| <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 iterator arguments have been replaced by |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> |