Insert elements in an empty matrix python using FOR loop

Question

I am trying to do matrix multiplication. First i created an empty matrix C and then using for loop I am trying to do matrix multiplication and assign the results to matrix C.

# Matrix Multiplication
A = [[1, 2] , [3, 4]]
B = [[2, 3] , [2, 1]]
n = len(A) # No. of rows
j = len(A[0]) # No. of columns
C =[[]]
for i in range(len(A)):
    for j in range(len(A[0])):
        C[i][j] = 0
        for k in range(len(A)):
            C[i][j] = C[i][j] + A[i][k] * B[k][j]
print(C)

I am getting the error "list assignment index out of range".

Why are you not using numpy for actual matrix processing? Only lists are used in your posted example. — Parfait
– Parfait, Commented Sep 1, 2019 at 2:53
I am studying Algorithms. So I am trying to get my concept right by trying to code without the inbuilt functions/libraries. — VAIBBHAV DEVENDER KALRA
– VAIBBHAV DEVENDER KALRA, Commented Sep 1, 2019 at 3:05
Python is not one of those languages that creates a new list element by simple assignment. — hpaulj
– hpaulj, Commented Sep 1, 2019 at 5:12

ComplicatedPhenomenon · Accepted Answer · 2019-09-01 02:31:41Z

2

You need to create C which has the number of rows same with A's and the number of columns same as B's.

# Matrix Multiplication
A = [[1, 2] , [3, 4]]
B = [[2, 3] , [2, 1]]
n = len(A) # No. of rows
j = len(A[0]) # No. of columns
C =[[0 for j in range(len(B[0]))] for i in range(len(A))]
for i in range(len(A)):
    for j in range(len(A[0])):
        for k in range(len(A)):
            C[i][j] = C[i][j] + A[i][k] * B[k][j]
print(C)

Output

[[6, 5], [14, 13]]

The matrix multiplication can be done via

import numpy as np
A = np.array([[1, 2] , [3, 4]])
B = np.array([[2, 3] , [2, 1]])
np.dot(A,B)

edited Sep 1, 2019 at 2:31

answered Sep 1, 2019 at 2:26

ComplicatedPhenomenon

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