Matrix multiplication in 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.

Answer 1

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;
  }
}