TheAlgorithms/Python · error · Exception

Unable to multiply these matrices, please check the dimensio

Error message

Unable to multiply these matrices, please check the dimensions.
Matrix A: {matrix1}
Matrix B: {matrix2}

What it means

Raised by strassen() when the inner dimension mismatches: the column count of matrix1 (matrix_dimensions(matrix1)[1]) must equal the row count of matrix2 (matrix_dimensions(matrix2)[0]). This is the standard matrix-multiplication conformability rule; the error message embeds both matrices for debugging. It is raised as a bare Exception before any padding or recursion starts.

Source

Thrown at divide_and_conquer/strassen_matrix_multiplication.py:120

    for i in range(len(bot_right)):
        new_matrix.append(bot_left[i] + bot_right[i])
    return new_matrix


def strassen(matrix1: list, matrix2: list) -> list:
    """
    >>> strassen([[2,1,3],[3,4,6],[1,4,2],[7,6,7]], [[4,2,3,4],[2,1,1,1],[8,6,4,2]])
    [[34, 23, 19, 15], [68, 46, 37, 28], [28, 18, 15, 12], [96, 62, 55, 48]]
    >>> strassen([[3,7,5,6,9],[1,5,3,7,8],[1,4,4,5,7]], [[2,4],[5,2],[1,7],[5,5],[7,8]])
    [[139, 163], [121, 134], [100, 121]]
    """
    if matrix_dimensions(matrix1)[1] != matrix_dimensions(matrix2)[0]:
        msg = (
            "Unable to multiply these matrices, please check the dimensions.\n"
            f"Matrix A: {matrix1}\n"
            f"Matrix B: {matrix2}"
        )
        raise Exception(msg)
    dimension1 = matrix_dimensions(matrix1)
    dimension2 = matrix_dimensions(matrix2)

    if dimension1[0] == dimension1[1] and dimension2[0] == dimension2[1]:
        return [matrix1, matrix2]

    maximum = max(*dimension1, *dimension2)
    maxim = int(math.pow(2, math.ceil(math.log2(maximum))))
    new_matrix1 = matrix1
    new_matrix2 = matrix2

    # Adding zeros to the matrices to convert them both into square matrices of equal
    # dimensions that are a power of 2
    for i in range(maxim):
        if i < dimension1[0]:
            for _ in range(dimension1[1], maxim):
                new_matrix1[i].append(0)
        else:

View on GitHub (pinned to f5988cc097)

Solutions

1. Fix operand order/orientation so columns(A) == rows(B), e.g. pass b transposed if you computed b.T by mistake.
2. Add a precondition check: if len(a[0]) != len(b): raise/transpose before calling.
3. Inspect the printed matrices in the message to see which operand has the wrong shape.

Example fix

# before
product = strassen(a, b)  # a: 2x2, b: 3x2 -> Exception

# after
if len(a[0]) != len(b):
    raise ValueError(f'incompatible: {len(a)}x{len(a[0])} @ {len(b)}x{len(b[0])}')
product = strassen(a, b)

def multipliable(a, b) -> bool:
    return all(len(row) == len(a[0]) for row in a) and len(a[0]) == len(b)

if multipliable(matrix1, matrix2):
    product = strassen(matrix1, matrix2)

def matrices_conformable(a: list, b: list) -> bool:
    return bool(a) and bool(b) and len(a[0]) == len(b)

try:
    product = strassen(matrix1, matrix2)
except Exception as exc:
    if 'check the dimensions' in str(exc):
        raise ValueError(f'cannot multiply {len(matrix1)}x{len(matrix1[0])} by {len(matrix2)}x...') from exc
    raise

When it happens

Trigger: strassen([[1,2],[3,4]], [[1,2,3]]) — a 2x2 times a 1x3 (2 columns vs 1 row); any A (mxn) times B (pxq) with n != p. Square matrices of equal size, and any n == p pair (including rectangulars like 4x3 * 3x4), pass this check.

Common situations: Transposed second operand (B.T vs B); loading matrices from files with wrong orientation; assuming the function requires square equal matrices (it does not — it pads automatically) and pre-transposing data incorrectly.

Related errors

• Matrices are not 2x2
• Odd matrices are not supported!
• Expecting a list of points but got {points}
• the value of both inputs must be positive
• both inputs must be positive integers

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/9d3206339908b168. Report an issue: GitHub.