # Prints [1 2]
np.unique()
supports an optionalreturn_index
argument which returns the indices with unique elements in addition to the unique elements.
import numpy as np
a = np.array([[1, 1, 1], [2, 2, 2]])
unique_els, indices = np.unique(a, return_index=True)
unique_els # Prints array([1, 2])
indices # Prints array([0, 3])
np.unique()
also supports an optionalreturn_counts
argument which returns the counts corresponding to the unique elements in addition to the unique elements.
import numpy as np
a = np.array([[1, 1, 1], [2, 2, 2]])
unique_els, counts = np.unique(a, return_counts=True)
unique_els # Prints array([1, 2])
counts # Prints array([3, 3])
Bincount
 Count number of occurrences of each value in a onedimensional array of nonnegative integers.
import numpy as np
a = np.array([1, 1, 1, 2, 2, 2])
print(np.bincount(a)) # Prints [0 3 3]
Elementwise square
 Return the elementwise square of the input.
import numpy as np
a = np.array([[1, 2, 3], [4, 5, 6]])
print(np.square(a)) # Prints [[ 1 4 9]
# [[16 25 36]]
Elementwise square root
 Return the elementwise nonnegative square root of an array.
import numpy as np
a = np.array([[1, 4, 9], [16, 25, 36]])
print(np.sqrt(a)) # Prints [[1. 2. 3.]
# [4. 5. 6.]]
Split
 Split an array into multiple subarrays.
import numpy as np
a = np.arange(8)
print(np.split(a, 2)) # Prints [array([0, 1, 2, 3]), array([4, 5, 6, 7])]
Swapaxes
 Interchange two axes of an array.
import numpy as np
a = np.zeros((2, 3, 5))
print(a.shape) # Prints (2, 3, 5)
print(np.swapaxes(a, 0, 1).shape) # Prints (5, 3, 2)
Moveaxis
 Compared to
np.transpose()
for reordering axes which needs positions of all axes to be explicitly specified, moving one axis while keeping the relative positions of all others is a common enough usecase to warrant its own syntactic sugar. This is the functionality that is offered bynp.moveaxis()
.
import numpy as np
a = np.zeros((2, 3, 5))
print(a.shape) # Prints (2, 3, 5)
print(np.moveaxis(a, 0, 1).shape) # Prints (3, 5, 2)
Horizontal split
 Split an array into multiple subarrays horizontally (columnwise).
 Note that
np.hsplit()
is equivalent tonp.split
withaxis=1
. Put simply, withnp.hsplit()
, the array is always split along the second axis regardless of the array dimension.
import numpy as np
a = np.arange(4).reshape(2, 2)
print(a) # Prints [[0 1]
# [2 3]]
print(np.hsplit(a, 2)) # Prints [array([[0],
# [2]]),
# [array([[1],
# [3]])]
Vertical split
 Split an array into multiple subarrays vertically (rowwise).
 Note that
np.vsplit()
is equivalent tonp.split
withaxis=0
(which is the default fornp.split
). Put simply, withnp.vsplit()
, the array is always split along the second axis regardless of the array dimension.
import numpy as np
a = np.arange(4).reshape(2, 2)
print(a) # Prints [[0 1]
# [2 3]]
print(np.vsplit(a, 2)) # Prints [array([[0, 1]]), array([[2, 3]])]
# np.split() with axis=0 (which is the defauly) yields the same output as np.vsplit()
print(np.split(a, 2)) # Prints [array([[0, 1]]), array([[2, 3]])]
Append
 Append values to the end of an array. If the
axis
argument is not given, both the input array and values are flattened before use.
