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do-while

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

do-while 循环和 while 循环非常相似,区别在于表达式的值是在每次循环结束时检查而不是开始时。和一般的 while 循环主要的区别是 do-while 的循环语句保证会执行一次(表达式的真值在每次循环结束后检查),然而在一般的 while 循环中就不一定了(表达式真值在循环开始时检查,如果一开始就为 false 则整个循环立即终止)。

do-while 循环只有一种语法:

<?php
$i
= 0;
do {
echo
$i;
} while (
$i > 0);
?>

以上循环将正好运行一次,因为经过第一次循环后,当检查表达式的真值时,其值为 false$i 不大于 0)而导致循环终止。

资深的 C 语言用户可能熟悉另一种不同的 do-while 循环用法,把语句放在 do-while(0) 之中,在循环内部用 break 语句来结束执行循环。以下代码片段示范了此方法:

<?php
do {
if (
$i < 5) {
echo
"i is not big enough";
break;
}
$i *= $factor;
if (
$i < $minimum_limit) {
break;
}
echo
"i is ok";

/* process i */

} while(0);
?>

可以使用 goto 跳出循环,取代这种 hack 的方式。

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User Contributed Notes 1 note

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jayreardon at gmail dot com
17 years ago
There is one major difference you should be aware of when using the do--while loop vs. using a simple while loop: And that is when the check condition is made.

In a do--while loop, the test condition evaluation is at the end of the loop. This means that the code inside of the loop will iterate once through before the condition is ever evaluated. This is ideal for tasks that need to execute once before a test is made to continue, such as test that is dependant upon the results of the loop.

Conversely, a plain while loop evaluates the test condition at the begining of the loop before any execution in the loop block is ever made. If for some reason your test condition evaluates to false at the very start of the loop, none of the code inside your loop will be executed.
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