Python Recursive Matrix Multiplier-CodePudding

I have made a matrix multiplier with recursion. Problem is that the return value only remembers the first iteration. The function works. It actually produces correct matrix multiplication for more than 2 matrices. I can probably fix this if I use a global variable but I want to know if there is another way to do this.

def m_multiple(*args):
    storage = [matrix for matrix in args]
    for no_matrix in range(0, len(storage)): # Number of matrix
        result = [[0 for _ in range(len(storage[no_matrix   1][0]))] for _ in range(len(storage[no_matrix]))]
        for row in range(len(storage[no_matrix])): # row of first matrix
            for column in range(len(storage[no_matrix   1][0])): # column of second matrix
                for row2 in range(len(storage[no_matrix   1])): # row of second matrix
                    result[row][column]  = storage[no_matrix][row][row2] * storage[no_matrix   1][row2][column]
        break
    if len(storage) > 2:
        storage.pop(0)
        storage.pop(0)
        storage.insert(0, result)
        m_multiple(*storage)
    return result

For example:

def main():
    x = [[2, 2], [2, 2]]
    y = [[3], [2]]
    z = [[1, 2]]
    a = [[1], [1]]
    print(m_multiple(x, y, z, a))

if __name__ == '__main__':
    main()

# Answer: (With print values at every stage of the recursion in m_multiple())
# [[10], [10]]
# [[10, 20], [10, 20]]
# [[30], [30]]
# [[10], [10]] <- Actual return output

CodePudding user response：

A minute change on your code will make the program work fine

Solution:

You should return the value returned by recursive call, instead you were just performing the function.

This line has been changed: return m_multiple(*storage)

Here is the code:

def m_multiple(*args):
    storage = [matrix for matrix in args]
    for no_matrix in range(0, len(storage)):  # Number of matrix
        result = [
            [0 for _ in range(len(storage[no_matrix   1][0]))]
            for _ in range(len(storage[no_matrix]))
        ]
        for row in range(len(storage[no_matrix])):  # row of first matrix
            for column in range(
                len(storage[no_matrix   1][0])
            ):  # column of second matrix
                for row2 in range(len(storage[no_matrix   1])):  # row of second matrix
                    result[row][column]  = (
                        storage[no_matrix][row][row2]
                        * storage[no_matrix   1][row2][column]
                    )
        break
    if len(storage) > 2:
        storage.pop(0)
        storage.pop(0)
        storage.insert(0, result)
        return m_multiple(*storage)
    return result


def main():
    x = [[2, 2], [2, 2]]
    y = [[3], [2]]
    z = [[1, 2]]
    a = [[1], [1]]
    print(m_multiple(x, y, z, a))


if __name__ == "__main__":
    main()

Output:

[[30], [30]]