Matrix multiplied by a vector without using numpy

Question

Without using numpy in python

I am running into the error "object of type 'int' has no len()" when running the code below when multiplying the matrix C with the vector A.

Any help would be appreciated

A = [3, 4, 8]
C = [[2, 9, 7], [3, 4, 1], [1, 6, 5]]

def MM(C, A):
  c = []
  for i in range(0, len(C)):
    temp=[]
    for j in range(0, len(A[0])):
        s = 0
        for k in range(0, len(C[0])):
            s += C[i][k] * A[k][j]
        temp.append(s)
    c.append(temp)
  return c
print("Matrix C multiplied by vector A is: \n", MM(C, A))

Answer 1

The problem is here:

for j in range(0, len(A[0])):

A[0] is 3, which is an integer and has no attribute len. A is also a 1D array so A[k][j] will also raise an error.

Answer 2

I haven't stepped through your entire logic to fix the code, but I see the error.

In this line you try to take the length of A[0] for j in range(0, len(A[0]))

You have defined A as: A = [3,4,8]

Therefore A[0] = 3 and is an integer, which doesn't have a len()

Answer 3

I think you need A = [[3], [4], [8]]. When the matrix is on the left side of the multiplication, you are multiplying it with a column vector instead of a row vector.

Keeping your code the same, but changing A as mentioned, I get the output:

[[98], [33], [67]]

which corresponds to the 3x1 column vector you would expect to get by doing this matrix multiplication.

If you instead wanted A to be a row vector on the left side of the multiplication, you could set:

A = [[3, 4, 8]]
C = [[2, 9, 7], [3, 4, 1], [1, 6, 5]]

and print

print("The vector A multiplied by the matrix C is: \n", MM(A, C))

to give the output:

[[26, 91, 65]]

This would be a 1x3 row vector multiplied by a 3x3 matrix to give a 1x3 result.