import numpy as np
a = np.array([[1, 2, 3], [4, 5, 6]])
b = np.array([[7, 8, 9]])
print(np.append(a, b)) # Prints [1 2 3 4 5 6 7 8 9]
print(np.append(a, b, axis=0)) # Prints [[1 2 3]
# [4 5 6]
# [7 8 9]]
 As another example:
a = np.arange(4).reshape(2, 2) # Prints [[0 1]
# [2 3]]
print(np.append(a, a)) # Prints [0 1 2 3 0 1 2 3]
print(np.append(a, a, axis=0)) # Prints [[0 1]
# [2 3]
# [0 1]
# [2 3]]
 With
np.append()
, it is important to note that both input arrays must have the same shape. However, ifaxis
is not specified, the input arrays can be any shape (since they will be flattened before use).
import numpy as np
a = np.array([[1, 2, 3], [4, 5, 6]])
b = np.array([7, 8, 9])
print(np.append(a, b, axis=0)) # ValueError: all the input arrays must have same number of dimensions
print(np.append(a, b)) # Prints [1 2 3 4 5 6 7 8 9]
Stack
 Join a sequence of arrays along a new axis.
 The
axis
parameter specifies the index of the new axis in the dimensions of the result. For e.g.,axis=0
denotes the first dimension andaxis=1
denotes the last dimension. Note that by default,axis
is set to 0.
import numpy as np
a = np.array([1, 2, 3])
b = np.array([4, 5, 6])
print((a, b)) # Prints (array([1, 2, 3]), array([4, 5, 6]))
print(np.stack((a, b))) # Prints [[1 2 3]
# [4 5 6]]
# This is the same as above since axis is set to 0 by default
print(np.stack((a, b), axis=0)) # Prints [[1 2 3]
# [4 5 6]]
 For the given input arrays
a
andb
, note thatnp.vstack()
command yields the same output asnp.stack()
, sincenp.vstack()
simply stacks them vertically (rowwise) thereby implicitly creating a new dimension (since the input arrays are 1D and cannot be stacked without adding a dimension), whilenp.stack()
adds a new dimension at index 0 explicitly and stacks them along that dimension.
import numpy as np
a = np.array([1, 2, 3])
b = np.array([4, 5, 6])
# This is the same as above since axis is set to 0 by default
print(np.stack((a, b), axis=0)) # Prints [[1 2 3]
# [4 5 6]]
# np.vstack() yields the same output as np.stack()
# Note that np.vstack() is discussed in more detail below
print(np.vstack((a, b))) # Prints [[1 2 3]
# [4 5 6]]
 For the given input arrays
a
andb
, note thatnp.vstack()
yields an output dissimilar tonp.stack()
, sincenp.vstack()
simply stacks them vertically (rowwise), whilenp.stack()
adds a new dimension at index 0 explicitly and stacks them along that dimension.
import numpy as np
a = np.array([[1, 2], [3, 4]])
b = np.array([[5, 6], [7, 8]])
c = np.stack((a, b))
print(c) # Prints [[[1 2]
# [3 4]]
#
# [[5 6]
# [7 8]]]
print(c.shape) # Prints (2, 2, 2)
d = np.vstack((a, b))
print(d) # Prints [[1 2]
# [3 4]
# [5 6]
# [7 8]]
print(d.shape) # Prints (4, 2)
 Joining arrays columnwise by doing horizontal stacking:
import numpy as np
a = np.array([1, 2, 3])
b = np.array([4, 5, 6])
print(np.stack((a, b), axis=1)) # Prints [[1 4]
# [2 5]
# [3 6]]
# Note np.hstack() does *not* match the output of np.stack()
print(np.hstack((a, b))) # Prints [1 2 3 4 5 6]
 With
np.stack()
, it is important to note that all input arrays must have exactly the same shape. Specifically, not only do the input arrays need to have the same number of dimensions (i.e., equal ranks), but also all the dimensions themselves should be equallength. This is an important point of differentiation compared tonp.vstack()
andnp.hstack()
.
