do-while

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(PHP 4, PHP 5)

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" ;

     

 } while( 0 );
 ?>

如果还不能立刻理解也不用担心。即使不用此“特性”也照样可以写出强大的代码来。自 PHP 5.3.0 起，还可以使用 goto 来跳出循环。

用户评论:

[#1] edusardo at artificiedaweb dot com [2015-10-04 14:22:47]

Outro exemplo simples do while com array:

<?php //imprime dias da semana em italiano =P $i = 0; $giorni = array("luned??","marted??","mercoled","gioved??","venerd??","sabato","domenica"); do{ echo "" .$giorni[$i]. " ?? un giorno della settimana. "; $i ++; } while($i < 7); echo "I giorni della settimana sono " .$i. "" ?>

[#2] Martin [2015-04-28 15:51:21]

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>'; ?>

[#3] info at pkrules dot in [2013-10-29 11:03:31]

Notice the way of using the loops:-
<?php $arr=array("orange","banana","apple","raspberry"); $i=0; while($i < count($arr)){ $a=$arr[$i]; echo $a." "; $i++; } echo "  "; $i=0; $c=count($arr); while($i < $c){ $a=$arr[$i]; echo $a." "; $i++; } echo "  "; while($a =$arr[1*--$i]){ echo $a." "; if($i<=0)break; } echo " "; foreach($arr as $i){ print $i; echo " "; } ?>

[#4] david dot schueler at tel-billig dot de [2011-03-18 05:12:48]

If you are trying to use a construct like this:
<?php do { // some code to run only one more time if expression is true if ($your_expression == true) continue; } while (false); ?>
It will NOT loop as expected, because the continue tries to run the "next" loop, but the expression says just "false" so there is no next loop. The continue exits the while loop.
To get around this you may use an other expression, like this
<?php do { // some code to run only one more time if expression is true if ($your_expression == true) continue; break; } while (true); ?>
or use the goto statement since PHP 5.3.

[#5] xiaomao5 at live dot com [2011-03-13 07:48:15]

Another hack
If you want $type to only have a value of 0 or 1, you can do this:

<?php do { echo 'Choose a type, 0 or 1: '; $type = trim(fgets(STDIN)); if ($type == 0) { } elseif ($type == 1) { } } while ($type != 0 && $type != 1); ?>

[#6] shaida dot mca at gmail dot com [2010-08-18 23:01:40]

Example of Do while :-

<?php $i = 0; echo 'This code will run at least once because i default value is 0. '; do { echo 'i value is ' . $i . ', so code block will run. '; ++$i; } while ($i < 10); ?>

[#7] andrew at NOSPAM dot devohive dot com [2008-09-15 10:44:59]

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 ?>

[#8] Ryan [2008-04-17 22:59:39]

I've found that the most useful thing to use do-while loops for is multiple checks of file existence. The guaranteed iteration means that it will check through at least once, which I had trouble with using a simple "while" loop because it never incremented at the end.

My code was:

<?php $filename = explode(".", $_FILES['file']['name']); // File being uploaded $i=0; // Number of times processed (number to add at the end of the filename) do { if($i > 0) $filename[0]++; else $filename[0] = $filename[0].$i; $i++; } while(file_exists("uploaded/".$filename[0].".".$filename[1])); move_uploaded_file($_FILES['file']['tmp_name'], "uploaded/".$filename[0].".".$filename[1]); ?>

I'm sure there are plenty of ways of doing this without using the do-while loop, but I managed to toss this one together in no-time flat, and I'm not a great PHP programmer. =) It's simple and effective, and I personally think it works better than any "for" or "while" loop that I've seen that does the same thing.

[#9] jantsch at gmail dot com [2007-11-28 17:56:56]

Useful when you want to continue to read a recordset that was already being read like in:

<?php $sql = "select * from customers"; $res = mysql_query( $sql ); // read the first record if( $rs = mysql_fetch_row( $res ) ){ // do something with this record } // do another stuff here // keep reading till the end if( mysql_num_rows( $res )>1 ){ do{ // processing the records till the end }while( $rs = mysql_fetch_row( $res )); } ?>

[#10] jayreardon at gmail dot com [2007-04-10 15:36:13]

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.