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.
do-while
do-while 루프는 시작부분이 아니라 각 반복(iteration)의
끝부분에서 표현식이 참인지 체크한다는것을 제외하고 while루프와
매우 비슷하다. 일반적인 while문과의 주요한 차이점은
do-while문의 첫번째 반복이 반드시 수행된다는것이다
(반복의 끝부분에서 표현식이 참인지 체크한다), 이와 같은 경우는
일반 while루프로 수행시킬수 없을것이다. (while루프에서는
각 반복의 시작부분에서 표현식이 참인지 체크되고, 시작부터 바로 그 값이
FALSE이면 그 루프는 즉시 수행을 멈추게 된다)
다음에 do-while루프의 한가지 문법을 보인다:
<?php
$i = 0;
do {
echo $i;
} while ($i > 0);
?>
위 루프는 정확히 한번 수행된다. 첫번째 반복(iteration) 이후에 표현식이
참인지 체크할때, FALSE가 되므로 ($i는 0보다 크지 않다) 루프 수행이 멈춘다.
고급 C 유저는 do-while루프의 다른 사용법에 익숙할것이다. 즉, do-while (0)으로 감싸고, break절을 사용하여 코드 블록의 중간에서 수행을 멈출수 있습니다. 다음 코드 예는 이런 경우를 보여준다:
<?php
do {
if ($i < 5) {
echo "i는 충분히 크지 않다";
break;
}
$i *= $factor;
if ($i < $minimum_limit) {
break;
}
echo "i is ok";
/* process i */
} while (0);
?>
이 코드를 바로 또는 전혀 이해할수 없다고 걱정하지 마라. 이런 '기능'을 사용하지 않고도 일반 스크립트나 심지어 훌륭한 스크립트를 작성할 수 있다.
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User Contributed Notes 9 notes
jayreardon at gmail dot com ¶
17 years ago
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>';
?>
mparsa1372 at gmail dot com ¶
3 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);
?>
anonymous at example dot com ¶
7 years ago
The last example on this page is simply abuse of the `break` keyword. Also, the suggestion to use `goto` if you don't understand the abuse of `break` is unsettling. (See the manual page for `goto` for more than enough reasons not to use it.)
The final example is generally better expressed using a typical if-else statement.
<?php
if ($i < 5) {
echo "i is not big enough";
} else {
$i *= $factor;
if ($i >= $minimum_limit) {
echo "i is ok";
/* process i */
}
}
?>
This version is easier to read and understand. And arguments for code golf are invalid as well as this version is 3 lines shorter.
In conclusion, although you can certainly write code that abuses the `break` keyword, you shouldn't in practice. Keep the code easy to read and understand for whoever inherits your code. And remember, code is for humans not computers.
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 = $child->next_sibling() ); // we have to use next_sibling for everything after first_child
?>
fuhse at data-quest dot de ¶
7 years ago
What actually surprised me: There is no alternative-syntax or template syntax for a do-while-loop.
So you can write
<?php
while ($a < 10) :
$a++;
endwhile;
?>
But this won't work:
<?php
do :
$a++
while ($a <= 10);
?>
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.
iamjeffjack at gmail dot com ¶
6 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"
?>
shaida dot mca at gmail dot com ¶
13 years ago
Example of Do while :-
<?php
$i = 0;
echo 'This code will run at least once because i default value is 0.<br/>';
do {
echo 'i value is ' . $i . ', so code block will run. <br/>';
++$i;
} while ($i < 10);
?>