import numpy as np
a = np.array([[[1, 2]]])
b = np.array([[3, 4]])
print(np.stack((a, b))) # ValueError: all input arrays must have the same shape
a = np.array([[1, 2], [3, 4]])
b = np.array([[5, 6]])
print(np.stack((a, b))) # ValueError: all input arrays must have the same shape
print(np.vstack((a, b))) # Prints [[1 2]
# [3 4]
# [5 6]]
 For arrays of rank \(\geq 2\) (i.e., tensors),
np.stack()
works well too:
import numpy as np
arrays = [np.random.randn(3, 4) for _ in range(10)]
np.stack(arrays, axis=0).shape # Prints (10, 3, 4)
np.stack(arrays, axis=1).shape # Prints (3, 10, 4)
np.stack(arrays, axis=2).shape # Prints (3, 4, 10)
Vertical stack
 Stack arrays in sequence vertically (rowwise for 2D arrays). In general, for arrays of rank \(\geq 2\) (i.e., tensors), stack arrays along the first dimension.
 Note that
np.vstack()
does not accept anaxis
argument. np.vstack()
is equivalent tonp.concatenate(<ndarray>, axis=0)
along the first axis (after 1D arrays of shape(N,)
have been reshaped to(1, N)
).
import numpy as np
a = np.array([1, 2, 3])
b = np.array([4, 5, 6])
print(np.vstack((a, b))) # Prints [[1 2 3]
# [4 5 6]]
a = np.array([[1], [2], [3]])
b = np.array([[4], [5], [6]])
print(np.vstack((a, b))) # Prints [[1]
# [2]
# [3]
# [4]
# [5]
# [6]]
 With
np.vstack()
, it is important to note that all input arrays must have: The same number of dimensions (i.e., equal ranks) and,
 Equallength concatenation axes (their respective second dimensions), i.e., the same number of columns. This is an important point of differentiation compared to
np.stack()
andnp.hstack()
.
a = np.array([[1, 2], [3, 4]])
b = np.array([[5, 6]])
print(np.vstack((a, b))) # Prints [[1 2]
# [3 4]
# [5 6]]
a = np.array([[[1, 2]]])
b = np.array([[3, 4]])
print(np.vstack((a, b))) # ValueError: all the input arrays must have same number of dimensions
a = np.array([1, 2, 3])
b = np.array([4, 5])
print(np.vstack((a, b))) # ValueError: all the input array dimensions for the concatenation axis must match exactly
 For arrays of rank \(\geq 2\) (i.e., tensors),
np.vstack()
works well too:
a = np.array([[[1, 2]]])
b = np.array([[[3, 4]]])
print(a.shape) # Prints (1, 1, 2)
c = np.vstack((a, b))
print(c) # Prints [[[1 2]]
# [[3 4]]]
print(c.shape) # Prints (2, 1, 2)
Horizontal stack
 Stack arrays in sequence horizontally (columnwise for 2D arrays). In general, for arrays of rank \(\geq 2\) (i.e., tensors), stack arrays along the second dimension.
 Note that
np.hstack()
does not accept anaxis
argument. np.hstack()
is equivalent tonp.concatenate(<ndarray>, axis=1)
, except for 1D arrays where it concatenates along the first axis.
import numpy as np
a = np.array([1, 2, 3])
b = np.array([4, 5, 6])
print(np.hstack((a, b))) # Prints [1 2 3 4 5 6]
a = np.array([[1], [2], [3]])
b = np.array([[4], [5], [6]])
print(np.hstack((a, b))) # Prints [[1 4]
# [2 5]
# [3 6]]
 With
np.hstack()
, it is important to note that all input arrays must have: The same number of dimensions (i.e., equal ranks) and,
 Equallength concatenation axes (their respective first dimensions), i.e., the same number of rows. This is an important point of differentiation compared to
np.stack()
andnp.vstack()
.
