python 3.x中矩阵乘法的列表理解

问题描述

我在 Python 3.x 中找到了矩阵乘法的代码，但我无法理解列表推导在下面的代码中是如何工作的。

# Program to multiply two matrices using list comprehension

# 3x3 matrix
X = [[12,7,3],[4,5,6],[7,8,9]]

# 3x4 matrix
Y = [[5,1,2],[6,3,0],9,1]]

# result is 3x4
result = [[sum(a*b for a,b in zip(X_row,Y_col)) for Y_col in zip(*Y)] for X_row in X]

for r in result:
   print(r)

解决方法

@Santosh，从纯粹的 List Comprehension 方式可能更容易理解这个 loop，如下所示：

# 3x3 matrix
X = [[12,7,3],[4,5,6],[7,8,9]]
# 3x4 matrix
Y = [[5,1,2],[6,3,0],9,1]]
# result is 3x4
result = [[0,[0,0]]

# iterate through rows of X
for r in range(len(X)):
   # iterate through columns of Y
   for c in range(len(Y[0])):
       # iterate through rows of Y
       for k in range(len(Y)):
           result[r][c] += X[r][k] * Y[k][c]
           
print(result)

那你猜。可以找到与 List Comprehension 版本的相似之处，几乎不需要重新格式化：

def matrix_mul(X,Y):
    zip_b = list(zip(*Y))
    return [[sum(a * b for a,b in zip(row_a,col_b))
                 for col_b in zip_b]
            for row_a in X]