Операторы инкремента и декремента
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)
?>
Логические операторы
| Пример | Название | Результат |
|---|---|---|
| $a and $b | Логическое 'и' | TRUE если и $a, и $b TRUE. |
| $a or $b | Логическое 'или' | TRUE если или $a, или $b TRUE. |
| $a xor $b | Исключающее 'или' | TRUE если $a, или $b TRUE, но не оба. |
| ! $a | Отрицание | TRUE если $a не TRUE. |
| $a && $b | Логическое 'и' | TRUE если и $a, и $b TRUE. |
| $a || $b | Логическое 'или' | TRUE если или $a, или $b TRUE. |
Смысл двух разных вариантов для операторов "and" и "or" в том, что они работают с различными приоритетами (смотрите таблицу Приоритет выполнения операторов).
add a note
User Contributed NotesЛогические операторы
Benjamin
29-Feb-2008 09:48
29-Feb-2008 09:48
Richard
30-Jan-2008 06:22
30-Jan-2008 06:22
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
17-Jan-2008 07:02
17-Jan-2008 07:02
$a = false xor true;
var_dump($a); // bool(false)
$a = (false xor true);
var_dump($a); // bool(true)
pepesantillan at gmail dot com
23-Dec-2007 03:23
23-Dec-2007 03:23
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
$e = (false || true);
?>
same goes for "&&" and "AND".
If you find it hard to remember operators precedence you can always use parenthesys - "(" and ")". And even if you get to learn it remember that being a good programmer is not showing you can do code with fewer words. The point of being a good programmer is writting code that is easy to understand (comment your code when necessary!), easy to maintain and with high efficiency, among other things.
paranoiq at centrum dot cz
19-Nov-2007 08:00
19-Nov-2007 08:00
and, or and xor can be used as conditional constructs:
<?php
// do_that() is executed only if do_this() returns false
if($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
01-Oct-2007 12:36
01-Oct-2007 12:36
$test = true and false; ---> $test === true
$test = (true and false); ---> $test === false
$test = true && false; ---> $test === false
Lawrence
28-Aug-2007 12:04
28-Aug-2007 12:04
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
13-Aug-2007 08:49
13-Aug-2007 08:49
> <?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
17-Jun-2007 07:46
17-Jun-2007 07:46
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 06:16
14-Jun-2007 06:16
;;;;;;;;;;;;;;;;;;;;;;;;;
; 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
?>