a = np.array([1, 2, 3])
b = np.array([4, 5])
print(np.hstack((a, b))) # Prints [1 2 3 4 5]
a = np.array([[[1, 2]]])
b = np.array([[3, 4]])
print(np.hstack((a, b))) # ValueError: all the input arrays must have same number of dimensions
a = np.array([[1, 2], [3, 4]])
b = np.array([[5, 6]])
print(np.hstack((a, b))) # ValueError: all the input array dimensions for the concatenation axis must match exactly
 For arrays of rank \(\geq 2\) (i.e., tensors),
np.hstack()
works well too:
a = np.array([[[1, 2]]])
b = np.array([[[3, 4]]])
print(a.shape) # Prints (1, 1, 2)
c = np.hstack((a, b))
print(c) # Prints [[[1 2]
# [3 4]]]
print(c.shape) # Prints (1, 2, 2)
Column Stack
 Take a sequence of 1D arrays and stack them as columns to make a single 2D array. 2D arrays are stacked asis, just like with
np.hstack
. 1D arrays are turned into 2D column vectors first.
import numpy as np
a = np.array([1, 2, 3])
b = np.array([4, 5, 6])
print(np.column_stack((a, b))) # Prints [[1 4]
# [2 5]
# [3 6]]
# The following np.stack() and np.hstack() commands
# return the same output as well
print(np.stack((a, b), axis=1)) # Prints [[1 4]
# [2 5]
# [3 6]]
print(np.hstack(([[1], [2], [3]], [[4], [5], [6]]))) # Prints [[1 4]
# [2 5]
# [3 6]]
 Note that
np.column_stack()
is equivalent tonp.vstack().T
:
import numpy as np
a = np.array([1, 2, 3])
b = np.array([4, 5, 6])
# np.vstack().T returns the same output as np.column_stack()
print(np.vstack((a, b)).T) # Prints [[1 4]
# [2 5]
# [3 6]]
Concatenate
 Join a sequence of arrays along an existing axis.
 The
axis
argument specifies the axis along which the arrays will be joined. If axis isNone
, the arrays are flattened before use. The default is0
(vertical/rowwise stacking).  Joining arrays rowwise by doing vertical stacking:
import numpy as np
a = np.array([[1, 2], [3, 4]])
b = np.array([[5, 6]])
print(np.concatenate((a, b))) # Prints [[1 2]
# [3 4]
# [5 6]]
# The following np.vstack() command returns the same output
print(np.vstack((a, b))) # Prints [[1 2]
# [3 4]
# [5 6]]
 Joining arrays columnwise by doing horizontal stacking:
print(np.concatenate((a, b.T), axis=1)) # Prints [[1 2 5]
# [3 4 6]]
# The following np.hstack() and np.column_stack() command
# return the same output as well
print(np.hstack((a, b.T))) # Prints [[1 2 5]
# [3 4 6]]
print(np.column_stack((a, b.T))) # Prints [[1 2 5]
# [3 4 6]]
 Joining arrays after flattening them:
print(np.concatenate((a, b), axis=None)) # Prints [1 2 3 4 5 6]
# The following np.append() command returns the same output as well
print(np.append(a, b)) # Prints [1 2 3 4 5 6]
Concatenate vs. hstack/vstack vs. append vs. column_stack
 All the aforementioned functions, except
np.concatenate()
, are written in Python and work by tweaking the dimensions of the input arrays, and then concatenating on the right axis. In other words, they are convenience functions since they’re just wrappers aroundnp.concatenate()
. Here’s a summary of their code:
vstack:
# Turn all inputs into 2D (or more) and concatenate on first dimension
concatenate([atleast_2d(_m) for _m in tup], axis=0)
# Equivalent to np.concatenate(tup, axis=0) if input tuple contains arrays that are at least 2D
hstack:
concatenate([atleast_1d(_m) for _m in tup], axis=<0 or 1>)
column_stack:
if needed, transform arrays using array(arr, copy=False, subok=True, ndmin=2).T
concatenate([atleast_1d(_m) for _m in tup], axis=1)
append:
concatenate((asarray(arr), values), axis=axis)
 As a side note, with an IPython shell you can use
??
to query the source code of a particular function.
