PHP: 논리 연산자

Last updated: Fri, 24 Jul 2009

논리 연산자

**논리 연산자**
예제	이름	결과
$a and $b	And	$a와 $b가 모두 `TRUE`이면 `TRUE`.
$a or $b	Or	$a나 $b가 `TRUE`이면 `TRUE`.
$a xor $b	Xor	$a와 $b중 하나만 `TRUE`일 때만 `TRUE`.
! $a	Not	$a가 `TRUE`가 아니면 `TRUE`.
$a && $b	And	$a와 $b가 모두 `TRUE`이면 `TRUE`.
$a \|\| $b	Or	$a나 $b가 `TRUE`이면 `TRUE`.

"and"와 "or" 연산자가 두 종류가 있는 것은, 다른 우선권을 가지기 때문입니다. (연산자 우선권 참고)

Example #1 논리 연산자 설명


<?php

// foo() will never get called as those operators are short-circuit
$a = (false && foo());
$b = (true  || foo());
$c = (false and foo());
$d = (true  or  foo());

// "||" has a greater precedence than "or"
$e = false || true; // $e will be assigned to (false || true) which is true
$f = false or true; // $f will be assigned to false
var_dump($e, $f);

// "&&" has a greater precedence than "and"
$g = true && false; // $g will be assigned to (true && false) which is false
$h = true and false; // $h will be assigned to true
var_dump($g, $h);
?>

위 예제의 출력 예시:

bool(true)
bool(false)
bool(false)
bool(true)

문자열 연산자

증가/감소 연산자

Last updated: Fri, 24 Jul 2009

add a note User Contributed Notes
논리 연산자

Rob
24-Oct-2009 02:16


Another example that might help.



<?php

(isset($panelemail) && !empty($panelemail) ? $panelemail : $userdata['email']);

?>

returns the userdata email address, but this



<?php

(isset($panelemail) AND !empty($panelemail) ? $panelemail : $userdata['email']);

?>

returns false.



The reason is that the two types of ands have a different order of precedence.  "&&" is higher than "AND", and the "?:" operator just happens to come between the two.  Also, since "||" (or) is actually higher than "AND," you should never mix &&s and ||s with ANDs and ORs without paretheses.



For example:



<?php

     true && false || false

?>

returns false, but



<?php

     true AND false || false

?>

returns true.

daevid at daevid dot com
11-Aug-2009 10:33


If you ever need to alternate row colors for data output in a table, do this:





in your CSS put these two classes:


    .dataRow1 { background-color: #DFDFDF; }


    .dataRow2 { background-color: #FFFFFF; }





then in your PHP, loop over each row:





<?php


// ....


foreach ($foo_array as $foo) {


   ?><tr class="<?php echo ($dr = !$dr) ? "dataRow1" : "dataRow2"; ?>"><td><?php echo $foo


?></td></tr><?php


}


?>





No need to initialize $dr as by default PHP will make it a boolean "false", then each iteration, it will toggle true/false and substitute the CSS class

momrom at freenet dot de
20-Apr-2009 12:32


Evaluation of logical expressions is stopped as soon as the result is known.

If you don't want this, you can replace the and-operator by min() and the or-operator by max().



<?php

function a($x) { echo 'Expression '; return $x; }

function b($x) { echo 'is '; return $x; }

function c($x) { echo $x ? 'true.' : 'false.' ;}



c( a( false ) and b( true ) ); // Output: Expression false.

c( min( a( false ), b( true ) ) ); // Output: Expression is false.



c( a( true ) or b( true ) ); // Output: Expression true.

c( max( a( true ), b( true ) ) ); // Output: Expression is true.

?>



This way, values aren't automaticaly converted to boolean like it would be done when using and or or. Therefore, if you aren't sure the values are already boolean, you have to convert them 'by hand':



<?php

c( min( (bool) a( false ), (bool) b( true ) ) );

?>

sandaimespaceman at gmail dot com
12-Sep-2008 12:07


There's one userful way to use the or operator:



<?php

//If the connection was success, "Connected to database" will be shown. 

