<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Introduction and motivation</title> <link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> <link rel="home" href="../../../index.html" title="Chapter 1. Range 2.0"> <link rel="up" href="../algorithms.html" title="Range Algorithm"> <link rel="prev" href="../algorithms.html" title="Range Algorithm"> <link rel="next" href="mutating.html" title="Mutating algorithms"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td> <td align="center"><a href="../../../../../../../index.html">Home</a></td> <td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../../../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="../algorithms.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../algorithms.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="mutating.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="range.reference.algorithms.range_algorithm_introduction"></a><a class="link" href="range_algorithm_introduction.html" title="Introduction and motivation"> Introduction and motivation</a> </h4></div></div></div> <p> In its most simple form a <span class="bold"><strong>Range Algorithm</strong></span> (or range-based algorithm) is simply an iterator-based algorithm where the <span class="emphasis"><em>two</em></span> iterator arguments have been replaced by <span class="emphasis"><em>one</em></span> range argument. For example, we may write </p> <p> </p> <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">range</span><span class="special">/</span><span class="identifier">algorithm</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">vec</span> <span class="special">=</span> <span class="special">...;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">sort</span><span class="special">(</span><span class="identifier">vec</span><span class="special">);</span> </pre> <p> </p> <p> instead of </p> <p> </p> <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">sort</span><span class="special">(</span><span class="identifier">vec</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">vec</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span> </pre> <p> </p> <p> However, the return type of range algorithms is almost always different from that of existing iterator-based algorithms. </p> <p> One group of algorithms, like <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">sort</span><span class="special">()</span></code>, will simply return the same range so that we can continue to pass the range around and/or further modify it. Because of this we may write </p> <pre class="programlisting"><span class="identifier">boost</span><span class="special">:</span><span class="identifier">unique</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">sort</span><span class="special">(</span><span class="identifier">vec</span><span class="special">));</span> </pre> <p> to first sort the range and then run <code class="computeroutput"><span class="identifier">unique</span><span class="special">()</span></code> on the sorted range. </p> <p> Algorithms like <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special">()</span></code> fall into another group of algorithms that return (potentially) narrowed views of the original range. By default <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special">(</span><span class="identifier">rng</span><span class="special">)</span></code> returns the range <code class="computeroutput"><span class="special">[</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">begin</span><span class="special">(</span><span class="identifier">rng</span><span class="special">),</span> <span class="identifier">found</span><span class="special">)</span></code> where <code class="computeroutput"><span class="identifier">found</span></code> denotes the iterator returned by <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">begin</span><span class="special">(</span><span class="identifier">rng</span><span class="special">),</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">end</span><span class="special">(</span><span class="identifier">rng</span><span class="special">))</span></code> </p> <p> Therefore exactly the unique values can be copied by writing </p> <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">copy</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">sort</span><span class="special">(</span><span class="identifier">vec</span><span class="special">)),</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream_iterator</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">));</span> </pre> <p> </p> <p> Algorithms like <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span></code> usually return the same range: <code class="computeroutput"><span class="special">[</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">begin</span><span class="special">(</span><span class="identifier">rng</span><span class="special">),</span> <span class="identifier">found</span><span class="special">)</span></code>. However, this behaviour may be changed by supplying the algorithms with a template argument: </p> <div class="informaltable"><table class="table"> <colgroup> <col> <col> </colgroup> <thead><tr> <th> <p> Expression </p> </th> <th> <p> Return </p> </th> </tr></thead> <tbody> <tr> <td> <p> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_found</span><span class="special">>(</span><span class="identifier">rng</span><span class="special">)</span></code> </p> </td> <td> <p> returns a single iterator like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique</span></code> </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_begin_found</span><span class="special">>(</span><span class="identifier">rng</span><span class="special">)</span></code> </p> </td> <td> <p> returns the range <code class="computeroutput"><span class="special">[</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">begin</span><span class="special">(</span><span class="identifier">rng</span><span class="special">),</span> <span class="identifier">found</span><span class="special">)</span></code> (this is the default) </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_begin_next</span><span class="special">>(</span><span class="identifier">rng</span><span class="special">)</span></code> </p> </td> <td> <p> returns the range <code class="computeroutput"><span class="special">[</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">begin</span><span class="special">(</span><span class="identifier">rng</span><span class="special">),</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">next</span><span class="special">(</span><span class="identifier">found</span><span class="special">))</span></code> </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_found_end</span><span class="special">>(</span><span class="identifier">rng</span><span class="special">)</span></code> </p> </td> <td> <p> returns the range <code class="computeroutput"><span class="special">[</span><span class="identifier">found</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">end</span><span class="special">(</span><span class="identifier">rng</span><span class="special">))</span></code> </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_next_end</span><span class="special">>(</span><span class="identifier">rng</span><span class="special">)</span></code> </p> </td> <td> <p> returns the range <code class="computeroutput"><span class="special">[</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">next</span><span class="special">(</span><span class="identifier">found</span><span class="special">),</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">end</span><span class="special">(</span><span class="identifier">rng</span><span class="special">))</span></code> </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_begin_end</span><span class="special">>(</span><span class="identifier">rng</span><span class="special">)</span></code> </p> </td> <td> <p> returns the entire original range. </p> </td> </tr> </tbody> </table></div> <p> This functionality has the following advantages: </p> <div class="orderedlist"><ol class="orderedlist" type="1"> <li class="listitem"> it allows for <span class="emphasis"><em><span class="bold"><strong>seamless functional-style programming</strong></span></em></span> where you do not need to use named local variables to store intermediate results </li> <li class="listitem"> it is very <span class="emphasis"><em><span class="bold"><strong>safe</strong></span></em></span> because the algorithm can verify out-of-bounds conditions and handle tricky conditions that lead to empty ranges </li> </ol></div> <p> For example, consider how easy we may erase the duplicates in a sorted container: </p> <p> </p> <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">vec</span> <span class="special">=</span> <span class="special">...;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">erase</span><span class="special">(</span><span class="identifier">vec</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_found_end</span><span class="special">>(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">sort</span><span class="special">(</span><span class="identifier">vec</span><span class="special">)));</span> </pre> <p> </p> <p> Notice the use of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_found_end</span></code>. What if we wanted to erase all the duplicates except one of them? In old-fashined STL-programming we might write </p> <p> </p> <pre class="programlisting"><span class="comment">// assume 'vec' is already sorted </span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">iterator</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">unique</span><span class="special">(</span><span class="identifier">vec</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">vec</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span> <span class="comment">// remember this check or you get into problems </span><span class="keyword">if</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">!=</span> <span class="identifier">vec</span><span class="special">.</span><span class="identifier">end</span><span class="special">())</span> <span class="special">++</span><span class="identifier">i</span><span class="special">;</span> <span class="identifier">vec</span><span class="special">.</span><span class="identifier">erase</span><span class="special">(</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">vec</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span> </pre> <p> </p> <p> The same task may be accomplished simply with </p> <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">erase</span><span class="special">(</span><span class="identifier">vec</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_next_end</span><span class="special">>(</span><span class="identifier">vec</span><span class="special">));</span> </pre> <p> and there is no need to worry about generating an invalid range. Furthermore, if the container is complex, calling <code class="computeroutput"><span class="identifier">vec</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span></code> several times will be more expensive than using a range algorithm. </p> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2003 -2010 Thorsten Ottosen, Neil Groves<p> Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="../algorithms.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../algorithms.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="mutating.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>