CapÃtulo 15. Operadores

Tabla de contenidos
Precedencia de Operadores
Operadores de AritmÃ©tica
Operadores de AsignaciÃ³n
Operadores Bit a Bit
Operadores de ComparaciÃ³n
Operadores de Control de Errores
Operadores de ejecuciÃ³n
Operadores de Incremento/Decremento
Operadores de LÃ³gica
Operadores de Cadena
Operadores de Matrices
Operadores de Tipo

Un operador es algo a lo que usted entrega uno o mÃ¡s valores (o expresiones, en jerga de programaciÃ³n) y produce otro valor (de modo que la construcciÃ³n misma se convierte en una expresiÃ³n). AsÃ que puede pensar sobre las funciones o construcciones que devuelven un valor (como print) como operadores, y en aquellas que no devuelven nada (como echo) como cualquier otra cosa.

Existen tres tipos de operadores. En primer lugar se encuentra el operador unario, el cual opera sobre un Ãºnico valor, por ejemplo ! (el operador de negaciÃ³n) o ++ (el operador de incremento). El segundo grupo se conoce como operadores binarios; Ã©ste grupo contiene la mayorÃa de operadores que soporta PHP, y una lista se encuentra disponible mÃ¡s adelante en la secciÃ³n Precedencia de Operadores.

El tercer grupo consiste del operador ternario: ?:. Ã‰ste debe ser usado para seleccionar entre dos expresiones, en base a una tercera, en lugar de seleccionar dos sentencias o rutas de ejecuciÃ³n. Rodear las expresiones ternarias con parÃ©ntesis es una muy buena idea.

Precedencia de Operadores

La precedencia de un operador indica quÃ© tan "cerca" se agrupan dos expresiones. Por ejemplo, en la expresiÃ³n 1 + 5 * 3, la respuesta es 16 y no 18, ya que el operador de multiplicaciÃ³n ("*") tiene una mayor precedencia que el operador de adiciÃ³n ("+"). Los parÃ©ntesis pueden ser usados para marcar la precedencia, si resulta necesario. Por ejemplo: (1 + 5) * 3 evalÃºa a 18. Si la precedencia de los operadores es la misma, se utiliza una asociaciÃ³n de izquierda a derecha.

La siguiente tabla lista la precedencia de los operadores, con aquellos de mayor precedencia listados al comienzo de la tabla. Los operadores en la misma lÃnea tienen la misma precedencia, en cuyo caso su asociatividad decide el orden para evaluarlos.

Tabla 15-1. Precedencia de Operadores

Asociatividad	Operadores	InformaciÃ³n Adicional
no-asociativo	new	new
izquierda	[	array()
no-asociativos	++ --	incremento/decremento
no-asociativos	! ~ - (int) (float) (string) (array) (object) @	tipos
izquierda	* / %	aritmÃ©tica
izquierda	+ - .	aritmÃ©tica, y cadena
izquierda	<< >>	manejo de bits
no-asociativos	< <= > >=	comparaciÃ³n
no-asociativos	== != === !==	comparaciÃ³n
izquierda	&	manejo de bits, y referencias
izquierda	^	manejo de bits
izquierda	\|	manejo de bits
izquierda	&&	lÃ³gicos
izquierda	\|\|	lÃ³gicos
izquierda	? :	ternario
derecha	= += -= *= /= .= %= &= \|= ^= <<= >>=	asignaciÃ³n
izquierda	and	lÃ³gicos
izquierda	xor	lÃ³gicos
izquierda	or	lÃ³gicos
izquierda	,	varios usos

La asociatividad de izquierda quiere decir que la expresiÃ³n es evaluada desde la izquierda a la derecha, la asociatividad de derecha quiere decir lo contrario.
Ejemplo 15-1. Asociatividad

<?php $a = 3 * 3 % 5; // (3 * 3) % 5 = 4 $a = true ? 0 : true ? 1 : 2; // (true ? 0 : true) ? 1 : 2 = 2 $a = 1; $b = 2; $a = $b += 3; // $a = ($b += 3) -> $a = 5, $b = 5 ?>
Use parÃ©ntesis para incrementar la legibilidad del cÃ³digo.

Nota: Aunque ! tiene una mayor precedencia que =, PHP permitirÃ¡ aun expresionas similares a la siguiente: if (!$a = foo()), en cuyo caso la salida de foo() va a dar a $a.

add a note User Contributed Notes
Operadores

T Chan
19-Aug-2006 10:30


In response to 26-Mar-2001 08:53, parens don't need to have precedence. There's only one way to convert them to a syntax tree. I can't think of a sensible reason it could be ambiguous either (and sensible language designers will make ( always pair with ) so there can't be any ambiguity).