//If the connection was failed, "Unable to connect" will be shown.(NOTE: The @ will hide error messages)

@mysql_connect("localhost", "root", "password") or die("Unable to connect");

echo "Connected to database";

?>



So you don't need to use if operators to add more lines.

jeffjeffleelee at hotmail dot com
07-Jul-2008 01:30


@zhustar:



I will verify the following:



<?php

$a = false xor true;

var_dump($a);  // bool(false)



$a = (false xor true);

var_dump($a);  // bool(true)

?>



At first I thought this was some egregious bug, and then I realized that this is merely an issue of operator precedence. English-word boolean operators have relatively low precedence--in fact they take place after assignment. So the first example assigns false to $a, and then does an xor operation on $a and boolean true.

loaded67 at hotmail dot com
13-Jun-2008 02:40


Someone ever noted that C type operators are applicable in php as well?

more like a (bool / boolean) cast...



See:



<?php

ini_set('display_errors', 1);

ini_set('error_reporting', 8191);// php5



$whatever = NULL;

$Iwaslike = 'WTF!';



if(!!($whatever)){

    echo '$whatever'.PHP_EOL;

}

if(!!($Iwaslike)){

    echo '$Iwaslike'.PHP_EOL;

}

if(!!!($whatever)){

    echo '$whatever'.PHP_EOL;

}

if(!!!($Iwaslike)){

    echo '$Iwaslike'.PHP_EOL;

}

?>

momrom at freenet dot de
12-Jun-2008 11:10


Some examples from the english manual that aren't avaiable in all languages:



Like in C, logical expressions are evaluated

from left to right until the result is known.

foo() will never get called in the following cases.

<?php

$a = (false && foo());

$b = (true  || foo());

$c = (false and foo());

$d = (true  or  foo());

?>



"||" has a greater precedence than "="

which has a greater one than "or"

<?php

// same as ($e = (false || true)),

// expression is true and $e ist assigned to true

$e = false || true; 



// same as (($e = false) or true),

// expression is true but $e is assigned to false

$f = false or true;

?>



"&&" has a greater precedence than "="

which has a greater one than "and"

<?php

// same as ($e = (true || false)),

// expression is false and $e ist assigned to false

$g = true && false; 



// same as (($e = true) and false),

// expression is false but $e is assigned to true

$h = true and false; 

?>

Benjamin
01-Mar-2008 02:48


Re:Richard



I show $b printing 1 rather than "banana". Here's how I understand what's going on.



<?php

//"||" has a greater precedence than "or"



$a=0 or $a="avocado"; //evaluated as ($a=0) or ($a="avacado")

//Since $a=0 is false, $a="avocado" is evaluated and $a is assigned the string value "avocado".

echo "$a"; //prints "avocado"

var_dump ($a); // string(7) "avocado" 



$b=0 || $b="banana"; // evaluated as $b = (0 || $b = "banana")

echo $b; //prints "1" 

var_dump ($b); // bool(true) 

?>

Richard
31-Jan-2008 11:22


Re Lawrence:



You sort of can do conditional evaluation:



$a=0 or $a="avocado";

echo "$a";                              #Prints "avocado"



But oddly: 



$b=0 || $b="banana";

echo $b;                                 #Prints "banana"

zhustar at gmail dot com
18-Jan-2008 12:02


$a = false xor true;

var_dump($a);  // bool(false)



$a = (false xor true);

var_dump($a);  // bool(true)

pepesantillan at gmail dot com
24-Dec-2007 08:23


worth reading for people learning about php and programming: (adding extras <?php ?> to get highlighted code)



about the following example in this page manual: 

Example#1 Logical operators illustrated



...

