Dotproduct of 3×4 matrices

February 16, 2022

I started learning about ML and wanted to create a layer consisting of 3 Neurons and a small batch of inputs in Python.I use Numpy to calculate the dotproduct of two matrices.

import numpy as np



a =         [[1.7, 2.2, 3.1, 2.6],
            [2.3, 5.8, -1,5, 2.6],
            [-1.5, 2.7, 3.3, -0,8]]

b =         [[0.2, 0.8, -0.5, 1.0],
            [0.5, -0.91, 0.26, -0.5],
            [-0.26, -0.27, 0.17, 0.87]]

c  =        [4.0,2.0,0.5]


output = np.dot(a, np.array(b).T) + c
print(output)

But somehow eventhough i transpose the matrix b i end up with a shape error

line 16, in <module>
    output = np.dot(a, np.array(b).T) + c
  File "<__array_function__ internals>", line 180, in dot
ValueError: shapes (3,) and (4,3) not aligned: 3 (dim 0) != 4 (dim 0)

>Solution :

You have two rows with 5 instead of 4 elements in a, probably you meant 1.5, and 0.8 instead of 1,5 and 0,8

a = np.array([[1.7, 2.2, 3.1, 2.6],
            [2.3, 5.8, -1.5, 2.6],
            [-1.5, 2.7, 3.3, -0.8]])

b = np.array([[0.2, 0.8, -0.5, 1.0],
            [0.5, -0.91, 0.26, -0.5],
            [-0.26, -0.27, 0.17, 0.87]])

c  = np.array([4.0,2.0,0.5])

np.dot(a, b.T) + c