In response to 12-Aug-2005 08:47, you can do <?php $myvar OR ($myvar = 1); ?>, or the equivalent <?php !$myvar AND $myvar = 1; ?>, and if you have to use ifs, <?php if(!$myvar) { $myvar = 1; } ?> will do just fine (though in general, I'd be careful using boolean expressions as an indicator of success/failure when 0 could be a valid value. And you probably want to use isset($myvariable)).



In response to the manual, associativity doesn't determine the order of evaluation - it determines how it's "bracketed": a=b=c=1 is equivalent to a=(b=(c=1)), but a is evaluated first (an important point if evaluating a has side-effects)

<?php

function one($str) {

    echo "$str";

    return 1;

}



$a = array(1,2);

$a[one("A")] = $a[one("B")] = 1; // outputs AB

?>

golotyuk at gmail dot com
09-Jul-2006 09:51


Simple POST and PRE incremnt sample:



<?php



$b = 5;

$a = ( ( ++$b ) > 5 ); // Pre-increment test

echo (int)$a;



$b = 5;

$a = ( ( $b++ ) > 5 ); // Post-increment test

echo (int)$a;



?>



This will output 10, because of the difference in post- and pre-increment operations

dlyons at lyons42 dot com
26-Nov-2005 03:30


Re: Rick on 2-Sep-2005.



Actually, the C equivalent of "$a[$c++]=$b[$c++];" has undefined behavior, and the increments are by no means guaranteed to happen after the assignment in C.  (Search for a discussion of C "sequence points" for details.)

sm
02-Sep-2005 05:15


Note the highly unfortunate difference from Java, which associates the trinary operator right-to-left. 



---------------------------- source



function trinaryTest($foo){  // works as you think in Java, but not PHP



    $bar    = $foo > 20

            ? "greater than 20"

            : $foo > 10

                ? "greater than 10"

                : $foo > 5

                    ? "greater than 5"

                    : "not worthy of consideration";    

    echo $foo." =>  ".$bar."\n";

}



echo "\n\n\n----trinaryTest\n\n";

trinaryTest(21);

trinaryTest(11);

trinaryTest(6);

trinaryTest(4);



function trinaryTestParens($foo){

    

    $bar    = $foo > 20

            ? "greater than 20"

            : ($foo > 10

                ? "greater than 10"

                : ($foo > 5

                    ? "greater than 5"

                    : "not worthy of consideration"));    

    echo $foo." =>  ".$bar."\n";

}



echo "\n\n\n----trinaryTestParens\n\n";

trinaryTestParens(21);

trinaryTestParens(11);

trinaryTest(6);

trinaryTestParens(4);



---------------------------- output

----trinaryTest



21 =>  greater than 5

11 =>  greater than 5

6 =>  greater than 5

4 =>  not worthy of consideration



----trinaryTestParens



21 =>  greater than 20

11 =>  greater than 10

6 =>  greater than 5

4 =>  not worthy of consideration

rick at nomorespam dot fourfront dot ltd dot uk
02-Sep-2005 03:51


A quick note to any C developers out there, assignment expressions are not interpreted as you may expect - take the following code ;-



<?php

$a=array(1,2,3);

$b=array(4,5,6);

$c=1;



$a[$c++]=$b[$c++];



print_r( $a ) ;

?>



This will output;-

Array ( [0] => 1 [1] => 6 [2] => 3 )

as if the code said;-

$a[1]=$b[2];



Under a C compiler the result is;-

Array ( [0] => 1 [1] => 5 [2] => 3 )

as if the code said;-

$a[1]=$b[1];



It would appear that in php the increment in the left side of the assignment is processed prior to processing the right side of the assignment, whereas in C, neither increment occurs until after the assignment.

kit dot lester at lycos dot co dot uk
22-Aug-2005 05:38


D'oh! please ignore (& forgive) the first paragraph in my note yesterday - I said that a diadic operator had the same precedence & associativity as a bunch of monadics. A bit of early senility must have struck me.



When I find time I'll rework my test to find out what I should have said - but using instanceof monadically didn't cause any visible errors in the test, so it could take a while.



[I'd be delighted if someone else beats me to it/ spares me the difficulties of seeing what's wrong in something that shouldn't have worked.]

kit dot lester at lycos do co dot uk
21-Aug-2005 09:21


Table 15-1 omits the precedence of instanceof - testing suggests it to be of the same precedence as not, negate, casting, and @.



The table also omits the precedence (and associativity) of the "execution operator" - but since that's a sort of quoting, I don't think it meaningfully has a precedence or associativity - they explain what is to happen where there's an open-endedness in the sense of missing brackets, and the quoting is a sort of bracket. (At least: because of the execution operator's double-ended closedness, I can't figure out any code where it would matter, so I can't test it.)

webmaster AT cafe-clope DOT net
12-Aug-2005 12:47


I regularly use some syntax like :



<?php

if(!$myvar)

  $myvar = $value ;

?>



and



<?php

if($myvar)

  echo "myvar is $myvar today" ;

?>



(or <?php echo ($myvar ? "myvar is $myvar today" : "") ?>)



It's small, but can become heavy when used too much.

