else

(PHP 4, PHP 5, PHP 7, PHP 8)

经常需要在满足某个条件时执行一条语句,而在不满足该条件时执行其它语句,这正是 else 的功能。else 延伸了 if 语句,可以在 if 语句中的表达式的值为 false 时执行语句。例如以下代码在 $a 大于 $b 时显示 a is bigger than b,反之则显示 a is NOT bigger than b

<?php
if ($a > $b) {
echo
"a is greater than b";
} else {
echo
"a is NOT greater than b";
}
?>
else 语句仅在 if 以及 elseif(如果有的话)语句中的表达式的值为 false 时执行(参见 elseif)。

注意: 悬挂的 else

在多层嵌套 if-else 语句的情况下, else 总是与最近的 if 进行关联。

<?php
$a
= false;
$b = true;
if (
$a)
if (
$b)
echo
"b";
else
echo
"c";
?>
虽然存在缩进(对 PHP 来说,无关紧要), 但是 else 还是与 if ($b) 进行关联,所以以上示例不会产生任何输出。虽然可以依赖此特性,但是推荐使用花括号,避免潜在的歧义问题。

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dormeydo at gmail dot com
17 years ago
An alternative and very useful syntax is the following one:statement ? execute if true : execute if falseThs is very usefull for dynamic outout inside strings, for example:print('$a is ' . ($a > $b ? 'bigger than' : ($a == $b ? 'equal to' : 'smaler than' )) .  '  $b');This will print "$a is smaler than $b" is $b is bigger than $a, "$a is bigger than $b" if $a si bigger and "$a is equal to $b" if they are same.
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11
Caliban Darklock
20 years ago
If you're coming from another language that does not have the "elseif" construct (e.g. C++), it's important to recognise that "else if" is a nested language construct and "elseif" is a linear language construct; they may be compared in performance to a recursive loop as opposed to an iterative loop. <?php$limit=1000;for($idx=0;$idx<$limit;$idx++)  { $list[]="if(false) echo \"$idx;\n\"; else"; }$list[]=" echo \"$idx\n\";";$space=implode(" ",$list);| // if ... else if ... else$nospace=implode("",$list); // if ... elseif ... else$start=array_sum(explode(" ",microtime()));eval($space);$end=array_sum(explode(" ",microtime()));echo $end-$start . " seconds\n";$start=array_sum(explode(" ",microtime()));eval($nospace);$end=array_sum(explode(" ",microtime()));echo $end-$start . " seconds\n";?>This test should show that "elseif" executes in roughly two-thirds the time of "else if". (Increasing $limit will also eventually cause a parser stack overflow error, but the level where this happens is ridiculous in real world terms. Nobody normally nests if() blocks to more than a thousand levels unless they're trying to break things, which is a whole different problem.)There is still a need for "else if", as you may have additional code to be executed unconditionally at some rung of the ladder; an "else if" construction allows this unconditional code to be elegantly inserted before or after the entire rest of the process. Consider the following elseif() ladder:<?phpif($a) { conditional1(); }elseif($b) { conditional2(); }elseif($c) { conditional3(); }elseif($d) { conditional4(); }elseif($e) { conditional5(); }elseif($f) { conditional6(); }elseif($g) { conditional7(); }elseif($h) { conditional8(); }else { conditional9(); }?>To insert unconditional preprocessing code for $e onward, one need only split the "elseif":<?phpif($a) { conditional1(); }elseif($b) { conditional2(); }elseif($c) { conditional3(); }elseif($d) { conditional4(); }else {....unconditional();....if($e) { conditional5(); }....elseif($f) { conditional6(); }....elseif($g) { conditional7(); }....elseif($h) { conditional8(); }....else { conditional9(); }}?>The alternative is to duplicate the unconditional code throughout the construct.
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