php-doc-en/language/control-structures/do-while.xml
Christoph Michael Becker 7104ee97ce Removed literal tags from title for readability
Closes GH-250.


git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@351935 c90b9560-bf6c-de11-be94-00142212c4b1
2020-12-06 22:38:36 +00:00

98 lines
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XML

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<sect1 xml:id="control-structures.do.while" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
<title>do-while</title>
<?phpdoc print-version-for="dowhile"?>
<simpara>
<literal>do-while</literal> loops are very similar to
<literal>while</literal> loops, except the truth expression is
checked at the end of each iteration instead of in the beginning.
The main difference from regular <literal>while</literal> loops is
that the first iteration of a <literal>do-while</literal> loop is
guaranteed to run (the truth expression is only checked at the end
of the iteration), whereas it may not necessarily run with a
regular <literal>while</literal> loop (the truth expression is
checked at the beginning of each iteration, if it evaluates to
&false; right from the beginning, the loop
execution would end immediately).
</simpara>
<para>
There is just one syntax for <literal>do-while</literal> loops:
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
$i = 0;
do {
echo $i;
} while ($i > 0);
?>
]]>
</programlisting>
</informalexample>
</para>
<simpara>
The above loop would run one time exactly, since after the first
iteration, when truth expression is checked, it evaluates to
&false; (<varname>$i</varname> is not bigger than 0) and the loop
execution ends.
</simpara>
<para>
Advanced C users may be familiar with a different usage of the
<literal>do-while</literal> loop, to allow stopping execution in
the middle of code blocks, by encapsulating them with
<literal>do-while</literal> (0), and using the <link
linkend="control-structures.break"><literal>break</literal></link>
statement. The following code fragment demonstrates this:
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
do {
if ($i < 5) {
echo "i is not big enough";
break;
}
$i *= $factor;
if ($i < $minimum_limit) {
break;
}
echo "i is ok";
/* process i */
} while (0);
?>
]]>
</programlisting>
</informalexample>
</para>
<simpara>
It is possible to use the
<link linkend="control-structures.goto"><literal>goto</literal></link>
operator instead of this hack.
</simpara>
</sect1>
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