Isn't there some trick to better such syntaxes ?

I was wondering about using things like :



<?php $myvar ||= $value ; ?>

<?php $myvar ?= $value ; ?>

<?php echo ($myvar ? "myvar is $myvar today") ; ?>

edwardsbc at yahoo dot com
04-May-2005 10:26


In response to npeelman at cfl dot rr dot com

29-Dec-2004 06:22:



You have misunderstood the behaviour of the interpreter.



With out the curly braces and the single quoted key identifier, the interpreter "assumes" you meant your CONSTANT to be a string.  This ONLY works within a parsed (double quoted) string.  And it doesn't help you at all if your array is multi-dimensional. I consider this a very bad habbit because it will get you in trouble elsewhere. Try the following:



<?php

define('item','AnyOldThing');

define('b',12);



$arr['item']['b'] = 'string';

$arr['AnyOldThing'][12]= 'Maybe not what I intended.';



echo "This is a {$arr['item']['b']}"; // [1] prints "This is a string".

echo "This is a $arr[item][b]"; // [2] broken

echo $arr[item][b]; // [3] broken

?>

npeelman at cfl dot rr dot com
29-Dec-2004 03:22


Update to message by yasuo_ohgaki at hotmail dot com:



  I know this is an old message but when using an Associative array in a string you do not have to use { and  } to resolve ambiguity. 



ex:



Associative Array in string: 



$arr['item'] = 'string'; 



echo "This is {$arr['item']}"; //prints "This is string".



...does work but, so does:



echo "This is $arr[item]"; //prints "This is string".



... simply enclose the whole string with double quotes and leave out the single quotes from around the index name. This simplifies the code and makes things easier to read.

Stopping at the dot completely
01-Sep-2004 01:33


The low precedence of the OR operator is useful for error control statements such as this one:

$my_file = @file ('non_existent_file') or die ("Failed opening file: error was '$php_errormsg'");



Notice the good readability of the code.

22-Aug-2004 09:51


I think warhog's note about the differing precedence between && / AND and || / OR is worth repeating.  Since && and || evaluate before the assignment operator (=) while AND and OR evaluate after it, you can get COMPLETELY different results if you don't fully parenthesise.



I cannot imagine when it would ever be important that those two pairs have differing precedence, but they do.  And I just spent two hours discovering that the hard way because I broke my career-long rule:



*Always fully parenthesise!*

11-Jun-2004 07:22


Warhog wrote: "maybe usefull for some tricky coding and helpfull to prevent bugs :D"



I'm sure Warhog was being facetious, but for the new programmers in the audience I'd like to point out that 'tricky coding' and relying on precedence/order of evaluation are both well-known ways to *produce* bugs.



Use parentheses instead.

09-Jun-2004 05:58


of course this should be clear, but i think it has to be mentioned espacially:



AND is not the same like &&



for example:



<?php $a && $b || $c; ?>

is not the same like

<?php $a AND $b || $c; ?>



the first thing is

(a and b) or c



the second

a and (b or c)



'cause || has got a higher priority than and, but less than &&



of course, using always [ && and || ] or [ AND and OR ] would be okay, but than you should at least respect the following:



<?php $a = $b && $c; ?>

<?php $a = $b AND $c; ?>



the first code will set $a to the result of the comparison $b with $c, both have to be true, while the second code line will set $a like $b and THAN - after that - compare the success of this with the value of $c



maybe usefull for some tricky coding and helpfull to prevent bugs :D



greetz, Warhog

yasuo_ohgaki at hotmail dot com
26-Mar-2001 12:34


About "{" and "}".


Sometimes PHP programmers need to use "{" and "}" to resolve ambiguity. Here is some examples.





Variable Variables:





$foo = "test";


$$bar = "this is";





echo "${$bar} $foo"; // prints "this is test"


Note: it is the same as


echo "$test $foo";





Array in string:





$arr[10][10][10] = "string";





echo "This is {$arr[10][10][10]}"; // prints "This is string"





Associative Array in string:





$arr['item'] = 'string';





echo "This is {$arr['item']}"; //prints "This is string".

yasuo_ohgaki at hotmail dot com
25-Mar-2001 11:53


Other Language books' operator precedence section usually include "(" and ")" - with exception of a Perl book that I have. (In PHP "{" and "}" should also be considered also). However, PHP Manual is not listed "(" and ")" in precedence list. It looks like "(" and ")" has higher precedence as it should be.





Note: If you write following code, you would need "()" to get expected value.





$bar = true;


$str = "TEST". ($bar ? 'true' : 'false') ."TEST";





Without "(" and ")" you will get only "true" in $str. 


(PHP4.0.4pl1/Apache DSO/Linux, PHP4.0.5RC1/Apache DSO/W2K Server)


It's due to precedence, probably.

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