使用array_splice（）在矩阵中一次删除两列

Question

I'm trying to get the correct result for deleted columns and rows in a matrix, but whenever I remove the first column specified in the $columnsToRemove array, the array shifts, and I end up deleting the wrong column. in the following example, how would I delete both the 0th and the 2nd column in each submatrix in one go?

$matrix = [
    [1,0,0,2], 
    [0,5,0,1], 
    [0,0,3,5]
];

$rowsToDelete = [1];

$columnsToDelete = [0, 2];

function constructSubmatrix($matrix, $rowsToDelete, $columnsToDelete) {
    foreach($rowsToDelete as $row){
        array_splice($matrix, $row, 1);
    }
    foreach($columnsToDelete as $column){
        for($i = 0; $i < count($matrix); $i++){
            array_splice($matrix[$i], $column, 1);
        }
    }
    return $matrix;
}

The return value should be: [[0,2],[0,5]]

But I keep getting: [[0,0],[0,3]]

Answer 1

Because if you're deleting multiple rows or multiple columns, then deleting the lowest column in that list will mean that the higher column numbers aren't correct any more.

e.g. with $columnsToDelete = [0, 2]; once you've deleted column 0, then column 2 has become column 1, but your'e still trying to delete column 2

You need to delete in reverse order to avoid this; so use rsort() your $rowsToDelete and $columnsToDelete before doing the deletion.

function constructSubmatrix($matrix, $rowsToDelete, $columnsToDelete) {
    rsort($rowsToDelete);
    foreach($rowsToDelete as $row){
        array_splice($matrix, $row, 1);
    }
    rsort($columnsToDelete);
    foreach($columnsToDelete as $column){
        for($i = 0; $i < count($matrix); $i++){
            array_splice($matrix[$i], $column, 1);
        }
    }
    return $matrix;
}