Unexpected T_STRING error (PHP) - php

Question

I can normally catch these easy, but......
function linear_regression($x, $y) {
// calculate number points
$n = count($x);
// ensure both arrays of points are the same size
if ($n != count($y)) {
trigger_error("linear_regression(): Number of elements in coordinate arrays do not match.", E_USER_ERROR);
}
// calculate sums
$x_sum = array_sum($x);
$y_sum = array_sum($y);
$xx_sum = 0;
$xy_sum = 0;
for($i = 0; $i < $n; $i++) {
$xy_sum+=($x[$i]*$y[$i]);
$xx_sum+=($x[$i]*$x[$i]);
}
// calculate slope
//$m = (($n * $xy_sum) - ($x_sum * $y_sum)) / (($n * $xx_sum) - ($x_sum * $x_sum));
$divisor = (($n * $xx_sum) – ($x_sum * $x_sum));
if ($divisor == 0){
$m = 0;
} else {
$m = (($n * $xy_sum) – ($x_sum * $y_sum)) / $divisor;
}
// calculate intercept
$b = ($y_sum - ($m * $x_sum)) / $n;
// return result
return array("m"=>$m, "b"=>$b);
}
var_dump( linear_regression(array(1, 2, 3, 4, 4), array(1.5, 1.6, 2.1, 3.0, 6)) );
The error is happening here $divisor = (($n * $xx_sum) – ($x_sum * $x_sum));
Any ideas why?

Answer 1

The minus sign is a fancy unicode dash (I think the em dash) instead of the regular ascii - character.

Answer 2

Don't ask me how I discovered that, but your "-" is not a real "-", is another character, but have the same apparence
Ok you didn't asked me, but I have discovered at http://writecodeonline.com/php/ , it doesn't recognize yours "-" character
Now working code with real "-", you can copy an paste, and see:
function linear_regression($x, $y) {
// calculate number points
$n = count($x);
// ensure both arrays of points are the same size
if ($n != count($y)) {
trigger_error("linear_regression(): Number of elements in coordinate arrays do not match.", E_USER_ERROR);
}
// calculate sums
$x_sum = array_sum($x);
$y_sum = array_sum($y);
$xx_sum = 0;
$xy_sum = 0;
for($i = 0; $i < $n; $i++) {
$xy_sum+=($x[$i]*$y[$i]);
$xx_sum+=($x[$i]*$x[$i]);
}
// calculate slope
//$m = (($n * $xy_sum) - ($x_sum * $y_sum)) / (($n * $xx_sum) - ($x_sum * $x_sum));
$divisor = (($n * $xx_sum) - ($x_sum * $x_sum));
if ($divisor == 0){
$m = 0;
} else {
$m = (($n * $xy_sum) - ($x_sum * $y_sum)) / $divisor;
}
// calculate intercept
$b = ($y_sum - ($m * $x_sum)) / $n;
// return result
return array("m"=>$m, "b"=>$b);
}
var_dump( linear_regression(array(1, 2, 3, 4, 4), array(1.5, 1.6, 2.1, 3.0, 6)) );

Answer 3

I retyped the offending line like this:
$divisor = (($n*$xx_sum)-($x_sum * $x_sum));
The error disappeared. After a bit of poking the error seems to be related to the minus sign in the middle. deleting it and retyping it seems to fix the problem.
See this fiddle:

Answer 4

Adam is right, but also, you didn’t close the for loop. Here try this.
Also, the minus sign was the wrong sign here:
$divisor = (($n * $xx_sum) - ($x_sum * $x_sum));
As well as here:
$m = (($n * $xy_sum) - ($x_sum * $y_sum)) / $divisor;
My cleaned up version of your function with the closed for loop in place as well here.
function linear_regression($x, $y) {
// calculate number points
$n = count($x);
// ensure both arrays of points are the same size
if ($n != count($y)) {
trigger_error("linear_regression(): Number of elements in coordinate arrays do not match.", E_USER_ERROR);
}
// calculate sums
$x_sum = array_sum($x);
$y_sum = array_sum($y);
$xx_sum = 0;
$xy_sum = 0;
for($i = 0; $i < $n; $i++) {
$xy_sum+=($x[$i]*$y[$i]);
$xx_sum+=($x[$i]*$x[$i]);
// calculate slope
//$m = (($n * $xy_sum) - ($x_sum * $y_sum)) / (($n * $xx_sum) - ($x_sum * $x_sum));
$divisor = (($n * $xx_sum) - ($x_sum * $x_sum));
if ($divisor == 0) {
$m = 0;
}
else {
$m = (($n * $xy_sum) - ($x_sum * $y_sum)) / $divisor;
}
// calculate intercept
$b = ($y_sum - ($m * $x_sum)) / $n;
}
// return result
return array("m"=>$m, "b"=>$b);
} // linear_regression