Challenge

A simple challenge: Given a two-dimensional matrix (an array of arrays) of real numbers, compute the determinant.

The determinant of a matrix is a mathematical construct used in many applications, such as solving polynomial equations, identifying the invertibility of the matrix, and finding the scaling factor under a matrix transformation. For more information about it, see this Wikipedia entry.

There are a couple of different ways to compute the determinant, and it is up to you how you implement it.

For instance, you may compute it using the Laplace expansion, a recursive algorithm which goes

1. Pick a row or column.
2. Start with a sum of zero.
3. For each entry of the row/column:
   1. Create a new matrix with the row and column of the entry removed. This new matrix is a square matrix of size one less that the original.
   2. Compute the determinant of that smaller matrix.
   3. Multiply that determinant by the entry.
   4. If the row index plus the column index is even^[1], add it to the sum, otherwise, subtract it.
4. The final sum is the determinant.

As an example, here is an ungolfed implementation along the first column.

function laplaceDet(matrix) {
	if (matrix.length === 1) return matrix[0][0];

	let sum = 0;
	for (let rowIndex = 0; rowIndex < matrix.length; ++rowIndex) {
		let minorMatrix = matrix.filter((_, index) => index !== rowIndex)
			          .map(row => row.slice(1));
		sum += ((-1) ** rowIndex) * matrix[rowIndex][0] * laplaceDet(minorMatrix);
	}
	return sum;
}

Try it online!

This is code-golf, so the program with the lowest byte-count wins!

Test cases

$$ \begin{aligned} \det\begin{bmatrix}1&0\\0&1\end{bmatrix}&=1 \\ \det\begin{bmatrix}1.5&2\\-3&4.5\end{bmatrix}&=12.75 \\ \det\begin{bmatrix}3&7\\1&-4\end{bmatrix}&=-19 \\ \det\begin{bmatrix}1&0&0\\0&1&0\\0&0&1\end{bmatrix}&=1 \\ \det\begin{bmatrix}1&2&3\\4&5&6\\7&8&9\end{bmatrix}&=0 \end{aligned} $$

In text form,

[[1,0],[0,1]] -> 1
[[1.5,2],[-3,4.5]] -> 12.75
[[3,7],[1,-4]] -> -19
[[1,0,0],[0,1,0],[0,0,1]] -> 1
[[1,2,3],[4,5,6],[7,8,9]] -> 0

posted about 3 years ago

CC BY-SA 4.0

3y ago

Moshi‭

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