Expresiones
Method chaining is read left to right (left associative):
<?php
class Test_Method_Chain
{
public function One()
{
echo "One" . PHP_EOL;
return $this;
}
public function Two()
{
echo "Two" . PHP_EOL;
return $this;
}
public function Three()
{
echo "Three" . PHP_EOL;
return $this;
}
}
$test = new Test_Method_Chain();
$test->One()->Two()->Three();
/* Ouputs:
One
Two
Three
*/
?>
Capítulo 15. Operadores
- Tabla de contenidos
- Precedencia de Operadores
- Operadores de Aritmética
- Operadores de Asignación
- Operadores Bit a Bit
- Operadores de Comparación
- Operadores de Control de Errores
- Operadores de ejecución
- Operadores de Incremento/Decremento
- Operadores de Lógica
- Operadores de Cadena
- Operadores de Matrices
- Operadores de Tipo
Un operador es algo a lo que usted entrega uno o más valores (o expresiones, en jerga de programación) y produce otro valor (de modo que la construcción misma se convierte en una expresión). Así 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 único valor, por ejemplo ! (el operador de negación) o ++ (el operador de incremento). El segundo grupo se conoce como operadores binarios; éste grupo contiene la mayoría de operadores que soporta PHP, y una lista se encuentra disponible más adelante en la sección Precedencia de Operadores.
El tercer grupo consiste del operador ternario: ?:. Éste debe ser usado para seleccionar entre dos expresiones, en base a una tercera, en lugar de seleccionar dos sentencias o rutas de ejecución. Rodear las expresiones ternarias con paréntesis es una muy buena idea.
Precedencia de Operadores
La precedencia de un operador indica qué tan "cerca" se agrupan dos expresiones. Por ejemplo, en la expresión 1 + 5 * 3, la respuesta es 16 y no 18, ya que el operador de multiplicación ("*") tiene una mayor precedencia que el operador de adición ("+"). Los paréntesis pueden ser usados para marcar la precedencia, si resulta necesario. Por ejemplo: (1 + 5) * 3 evalúa a 18. Si la precedencia de los operadores es la misma, se utiliza una asociación 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 línea tienen la misma precedencia, en cuyo caso su asociatividad decide el orden para evaluarlos.
Tabla 15-1. Precedencia de Operadores
| Asociatividad | Operadores | Información Adicional |
|---|---|---|
| no-asociativo | new | new |
| izquierda | [ | array() |
| no-asociativos | ++ -- | incremento/decremento |
| no-asociativos | ! ~ - (int) (float) (string) (array) (object) @ | tipos |
| izquierda | * / % | aritmética |
| izquierda | + - . | aritmética, y cadena |
| izquierda | << >> | manejo de bits |
| no-asociativos | < <= > >= | comparación |
| no-asociativos | == != === !== | comparación |
| izquierda | & | manejo de bits, y referencias |
| izquierda | ^ | manejo de bits |
| izquierda | | | manejo de bits |
| izquierda | && | lógicos |
| izquierda | || | lógicos |
| izquierda | ? : | ternario |
| derecha | = += -= *= /= .= %= &= |= ^= <<= >>= | asignación |
| izquierda | and | lógicos |
| izquierda | xor | lógicos |
| izquierda | or | lógicos |
| izquierda | , | varios usos |
La asociatividad de izquierda quiere decir que la expresión es evaluada desde la izquierda a la derecha, la asociatividad de derecha quiere decir lo contrario.
