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<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 iterator arguments have been replaced by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">&lt;</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">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&gt;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</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">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</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">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_found</span><span class="special">&gt;(</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">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_begin_found</span><span class="special">&gt;(</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">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_begin_next</span><span class="special">&gt;(</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">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_found_end</span><span class="special">&gt;(</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">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_next_end</span><span class="special">&gt;(</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">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_begin_end</span><span class="special">&gt;(</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">&lt;</span><span class="keyword">int</span><span class="special">&gt;</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">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_found_end</span><span class="special">&gt;(</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">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</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">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">return_next_end</span><span class="special">&gt;(</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>
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