Matrix Multiplication in pure Python?

Question

I'm trying to multiply two matrices together using pure Python. Input (X1 is a 3x3 and Xt is a 3x2):

X1 =  [[1.0016, 0.0, -16.0514],         [0.0, 10000.0, -40000.0],         [-16.0514, -40000.0, 160513.6437]] Xt =  [(1.0, 1.0),         (0.0, 0.25),         (0.0, 0.0625)] 

where Xt is the zip transpose of another matrix. Now here is the code:

def matrixmult (A, B):     C = [[0 for row in range(len(A))] for col in range(len(B[0]))]     for i in range(len(A)):         for j in range(len(B[0])):             for k in range(len(B)):                 C[i][j] += A[i][k]*B[k][j]     return C 

The error that python gives me is this:

IndexError: list index out of range.

Now I'm not sure if Xt is recognised as an matrix and is still a list object, but technically this should work.

Answer 1

If you really don't want to use numpy you can do something like this:

def matmult(a,b):     zip_b = zip(*b)     # uncomment next line if python 3 :      # zip_b = list(zip_b)     return [[sum(ele_a*ele_b for ele_a, ele_b in zip(row_a, col_b))               for col_b in zip_b] for row_a in a]  x = [[1,2,3],[4,5,6],[7,8,9],[10,11,12]] y = [[1,2],[1,2],[3,4]]  import numpy as np # I want to check my solution with numpy  mx = np.matrix(x) my = np.matrix(y)        

Result:

>>> matmult(x,y) [[12, 18], [27, 42], [42, 66], [57, 90]] >>> mx * my matrix([[12, 18],         [27, 42],         [42, 66],         [57, 90]])