CODE - The content of an object is changed after e

Question

The problem is that the content of $oldpop get magically changed after executing the function func, whereas the input of func is $matepop. Inside func, $oldpop is not used (I added the comment line to show the place - see the end of the code snippet of MAIN.PHP). Below I provide just some principal parts of the code. Maybe, someone could suggest the reason of the problem?

I should mention I don't use static variables.

File MAIN.PHP

include_once 'func.php';
include_once 'select.php';

class Individual {
    private $genes;
    private $rank;

    public function __construct() {
          $this->genes = array();
          $this->rank = 0;
    }

    public function setRank($val){
            $this->rank = $val;
    }

    public function setGene($i,$val){
        $this->genes[$i] = $val;
    }
}

class Population {
    private $ind;

    public function __construct()
    {
        $this->ind = array();
    }

    public function addIndividual(Individual $ind)
    {
        $this->ind[] = $ind;
    }

    public function getIndividual($i){
        return $this->ind[$i];
    }
  }


    $oldpop = new Population();

  for($i=0; $i<$popsize; $i++) {
    $oldpop->addIndividual(new Individual());
  }

    $oldpop = func($oldpop,$popsize); 

    for ($i = 0; $i < $gener; $i++) 
    {        
        $matepop = new Population();
        $matepop = nselect($matepop,$oldpop,$popsize);

        // !!! Here the $oldpop content is correct (original)
        $matepop = func($matepop,$popsize);
        // !!!! Here the original content of $oldpop is magically changed

    }

File SELECT.PHP

function nselect($matepop,$oldpop,$popsize) { 
  $select = array();
  //...
  $select[] = $oldpop->getIndividual($i);
  //...
  for ($i=0; $i < $popsize; $i++) {
       $matepop->addIndividual($select[$i]);
  }
  return $matepop;
}

File FUNC.PHP

function func($pop,$popsize) { 
  //...
  $pop->getIndividual($i)->setRank($val);
  //...
  return $pop;
}

Answer 1

PHP object variables don't contain a full copy of the object; rather, they contain a reference to the object. This means that when you used

$select[] = $oldpop->getIndividual($i);

in your code, you didn't make a new copy of the object $i; you simply copied the reference to the object $i into the array $select. This means that both $oldpop and $matepop contain references to the same objects in their $ind arrays. Then, when you later used

$pop->getIndividual($i)->setRank($val);

to set the rank of each Individual-class object in $matepop, they also changed for $oldpop.

Answer 2

In your nselect() function, you're getting individuals from $oldpop and then adding them to $matepop:

$select[] = $oldpop->getIndividual($i);
//...
for ($i=0; $i < $popsize; $i++) {
    $matepop->addIndividual($select[$i]);
}

When you do this, this does not "create a copy" of the individual and add the copy - it adds a reference to the original. So, whatever you do to any of the individuals added to $matepop through this method will consequently be updated in $oldpop as well because, well, they're the exact same object.

So, in your func() method, when you set the rank on the individuals in $matepop (and I'm assuming that's the only individual-modification you perform in that function) you're modifying the one and only instance of each individual, which happens to be referenced in $matepop and $oldpop. This is why you see $oldpop modified.

To resolve an issue like this, you'll need to implement some type of cloning/copying instead of a direct reference.