<?php

// "||" has a greater precedence than "or"

$e = false || true; // $e will be assigned to (false || true) which is true

$f = false or true; // $f will be assigned to false

var_dump($e, $f);



// "&&" has a greater precedence than "and"

$g = true && false; // $g will be assigned to (true && false) which is false

$h = true and false; // $h will be assigned to true

var_dump($g, $h); 

?>

_______________________________________________end of my quote...



If necessary, I wanted to give further explanation on this and say that when we write:

$f = false or true; // $f will be assigned to false

the explanation: 



"||" has a greater precedence than "or" 



its true. But a more acurate one would be



"||" has greater precedence than "or" and than "=", whereas "or" doesnt have greater precedence than "=", so



<?php

$f = false or true;



//is like writting



($f = false ) or true;



//and



$e = false || true;



is the same as



$e = (false || true);



?> 



same goes for "&&" and "AND". 



If you find it hard to remember operators precedence you can always use parenthesys - "(" and ")". And even if you get to learn it remember that being a good programmer is not showing you can do code with fewer words. The point of being a good programmer is writting code that is easy to understand (comment your code when necessary!), easy to maintain and with high efficiency, among other things.

paranoiq at centrum dot cz
20-Nov-2007 01:00


and, or and xor can be used as conditional constructs:



<?php

// do_that() is executed only if do_this() returns false

if($something) do_this() or do_that();

// $b is assigned to $b, do_that() is executed if $b is false

if($something) $a = $b or do_that();



// do_that() is executed only if do_this() returns true

if($something) do_this() and do_that();

// $b is assigned to $b, do_that() is executed if $b is true

if($something) $a = $b and do_that();



// both do_that() and do_this() are executed..

if($something) do_this() xor do_that();

// .. so the behaviour is same as:

if($something) {

    do_this();

    do_that();

}

?>



for understanding what happens if $b is NULL or do_this() returns NULL, read the avbentem's comment on NULL type. generaly speaking, NULL is threated like false in most cases.

peter dot kutak at NOSPAM dot gmail dot com
02-Oct-2007 04:36


$test = true and false;     ---> $test === true

$test = (true and false);  ---> $test === false

$test = true && false;      ---> $test === false

Lawrence
29-Aug-2007 04:04


Note that PHP's boolean operators *always* return a boolean value... as opposed to other languages that return the value of the last evaluated expression.



For example:



$a = 0 || 'avacado';

print "A: $a\n";



will print:



A: 1



in PHP -- as opposed to printing "A: avacado" as it would in a language like Perl or JavaScript.



This means you can't use the '||' operator to set a default value:



$a = $fruit || 'apple';



instead, you have to use the '?:' operator:



$a = ($fruit ? $fruit : 'apple');

Andrew
14-Aug-2007 12:49


> <?php

> your_function() or return "whatever";

> ?>



doesn't work because return is not an expression, it's a statement. if return was a function it'd work fine. :/

looris at gmail dot com
18-Jun-2007 11:46


Please note that while you can do things like:

<?php

your_function() or die("horribly");

?>



you can't do:

<?php

your_function() or return "whatever";

?>

(it will give you a syntax error).

eduardofleury at uol dot com dot br
14-Jun-2007 10:16


;;;;;;;;;;;;;;;;;;;;;;;;;

; P1 P2; And; OR  ; XOR ;

;;;;;;;;;;;;;;;;;;;;;;;;;

; V  V ; V  ; V   ; F   ;

; V  F ; F  ; V   ; V   ;

; F  V ; F  ; V   ; V   ;

; F  F ; F  ; F   ; F   ;

;;;;;;;;;;;;;;;;;;;;;;;;;



<?php 



$a = 2;

$b = 3;

$c = 6;



print !($a > $b && $b < $c);// true



print (($a > $b) and ($b < $c));// false



print ($a == $b or $b < $c); // true



print $a == $b || $b < $c; // true



$x = $a < $b; //$x = true



$y = $b === $c; //$y = false



print  $x xor $y; // true



?>

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문자열 연산자

증가/감소 연산자

Last updated: Fri, 24 Jul 2009