| Re: Doubt regarding an array of references... [message #177507 is a reply to message #177493] |
Tue, 03 April 2012 09:02   |
alvaro.NOSPAMTHANX
Messages: 277
Registered: September 2010
Karma:
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Senior Member |
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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
--
-- http://alvaro.es - Álvaro G. Vicario - Burgos, Spain
-- Mi sitio sobre programación web: http://borrame.com
-- Mi web de humor satinado: http://www.demogracia.com
--
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