MathPHP\LinearAlgebra\Matrix::diagonal PHP Method

    public function diagonal() : Matrix
    {
        $m = $this->m;
        $n = $this->n;
        $R = [];
        for ($i = 0; $i < $m; $i++) {
            for ($j = 0; $j < $n; $j++) {
                $R[$i][$j] = $i == $j ? $this->A[$i][$j] : 0;
            }
        }
        return MatrixFactory::create($R);
    }