Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.php > #5335

Re: Doubt regarding an array of references...

Show all headers | View raw

Am 03.04.2012 11:02, schrieb "Álvaro G. Vicario":
> El 02/04/2012 18:17, Leonardo Azpurua escribió/wrote:
>> I am coding a simple parser, where a construct is made of a fixed number
>> sequence of  varIDs or quoted literals that may or may not be separated by
>> punctuators.
>>
>> In order to avoid a repetition of similar statements, I decided to try if I
>> could store the references of the variables where each value should be
>> stored and then perform a loop in order to extract and store the symbols.
>>
>> The try was this little program:
> 
> As a general rule, I suggest you debug with var_dump(). Plain echos cast everything
> to string and hide most of the subtleties you are interested in when debugging.
> 
>>
>>   $a = $b = $c = "nil";
>>   echo "a = $a, b = $b, c = $c<br>\n";
>>
>>   $d = array(&$a,&$b,&$c);
>>   for ($n = 0; $n<  count($d); $n++) $d[$n] =  $n;
>>   echo "a = $a, b = $b, c = $c<br>\n";
>>
>>   for ($n = 0; $n<  count($d); $n++) $d[$n] =  ($n + 100);
>>   echo "a = $a, b = $b, c = $c<br>\n";
>>
>>   $n = 200;
>>   foreach ($d as&$nx) $nx = $n++;;        // why&$nx ?
> 
> Why «&$nx»? Because the foreach() construct does not create references by itself. This:
> 
> foreach($foo as $bar){
>     $bar = 'New value';
> }
> 
> ... is roughly equivalent to:
> 
> for($i=0; $i<count($foo); $i++){
>     $bar = $foo[$i]; // Good old copy, not reference
>     $bar = 'New value'; // $foo[$i] keeps old value
> }
> 
> (Please note we are talking about arrays and scalars; PHP objects follow their own
> rules).
> 
>>   echo "a = $a, b = $b, c = $c<br>\n";
> 
> Last but not least, an additional important bit -- Every time you use references in a
> foreach() construct, don't forget to unset the temporary variable. Otherwise, you'll
> face unexpected side effects if you use the same variable name in another loop. Run
> and compare this:
> 
>     $foo = range(1, 10);
>     foreach($foo as &$bar){
>         $bar *= 100;
>     }
> 
>     var_dump($foo);
>     foreach($foo as $bar){
>     }
>     var_dump($foo);
> 
> ... and this:
> 
>     $foo = range(1, 10);
>     foreach($foo as &$bar){
>         $bar *= 100;
>     }
>     unset($bar); // <--
> 
>     var_dump($foo);
>     foreach($foo as $bar){
>     }
>     var_dump($foo);
> 
> (In case you didn't see it, there's a full chapter dedicated to references [1])
> 
> [1] http://www.php.net/manual/en/language.references.php

My 2¢:

The problem with the counted for-loop is tedious writing, and off-by-one error.

The problem with the foreach-loop is: don't forget to use the key when you want to
change values!

This works:

#--------------------------------
strobel@s114-intel:~> php -a
Interactive shell

php > $a = array(1,2,3);
php > foreach ($a as $k=>$v) { $a[$k]++; }
php > var_dump($a);
array(3) {
  [0]=>
  int(2)
  [1]=>
  int(3)
  [2]=>
  int(4)
}
php > foreach ($a as $k=>$v) { $a[$k]*=4; }
php > var_dump($a);
array(3) {
  [0]=>
  int(8)
  [1]=>
  int(12)
  [2]=>
  int(16)
}
php >
#--------------------------

I see references in PHP as "a can of worms", in PHP5 they are only necessary in very
special cases.

/Str.

Back to comp.lang.php | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Doubt regarding an array of references... "Leonardo Azpurua" <leonardo@exmvps.org> - 2012-04-02 11:47 -0430
  Re: Doubt regarding an array of references... "Álvaro G. Vicario" <alvaro.NOSPAMTHANX@demogracia.com.invalid> - 2012-04-03 11:02 +0200
    Re: Doubt regarding an array of references... "M. Strobel" <sorry_no_mail_here@nowhere.dee> - 2012-04-03 13:39 +0200
      Re: Doubt regarding an array of references... Jerry Stuckle <jstucklex@attglobal.net> - 2012-04-03 08:03 -0400
        Re: Doubt regarding an array of references... "M. Strobel" <sorry_no_mail_here@nowhere.dee> - 2012-04-03 14:15 +0200
        Re: Doubt regarding an array of references... "M. Strobel" <sorry_no_mail_here@nowhere.dee> - 2012-04-04 10:48 +0200

csiph-web