Graphical treatment
 If you have two matrices,
np.hstack()
stacks horizontally whilenp.vstack()
stacks vertically:
 If you’re stacking a matrix and a vector,
np.hstack()
becomes tricky to use, sonp.column_stack()
is a better option. The problem withnp.hstack()
is that when you append a column, you need convert it from a 1D array to a 2D column vector first, because a 1D array is normally interpreted as a row vector in 2D context in NumPy:
a = np.ones(2) # 2D, shape = (2, 2)
b = np.array([0, 0]) # 1D, shape = (2,)
np.hstack((a, b)) # Dimensions mismatch error
 If you’re stacking two vectors, you’ve got three options:
np.concatenate()
in its raw form is useful for 3D arrays and above (i.e., tensors).
New axis
np.newaxis
is used to increase the dimension of the existing array by an additional dimension, for e.g., a 1D array gets transformed to a 2D array, a 2D array gets transformed to a 3D array, and so on. Essentially,
np.newaxis
is a pseudoindex that allows the temporary addition of an axis into a multiarray.  Here is a visual illustration which depicts the promotion of a 1D array to a 2D array (row/column vector):
Usecase: 1D array > 2D vector
np.newaxis
might come in handy when you wish to explicitly convert a 1D array to either a 2D row or a column vector, as depicted in the above picture. The section on Transpose also describes this usecase. As an example:
# Define a 1D array
a = np.arange(4)
a.shape # Prints (4,)
# Make "a" a row vector by inserting an axis along the first dimension
row_vec = a[np.newaxis, :] # or a[None, :]
row_vec.shape # Prints (1, 4)
# Make "a" a column vector by inserting an axis along the second dimension
col_vec = a[:, np.newaxis] # or arr[:, None]
col_vec.shape # Prints (4, 1)
Usecase: During broadcasting
 When making use of broadcasting as part of some operation, for instance while doing addition of some arrays,
np.newaxis
proves useful by inserting a new dimension with size 1 (which is one of the required conditions for broadcasting to work). As an example, let’s say you want to add the following two arrays:
import numpy as np
a = np.array([1, 2, 3, 4, 5])
b = np.array([5, 4, 3])
 If you try to add these 1D arrays just like that, NumPy will raise the following
ValueError
:
# ValueError: operands could not be broadcast together with shapes (5,) (3,)
 To remedy this, you can use
np.newaxis
to increase the dimension of one of the arrays so that NumPy can broadcast:
# Let's utilize np.newaxis with a
a_new = a[:, np.newaxis] # or a[:, None]
a_new.shape # Prints (5, 1)
print(a_new) # Prints [[1]
# [2]
# [3]
# [4]
# [5]]
# Now, add
print(a_new + b) # Prints [[ 6 5 4]
# [ 7 6 5]
# [ 8 7 6]
# [ 9 8 7]
# [10 9 8]]
# Alternatively, you can also utilize np.newaxis with b
b_new = b[:, np.newaxis] # or b[:, None]
b_new.shape # Prints (3, 1)
print(b_new) # Prints [[5]
# [4]
# [3]]
# Now, add
print(a + b_new) # Prints [[ 6 7 8 9 10]
# [ 5 6 7 8 9]
# [ 4 5 6 7 8]]
# Note that we get the same result in both cases (but one being the transpose of the other)
 In the example below, we inserted a temporary axis using
np.newaxis
between the first and second axes of B to enable broadcasting.
import numpy as np
a = np.ones((3, 4, 5, 6))
b = np.ones((4, 6))
print((a + b[:, np.newaxis, :]).shape) # Prints (3, 4, 5, 6)
# or b[:, None, :]
Usecase: Promote to higher dimensions
 This is similar to Usecase: 1D array > 2D vector with the only exception being that this usecase involves repeated use of
np.newaxis
more than once to promote the input array to higher dimensions. Such an operation is sometimes needed for higher order arrays (i.e. Tensors).
