PHP: do-while - PHP中文网

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 6 notes

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jayreardon at gmail dot com ¶

16 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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-9

Martin ¶

8 years ago

Do-while loops can also be used inside other loops, for example:

<?php
// generating an array with random even numbers between 1 and 1000

$numbers = array();
$array_size = 10;

// for loop runs as long as 2nd condition evaluates to true
for ($i=0;$i<$array_size;$i++) { 

      // always executes (as long as the for-loop runs)
      do { 
         $random = rand(1,1000);

     // if the random number is even (condition below is false), the do-while-loop execution ends
     // if it's uneven (condition below is true), the loop continues by generating a new random number
     } while (($random % 2) == 1);

     // even random number is written to array and for-loop continues iteration until original condition is met
     $numbers[] = $random; 
}

// sorting array by alphabet

asort($numbers);

// printing array

echo '<pre>';
print_r($numbers);
echo '</pre>';
?>

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-6

mparsa1372 at gmail dot com ¶

2 years ago

The example below first sets a variable $x to 1 ($x = 1). Then, the do while loop will write some output, and then increment the variable $x with 1. Then the condition is checked (is $x less than, or equal to 5?), and the loop will continue to run as long as $x is less than, or equal to 5:

<?php
$x = 1;

do {
  echo "The number is: $x <br>";
  $x++;
} while ($x <= 5);
?>

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-31

andrew at NOSPAM dot devohive dot com ¶

15 years ago

I'm guilty of writing constructs without curly braces sometimes... writing the do--while seemed a bit odd without the curly braces ({ and }), but just so everyone is aware of how this is written with a do--while...

a normal while:
<?php
   while ( $isValid ) $isValid = doSomething($input);
?>

a do--while:
<?php
   do $isValid = doSomething($input);
   while ( $isValid );
?>

Also, a practical example of when to use a do--while when a simple while just won't do (lol)... copying multiple 2nd level nodes from one document to another using the DOM XML extension

<?php
   # open up/create the documents and grab the root element
   $fileDoc  = domxml_open_file('example.xml'); // existing xml we want to copy
   $fileRoot = $fileDoc->document_element();
   $newDoc   = domxml_new_doc('1.0'); // new document we want to copy to
   $newRoot  = $newDoc->create_element('rootnode');
   $newRoot  = $newDoc->append_child($newRoot); // this is the node we want to copy to

   # loop through nodes and clone (using deep)
   $child = $fileRoot->first_child(); // first_child must be called once and can only be called once
   do $newRoot->append_child($child->clone_node(true)); // do first, so that the result from first_child is appended
   while ( $child = $child->next_sibling() ); // we have to use next_sibling for everything after first_child
?>

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-31

iamjeffjack at gmail dot com ¶

5 years ago

If you put multiple conditions in the while check, a do-while loop checks these conditions in order and runs again once it encounters a condition that returns true. This can be helpful to know when troubleshooting why a do-while loop isn't finishing. An (illustrative-only) example:

<?php
    $numberOne = 0;
    do {
        echo $numberOne;
        $numberOne++;
    } while( $numberOne < 5 || incrementNumberTwo() );
    function incrementNumberTwo() {
        echo "function incrementNumberTwo called";
        return false;
    }
    // outputs "01234function incrementNumberTwo called"
?>

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-30

M. H. S. ¶

4 years ago

<!-- if you write with WHILE: -->
<?php
$i = 100
while ($i < 10) :
    echo "\$i is $i.";
endwhile;
?>
<!-- returning: -->

<!-- if you write with DO/WHILE: -->
<?php
$i = 100;
do {
    echo "\$i is $i.";
} while ($i < 10);
?>
<!-- returning: -->
$i is 100.

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