学习Python并陷入矩阵乘法

Question

def matrixmulti(mat1,mat2):

result=[]
'''
for i in range(len(mat1)): 


    for j in range(len(mat2[0])): 


        for k in range(len(mat2)): 
            result[i][j] += mat1[i][k] * mat2[k][j]
            print (result[i][j])
'''
result = [[sum(a * b for a, b in zip(mat1_row, mat2_col))  
                    for mat2_col in zip(*mat2)] 
                            for mat1_row in mat1] 

for i in range(len(mat1)):
    for j in range(len(mat2[0])):
        print(mat[i][j],end=" ")
    print()

我首先尝试使用嵌套的for循环，由于某种原因它没有执行，但是嵌套列表也发生了同样的事情。谁能帮助我我在哪里做错了什么？我刚刚通过了两个全局初始化的矩阵来传递函数。

Answer 1

您在代码中犯了很少的错误，可以修复该错误以使其正常工作。我已经制作了一个固定的三环版本，并在此处添加了一些注释，以说明如何正确完成它：

def matrixmulti(mat1,mat2):
    # <-- code in functions needs to be indented in python for it to be considered inside the function!

    if len(mat1[0]) != len(mat2): # Optional: check if "inner" dimensions match and throw an exception if they don't
        raise ValueError("matrices dimensions does not match")

    # initializing the result matrix to all zeroes. "_" means we don't care about the value of a variable
    result = [ [ 0 for _ in range(len(mat1)) ] for _ in range(len(mat2[0])) ]

    # doing the naive triple for-loop calculation
    for i in range(len(mat1)): 
        for j in range(len(mat2[0])):
            for k in range(len(mat2)): 
                result[i][j] += mat1[i][k] * mat2[k][j]

    # don't forget to return the results! otherwise you function is not super helpful
    return result

# heres the matrix printing wrapped up in a function
def print_matrix(matrix):
    for i in range(len(matrix)):
        for j in range(len(matrix[0])):
            print(matrix[i][j],end=" ")
        print()

multiplied = matrixmulti(mat1, mat2) # do the calculation and store the results in "multiplied"

print_matrix(multiplied) # finally print the results

您犯了几个错误：

1. 您必须缩进函数代码，才能将其视为Python函数内部的代码
2. [如果执行三重循环计算，则必须以某种方式初始化结果矩阵（在示例中，我使用列表推导进行了此操作，因此我获得了正确的嵌套结构），否则，对结果矩阵的索引将失败，因为试图访问嵌套的部分将不存在
3. 您必须从matrixmulti函数返回结果，或者在函数内部打印结果才能获得帮助

希望这会有所帮助。