import numpy as np
a = np.arange(5*5).reshape(5,5)
a.shape # Prints (5, 5)
# Promoting 2D array to a 5D array
arr_5D = a[np.newaxis, ..., np.newaxis, np.newaxis] # or a[None, :, None, None]
arr_5D.shape # Prints (1, 5, 5, 1, 1)
np.newaxis
vs. np.reshape()
np.newaxis
uses the slicing operator to recreate the array whilenp.reshape()
reshapes the array to the desired layout (assuming that the dimensions match; which is a must for a reshape to happen). Using
np.reshape()
, we can achieve a similar end result as withnp.newaxis
:
import numpy as np
a = np.array([1, 2, 3])
print(a.shape) # Print (3,)
# Transform the 1D array to a row vector by reshaping with (1, 1), i.e., 1 row and 3 columns
print(a.reshape(1, 1).shape) # Prints (1, 3)
a = np.array([1, 2, 3])
# Both of the np.newaxis commands yield the same output
print(a[np.newaxis, ...].shape) # Prints (1, 3)
print(a[np.newaxis, :].shape) # Prints (1, 3)
a = np.array([1, 2, 3])
# Transform the 1D array to a column vector by reshaping with (1, 1), i.e., 3 rows and 1 column
print(a.reshape(1, 1).shape) # Prints (3, 1)
a = np.array([1, 2, 3])
# Both of the np.newaxis commands yield the same output
print(a[..., np.newaxis].shape) # Prints (3, 1)
print(a[:, np.newaxis].shape) # Prints (3, 1)
b = np.array([[1, 2, 3], [4, 5, 6]])
print(b.shape) # Print (2, 3)
# Transform the input array to dimensions (1, 2, 3, 1)
print(b.reshape(1, 2, 3, 1).shape) # Prints (1, 2, 3, 1)
b = np.array([[1, 2, 3], [4, 5, 6]])
# Transform the input array to dimensions (1, 2, 3, 1)
print(b[np.newaxis, ..., np.newaxis].shape) # Prints (1, 2, 3, 1)
None
as a shortcut for np.newaxis
 You can also use
None
in place ofnp.newaxis
; these are in fact the same objects.
np.newaxis is None # Prints True
Expand dims
 As an alternative to
np.newaxis
, you can usenp.expand_dims()
:
import numpy as np
a = np.arange(5*5).reshape(5,5)
# Use np.expand_dims to add an axis at index position 1
b = np.expand_dims(a, axis=1)
print(b.shape) # Prints (6, 1)
# Use np.expand_dims to add an axis at index position 0
c = np.expand_dims(a, axis=0)
print(c.shape) # Prints (1, 6)
# Using the intuitive `axis` argument to add new axes at the
# 1st, 4th, and last (1) dimension of the resulting array
arr_5D = np.expand_dims(a, axis=(0, 3, 1))
arr_5D.shape # Prints (1, 5, 5, 1, 1)
Inverse
 Compute the inverse of a matrix.
import numpy as np
from numpy.linalg import inv
a = np.array([[1., 2.], [3., 4.]])
print(inv(a)) # Prints [[2. 1. ]
# [ 1.5 0.5]]
Solve
 Solve a linear matrix equation, or system of linear scalar equations.
import numpy as np
from numpy.linalg import solve
# Solve the system of equations:
# x0 + 2 * x1 = 1
# 3 * x0 + 5 * x1 = 2
a = np.array([[1, 2], [3, 5]])
b = np.array([1, 2])
print(np.linalg.solve(a, b)) # Prints [1. 1.]