Nota: Aunque ! tiene una mayor precedencia que =, PHP permitirá aun expresionas similares a la siguiente: if (!$a = foo()), en cuyo caso la salida de foo() va a dar a $a.
add a note
User Contributed Notes
Operadores
pgarvin76+php dot net at NOSPAMgmail dot com
29-Dec-2008 04:43
ddascalescu at gmail dot com
23-Oct-2008 06:53
The -> operator, not listed above, is called "object operator" (T_OBJECT_OPERATOR).
figroc at gmail dot com
02-Aug-2008 03:30
The variable symbol '$' should be considered as the highest-precedence operator, so that the variable variables such as $$a[0] won't confuse the parser. [http://www.php.net/manual/en/language.variables.variable.php]
phpnet dot 20 dot dpnsubs at xoxy dot net
01-Nov-2007 02:13
Note that in php the ternary operator ?: has a left associativity unlike in C and C++ where it has right associativity.
You cannot write code like this (as you may have accustomed to in C/C++):
<?php
$a = 2;
echo (
$a == 1 ? 'one' :
$a == 2 ? 'two' :
$a == 3 ? 'three' :
$a == 4 ? 'four' : 'other');
echo "\n";
// prints 'four'
?>
You need to add brackets to get the results you want:
<?
$a = 2;
echo ($a == 1 ? 'one' :
($a == 2 ? 'two' :
($a == 3 ? 'three' :
($a == 4 ? 'four' : 'other') ) ) );
echo "\n";
//prints 'two'
?>
Gautam
10-Oct-2007 03:22
<?php
$result1 = 7 + 8 * 9/3 -4;
$result2 = 7 + 8 * (9/3 -4);
$result3 =(7 + 8)* 9/3 -4;
echo "Result1 for 7 + 8 * 9/3 -4 = $result1 Result2 for 7 + 8 * (9/3 -4) = $result2 and Result3 (7 + 8)* 9/3 -4 = $result3 "
/*
which gives results as under
Result1 for 7 + 8 * 9/3 -4 = 27 Result2 for 7 + 8 * (9/3 -4) = -1 and Result3 (7 + 8)* 9/3 -4 = 41
Execution Order is 1) expression in brackets 2) division 3) multiplication 4) addition and 5) subtraction
*/
?>
janturon at email dot cz
08-Oct-2007 06:42
This is very common problem: set one variable to another, if it is not empty. If it is, set it to something else.
For example: set $bar to $foo, if $foo is empty, set $bar to "undefined";
if(!empty($foo)) $bar= $foo; else $bar= "undefined";
OR operator can shorten it:
$bar= @$foo or $bar= "undefined";
me at robrosenbaum dot com
12-Jul-2007 12:16
The scope resolution operator ::, which is missing from the list above, has higher precedence than [], and lower precedence than 'new'. This means that self::$array[$var] works as expected.
madcoder at gmail dot com
09-Jun-2007 03:17
In response to mathiasrav at gmail dot com:
The reason for that behavior is the parentheses. From the description:
"Parentheses may be used to force precedence, if necessary. For instance: (1 + 5) * 3 evaluates to 18."
So the order of operations says that even though the equality operator has higher precedence, the parentheses in your statement force the assignment operator to a higher precedence than the equality operator.
That said, it still doesn't work the way you expect it to. Neither way works, for these reasons:
<?php
if ( $a != ($a = $b) )
?>
Order of operations says to do the parentheses first. So you end up with:
<?php
$a = $b;
if ( $a != $a )
?>
Which is obviously going to be false. Without the parentheses:
<?php
if ( $a != $a = $b )
?>
Order of operations says to do the inequality first, then the assignment, so you have:
<?php
if ( $a != $a );
$a = $b;
?>
Which again is not what you expected, and again will always be false. But because you are only working with values of 0 and 1, you can make use of the XOR operator:
<?php
if ( $a ^= $b )
?>
This will only be true if 1) $a is 0 and $b is 1, or 2) $a is 1 and $b is 0. That is precisely what you wanted, and it even does the assignment the way you expected it to.
<?php
foreach ($ourstring as $c) {
if ($bold ^= $c['bold']) $resstring .= bold;
if ($underline ^= $c['underline']) $resstring .= underline;
$resstring .= $c[0];
}
?>
That code now works and produces the output you expected.
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
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.
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
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.
<?php
$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.