Matrix multiplication in php - php

Question

Although the order of matrices should be fine, the following code throws back the exception. It might be a tiny thing I'm not being able to notice, but can't figure it out.
<?php
$mat1 = array(5,1);
$mat2 = array(1,5);
function matrixmult($m1,$m2){
$r=count($m1);
$c=count($m2[0]);
$p=count($m2);
if(count($m1[0])!=$p){throw new Exception('Incompatible matrixes');}
$m3=array();
for ($i=0;$i< $r;$i++){
for($j=0;$j<$c;$j++){
$m3[$i][$j]=0;
for($k=0;$k<$p;$k++){
$m3[$i][$j]+=$m1[$i][$k]*$m2[$k][$j];
}
}
}
}
return($m3);
}
matrixmult($mat1,$mat2);
?>

Answer 1

You're specifying your test matrices wrong, in two ways:
The arrays aren't two-dimensional (that is, arrays of arrays of numbers).
Even if you wrapped another array( ) around them, the condition that the width of the first matrix be equal to the height of the second matrix doesn't hold with [5 1] and [1 5], which are both 2 wide and 1 high.
What you need is something like
$mat1 = array(array(5,1));
$mat2 = array(array(1),array(5));

Answer 2

Just to round this out. Here is a working solution:
function M_mult($_A,$_B) {
// AxB outcome is C with A's rows and B'c cols
$r = count($_A);
$c = count($_B[0]);
$in= count($_B); // or $_A[0]. $in is 'inner' count
if ( $in != count($_A[0]) ) {
print("ERROR: need to have inner size of matrices match.\n");
print(" : trying to multiply a ".count($_A)."x".count($_A[0])." by a ".count($_B)."x".count($_B[0])." matrix.\n");
print("\n");
exit(1);
}
// allocate retval
$retval = array();
for($i=0;$i< $r; $i++) { $retval[$i] = array(); }
// multiplication here
for($ri=0;$ri<$r;$ri++) {
for($ci=0;$ci<$c;$ci++) {
$retval[$ri][$ci] = 0.0;
for($j=0;$j<$in;$j++) {
$retval[$ri][$ci] += $_A[$ri][$j] * $_B[$j][$ci];
}
}
}
return $retval;
}
}

Answer 3

You must delete the curly braces before return.
<?php
$mat1 = array(5,1);
$mat2 = array(1,5);
function matrixmult($m1,$m2){
$r=count($m1);
$c=count($m2[0]);
$p=count($m2);
if(count($m1[0])!=$p){throw new Exception('Incompatible matrixes');}
$m3=array();
for ($i=0;$i< $r;$i++){
for($j=0;$j<$c;$j++){
$m3[$i][$j]=0;
for($k=0;$k<$p;$k++){
$m3[$i][$j]+=$m1[$i][$k]*$m2[$k][$j];
}
}
}
return($m3);
}
matrixmult($mat1,$mat2);
?>