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	<title>BOOST_PP_WHILE</title>
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<body>
	<div style="margin-left:  0px;">
		The <b>BOOST_PP_WHILE</b> macro represents a looping construct.
	</div>
	<h4>Usage</h4>
		<div class="code">
			<b>BOOST_PP_WHILE</b>(<i>pred</i>, <i>op</i>, <i>state</i>)
		</div>
	<h4>Arguments</h4>
		<dl>
			<dt>pred</dt>
			<dd>
				A binary predicate of the form <i>pred</i>(<i>d</i>, <i>state</i>).&nbsp;
				This predicate is expanded by <b>BOOST_PP_WHILE</b> with the next available
				iteration <i>d</i> and the current <i>state</i>.&nbsp;
				This predicate must expand to value in the range of <i>0</i> to <b>BOOST_PP_LIMIT_MAG</b>.
				The construct continues to loop until this predicate returns <i>0</i>.&nbsp;
				When this predicate returns <i>0</i>, <b>BOOST_PP_WHILE</b> returns the current <i>state</i>.
			</dd>
			<dt>op</dt>
			<dd>
				A binary operation of the form <i>op</i>(<i>d</i>, <i>state</i>).&nbsp;
				This operation is expanded by <b>BOOST_PP_WHILE</b> with the next available
				iteration <i>d</i> and the current <i>state</i>.&nbsp;
				This macro is repeatedly applied to the <i>state</i>, each time producing a new <i>state</i>, until <i>pred</i> returns <i>0</i>.
			</dd>
			<dt>state</dt>
			<dd>
				The initial state.&nbsp;
				Often this argument is a <i>tuple</i>.
			</dd>
		</dl>
	<h4>Remarks</h4>
		<div>
			This macro iterates <i>op</i>(<i>d</i>, <i>state</i>) while <i>pred</i>(<i>d</i>, <i>state</i>) is non-zero.&nbsp;
			In other words expands to:
			<div>
				<i>op</i>(<i>d</i>, ... <i>op</i>(<i>d</i>, <i>op</i>(<i>d</i>, <i>state</i>)) ... ).
			</div>
		</div>
		<div>
			The <i>d</i> value that is passed to both <i>pred</i> and <i>op</i> represents the next available iteration.&nbsp;
			Other macros that have <b>_D</b> suffix variants internally use <b>BOOST_PP_WHILE</b>--for example, <b>BOOST_PP_ADD</b> and <b>BOOST_PP_ADD_D</b>.&nbsp;
			Using these <b>_D</b> versions is not strictly necessary, but passing the <i>d</i> value (that passed to <i>pred</i> or <i>op</i>) to these macros allows them to reenter <b>BOOST_PP_WHILE</b> with maximum efficiency.
		</div>
		<div>
			To directly use this <i>d</i> value, rather than simply passing it to another macro, see <b>BOOST_PP_WHILE_<i>d</i></b>.
		</div>
		<div>
			Previously, this macro could not be used recursively inside <b>BOOST_PP_WHILE</b>.&nbsp;
			This limitation no longer exists, as the library can automatically detect the next available iteration.
		</div>
	<h4>See Also</h4>
		<ul>
			<li><a href="limit_mag.html">BOOST_PP_LIMIT_MAG</a></li>
			<li><a href="while_d.html">BOOST_PP_WHILE_<i>d</i></a></li>
		</ul>
	<h4>Requirements</h4>
		<div>
			<b>Header:</b> &nbsp;<a href="../headers/control/while.html">&lt;boost/preprocessor/control/while.hpp&gt;</a>
		</div>
	<h4>Sample Code</h4>
<div><pre>
#include &lt;<a href="../headers/arithmetic/add.html">boost/preprocessor/arithmetic/add.hpp</a>&gt;
#include &lt;<a href="../headers/control/while.html">boost/preprocessor/control/while.hpp</a>&gt;
#include &lt;<a href="../headers/tuple/elem.html">boost/preprocessor/tuple/elem.hpp</a>&gt;

#define PRED(n, state) <a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>(2, 1, state)

#define OP(d, state) \
   OP_D( \
      d, \
      <a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>(2, 0, state), \
      <a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>(2, 1, state) \
   ) \
   /**/

#define OP_D(d, res, c) \
   ( \
      <a href="add_d.html">BOOST_PP_ADD_D</a>( \
         d, \
         res, \
         <a href="dec.html">BOOST_PP_DEC</a>(c) \
      ), \
      <a href="dec.html">BOOST_PP_DEC</a>(c) \
   ) \
   /**/

#define SUMMATION(n) \
   <a href="tuple_elem.html">BOOST_PP_TUPLE_ELEM</a>( \
      2, 0, \
      <a href="while.html">BOOST_PP_WHILE</a>(PRED, OP, (n, n)) \
   ) \
   /**/

SUMMATION(3) // expands to 6
SUMMATION(4) // expands to 10
</pre></div>
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	<div style="margin-left: 0px;">
		<i>© Copyright <a href="http://www.housemarque.com" target="_top">Housemarque Oy</a> 2002</i>
		</br><i>© Copyright Paul Mensonides 2002</i>
	</div>
	<div style="margin-left: 0px;">
		<p><small>Distributed under the Boost Software License, Version 1.0. (See
		accompanying file <a href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
		copy at <a href=
		"http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</small></p>
	</div>
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