Test any
 Test whether any array element along a given axis evaluates to
True
.  Returns a single boolean unless
axis
is notNone
(which is the default).  Note that not a number (NaN), \(\infty\) and \(\infty\) all evaluate to
True
because these are not equal to 0.
import numpy as np
a = [[True, False], [True, True]]
print(np.any(a)) # Prints True
a = [[True, False], [True, True]]
print(np.any(a, axis=0)) # Prints [ True, True]
a = np.array([[1, 2], [3, 4]])
print(np.any(a)) # Prints True
# A nifty usecase of np.any() is to select elements that satisfy
# atleast one condition from multiple given conditions
a = np.array([[1, 2], [3, 4]])
print(np.any([a > 1, a < 5], axis=0)) # Prints [[ True True]
# [ True True]]
print(a[np.any([a > 1, a < 5], axis=0)]) # Prints [1 2 3 4]
# If axis is None (default), np.any() yields a single boolean by performing
# a logical OR reduction over all the dimensions of the input array
print(np.any([a > 1, a < 5])) # Prints True
Test all
 Test whether all array elements along a given axis evaluate to
True
.  Similar to
np.any()
,np.all()
returns a single boolean unlessaxis
is notNone
(which is the default).  Again, similar to
np.any()
, note that not a number (NaN), \(\infty\) and \(\infty\) all evaluate toTrue
because these are not equal to 0.
import numpy as np
a = [[True, False], [True, True]]
print(np.all(a)) # Prints True
a = [[True, False], [True, True]]
print(np.all(a, axis=0)) # Prints [ True, False]
a = np.array([[1, 2], [3, 4]])
print(np.all(a)) # Prints True
# A nifty usecase of np.all() is to select elements that satisfy
# all given conditions
a = np.array([[1, 2], [3, 4]])
print(np.all([a > 1, a < 5], axis=0)) # Prints [[False True]
# [ True True]]
print(a[np.all([a > 1, a < 5], axis=0)]) # Prints [2 3 4]
# Note that if only one condition is specified, np.any() and np.all()
# are equivalent, and have identical outputs
print(a[np.any([a > 1], axis=0)]) # Prints [2 3 4]
print(a[np.all([a > 1], axis=0)]) # Prints [2 3 4]
# If axis is None (default), np.all() yields a single boolean by performing
# a logical AND reduction over all the dimensions of the input array
print(np.all([a > 1, a < 5])) # Prints False
Convert NumPy Array to Python List
<ndarray>.tolist()
returns the NumPy array as a nested list of Python scalars.
import numpy as np
a = np.arange(9).reshape(3, 3)
a.tolist() # Prints [[0, 1, 2], [3, 4, 5], [6, 7, 8]]
References and credits
 Parts of this tutorial were originally contributed by Justin Johnson.
 Stanford CS231n Python/NumPy Tutorial served as a major inspiration for this tutorial.
 NumPy documentation: the absolute basics for beginners
 NumPy documentation: Broadcasting
 NumPy Illustrated: The Visual Guide to NumPy
 What is an intuitive explanation of np.unravel_index?
 When should I use hstack/vstack vs. append vs. concatenate vs. column_stack?
 What is the difference between NumPy’s array() and asarray() functions?
 What does 1 mean in NumPy reshape?
 Find the most frequent number in a NumPy vector
 NumPy: From ND to 1D arrays
 Difference between functions generating random numbers in NumPy
 How do I select elements of an array given condition?
 How does NumPy’s transpose() method permute the axes of an array?
 What is the difference between contiguous and noncontiguous arrays?
 When should I use hstack/vstack vs append vs concatenate vs column_stack?
 NumPy vstack vs. column_stack
 How does numpy.newaxis work and when to use it?
 What does numpy.concatenate do with a single argument?
 NumPy: Add new dimensions to ndarray (np.newaxis, np.expand_dims)
 How to pad NumPy array with zeros?
 NumPy: Slicing ndarray
Citation
If you found our work useful, please cite it as:
@article{Chadha2020NumPyPrimer,
title = {NumPy Primer},
author = {Chadha, Aman},
journal = {Distilled AI},
year = {2020},
note = {\url{https://aman.ai}}
}