Add matrix multiplication function with validation

[ArsiyaTasleem90250]

Implement matrix multiplication function with validation.

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[algorithms-keeper]

Click here to look at the relevant links ⬇️

🔗 Relevant Links

Repository:

• Contributing guidelines
• Project Euler solution guidelines

Python:

• Formatted string literals (f-strings)
• Type hints
• doctest
• unittest
• pytest

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[algorithms-keeper]

Variable and function names should follow the snake_case naming convention. Please update the following name accordingly: rows_A

[algorithms-keeper]

Variable and function names should follow the snake_case naming convention. Please update the following name accordingly: cols_A

[algorithms-keeper]

Variable and function names should follow the snake_case naming convention. Please update the following name accordingly: rows_B

[algorithms-keeper]

Variable and function names should follow the snake_case naming convention. Please update the following name accordingly: cols_B

[algorithms-keeper]

Please provide descriptive name for the parameter: A

Please provide descriptive name for the parameter: B

[algorithms-keeper]

Please provide descriptive name for the parameter: M

[cclauss]

@priya-sundaram-dev Filename is matrix/matrix_diagonal_sum.py, but the algorithm is matrix multiplication.
Are they two different algorithms??

• https://docs.python.org/3/whatsnew/3.5.html#pep-465-a-dedicated-infix-operator-for-matrix-multiplication

[priya-sundaram-dev]

@cclauss you're right — they're mismatched. The file is named matrix/matrix_diagonal_sum.py (and DIRECTORY.md lists it as "Matrix Diagonal Sum"), but the code inside is matrix_multiply() — plain O(n³) matrix multiplication, nothing about a diagonal sum. So the name/content don't agree.

@ArsiyaTasleem90250 two things to sort out before this can go in:

1. It's a duplicate. We already have matrix multiplication in the repo: matrix/matrix_operation.py has multiply(matrix_a, matrix_b), and there's also matrix/matrix_multiplication_recursion.py. Per CONTRIBUTING we don't add a second implementation of an existing algorithm. If you actually meant to add a diagonal sum (that's a genuinely different, not-yet-present algorithm), rename the function to match the file and implement that instead — e.g. sum of the primary (and optionally secondary) diagonal.

2. If you do resubmit, a few style fixes the CI will want anyway: use built-in generics (list[list[float]]) rather than typing.List, and snake_case names — A/B will trip ruff's N806, use matrix_a/matrix_b.

Happy to re-review once it's pointed at a non-duplicate algorithm. Thanks for the PR!

[priya-sundaram-dev]

@cclauss good catch — yes, these look like two different things. The file is named matrix/matrix_diagonal_sum.py and the DIRECTORY.md entry says "Matrix Diagonal Sum", but the module docstring and the function (matrix_multiply) both implement matrix multiplication, not a diagonal sum. So the name and the algorithm don't match.

And matrix multiplication is already in the repo, so as written this is a duplicate:

• matrix/matrix_operation.py → multiply(matrix_a, matrix_b)
• matrix/matrix_multiplication_recursion.py

Two ways forward for @-the author:

1. If the intent was a diagonal sum algorithm (which isn't in the repo yet), keep the filename and rewrite the body to actually sum the main/anti-diagonals — that would be a genuinely new algorithm.
2. If the intent was matrix multiplication, this overlaps existing code and would likely be closed as a duplicate.

Two smaller style notes for whichever direction is taken (both will trip the ruff CI here):

• Use built-in generics list[list[float]] instead of typing.List (repo targets 3.10+, and from typing import List is flagged).
• Rename the uppercase locals/args (A, B, M) to lowercase (matrix_a, etc.) — ruff N803/N806 flags uppercase names.