1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Introduction and motivation</title> 5<link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../../index.html" title="Chapter 1. Range 2.0"> 8<link rel="up" href="../algorithms.html" title="Range Algorithms"> 9<link rel="prev" href="../algorithms.html" title="Range Algorithms"> 10<link rel="next" href="mutating.html" title="Mutating algorithms"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<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> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h4 class="title"> 27<a name="range.reference.algorithms.introduction"></a><a class="link" href="introduction.html" title="Introduction and motivation">Introduction 28 and motivation</a> 29</h4></div></div></div> 30<p> 31 In its most simple form a <span class="bold"><strong>Range Algorithm</strong></span> 32 (or range-based algorithm) is simply an iterator-based algorithm where 33 the <span class="emphasis"><em>two</em></span> iterator arguments have been replaced by 34 <span class="emphasis"><em>one</em></span> range argument. For example, we may write 35 </p> 36<p> 37</p> 38<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> 39<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></span> 40 41<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> 42<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> 43</pre> 44<p> 45 </p> 46<p> 47 instead of 48 </p> 49<p> 50</p> 51<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> 52</pre> 53<p> 54 </p> 55<p> 56 However, the return type of range algorithms is almost always different 57 from that of existing iterator-based algorithms. 58 </p> 59<p> 60 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 61 that we can continue to pass the range around and/or further modify it. 62 Because of this we may write 63</p> 64<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> 65</pre> 66<p> 67 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. 68 </p> 69<p> 70 Algorithms like <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span><span class="special">()</span></code> 71 fall into another group of algorithms that return (potentially) narrowed 72 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> 73 where <code class="computeroutput"><span class="identifier">found</span></code> denotes the 74 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> 75 </p> 76<p> 77 Therefore exactly the unique values can be copied by writing 78</p> 79<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> 80 <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> 81</pre> 82<p> 83 </p> 84<p> 85 Algorithms like <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unique</span></code> usually return 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>. However, this behaviour may be changed 86 by supplying a <code class="computeroutput"><span class="identifier">range_return_value</span></code> 87 as a template parameter to the algorithm: 88 </p> 89<div class="informaltable"><table class="table"> 90<colgroup> 91<col> 92<col> 93</colgroup> 94<thead><tr> 95<th> 96 <p> 97 Expression 98 </p> 99 </th> 100<th> 101 <p> 102 Return 103 </p> 104 </th> 105</tr></thead> 106<tbody> 107<tr> 108<td> 109 <p> 110 <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> 111 </p> 112 </td> 113<td> 114 <p> 115 returns a single iterator like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique</span></code> 116 </p> 117 </td> 118</tr> 119<tr> 120<td> 121 <p> 122 <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> 123 </p> 124 </td> 125<td> 126 <p> 127 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> 128 <span class="identifier">found</span><span class="special">)</span></code> 129 (this is the default) 130 </p> 131 </td> 132</tr> 133<tr> 134<td> 135 <p> 136 <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> 137 </p> 138 </td> 139<td> 140 <p> 141 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> 142 <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> 143 </p> 144 </td> 145</tr> 146<tr> 147<td> 148 <p> 149 <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> 150 </p> 151 </td> 152<td> 153 <p> 154 returns the range <code class="computeroutput"><span class="special">[</span><span class="identifier">found</span><span class="special">,</span> 155 <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> 156 </p> 157 </td> 158</tr> 159<tr> 160<td> 161 <p> 162 <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> 163 </p> 164 </td> 165<td> 166 <p> 167 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> 168 </p> 169 </td> 170</tr> 171<tr> 172<td> 173 <p> 174 <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> 175 </p> 176 </td> 177<td> 178 <p> 179 returns the entire original range. 180 </p> 181 </td> 182</tr> 183</tbody> 184</table></div> 185<p> 186 This functionality has the following advantages: 187 </p> 188<div class="orderedlist"><ol class="orderedlist" type="1"> 189<li class="listitem"> 190 it allows for <span class="emphasis"><em><span class="bold"><strong>seamless functional-style 191 programming</strong></span></em></span> where you do not need to use named 192 local variables to store intermediate results 193 </li> 194<li class="listitem"> 195 it is very <span class="emphasis"><em><span class="bold"><strong>safe</strong></span></em></span> 196 because the algorithm can verify out-of-bounds conditions and handle 197 tricky conditions that lead to empty ranges 198 </li> 199</ol></div> 200<p> 201 For example, consider how easy we may erase the duplicates in a sorted 202 container: 203 </p> 204<p> 205</p> 206<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> 207<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> 208</pre> 209<p> 210 </p> 211<p> 212 Notice the use of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_found_end</span></code>. 213 What if we wanted to erase all the duplicates except one of them? In old-fashioned 214 STL-programming we might write 215 </p> 216<p> 217</p> 218<pre class="programlisting"><span class="comment">// assume 'vec' is already sorted</span> 219<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> 220 221<span class="comment">// remember this check or you get into problems</span> 222<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> 223 <span class="special">++</span><span class="identifier">i</span><span class="special">;</span> 224 225<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> 226</pre> 227<p> 228 </p> 229<p> 230 The same task may be accomplished simply with 231</p> 232<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> 233</pre> 234<p> 235 and there is no need to worry about generating an invalid range. Furthermore, 236 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 237 than using a range algorithm. 238 </p> 239</div> 240<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 241<td align="left"></td> 242<td align="right"><div class="copyright-footer">Copyright © 2003-2010 Thorsten Ottosen, 243 Neil Groves<p> 244 Distributed under the Boost Software License, Version 1.0. (See accompanying 245 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>) 246 </p> 247</div></td> 248</tr></table> 249<hr> 250<div class="spirit-nav"> 251<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> 252</div> 253</body> 254</html> 255
