NumPy - matmul() function

The NumPy matmul() function is used to perform matrix product of two arrays. Specifically,

• If both a and b are 2-D arrays, it is matrix multiplication.
• If a or b is 1-D, it is promoted to a matrix by prepending a 1 to its dimensions. After matrix multiplication the prepended 1 is removed.
• If either argument is N-D, N > 2, it is treated as a stack of matrices residing in the last two indexes and broadcast accordingly.

Please note that multiplication by scalars is not allowed by this function.

Syntax

numpy.matmul(a, b, out=None)

Parameters

Return Value

Returns the matrix product of the two arrays.

Exception

Raises ValueError exception, if the last dimension of a is not the same size as the second-to-last dimension of b or a scalar value is passed in.

Example: matmul() function with 1-D arrays

When two 1-D arrays are used, the function returns inner product of the arrays.

import numpy as np
Arr1 = [5, 8]
Arr2 = [10, 20]

#returns 5*10 + 8*20 = 210
print(np.matmul(Arr1, Arr2))

The output of the above code will be:

210

Example: matmul() function with matrix

When two matrix are used, the function returns matrix multiplication.

import numpy as np
Arr1 = np.array([[1, 2], 
                 [3, 4]])
Arr2 = np.array([[10, 20], 
                 [30, 40]])
Arr3 = np.matmul(Arr1, Arr2)

print(Arr3)

The output of the above code will be:

[[ 70 100]
 [150 220]]

The matrix multiplication is calculated as:

[[1*10+2*30 1*20+2*40]
 [3*10+4*30 3*20+4*40]]

= [[ 70 100]
   [150 220]]