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<title>Iterators</title>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="python.iterators"></a>Iterators</h2></div></div></div>
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<p>
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In C++, and STL in particular, we see iterators everywhere. Python also has
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iterators, but these are two very different beasts.
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</p>
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<p>
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<span class="bold"><strong>C++ iterators:</strong></span>
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" type="disc">
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<li class="listitem">
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C++ has 5 type categories (random-access, bidirectional, forward, input,
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output)
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</li>
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<li class="listitem">
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There are 2 Operation categories: reposition, access
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</li>
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<li class="listitem">
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A pair of iterators is needed to represent a (first/last) range.
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</li>
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</ul></div>
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<p>
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<span class="bold"><strong>Python Iterators:</strong></span>
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" type="disc">
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<li class="listitem">
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1 category (forward)
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</li>
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<li class="listitem">
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1 operation category (next())
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</li>
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<li class="listitem">
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Raises StopIteration exception at end
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</li>
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</ul></div>
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<p>
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The typical Python iteration protocol: <code class="literal"><span class="bold"><strong>for y
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in x...</strong></span></code> is as follows:
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</p>
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<pre class="programlisting"><span class="identifier">iter</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">__iter__</span><span class="special">()</span> <span class="comment"># get iterator</span>
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<span class="keyword">try</span><span class="special">:</span>
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<span class="keyword">while</span> <span class="number">1</span><span class="special">:</span>
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<span class="identifier">y</span> <span class="special">=</span> <span class="identifier">iter</span><span class="special">.</span><span class="identifier">next</span><span class="special">()</span> <span class="comment"># get each item</span>
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<span class="special">...</span> <span class="comment"># process y</span>
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<span class="keyword">except</span> <span class="identifier">StopIteration</span><span class="special">:</span> <span class="keyword">pass</span> <span class="comment"># iterator exhausted</span>
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</pre>
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<p>
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Boost.Python provides some mechanisms to make C++ iterators play along nicely
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as Python iterators. What we need to do is to produce appropriate <code class="computeroutput"><span class="identifier">__iter__</span></code> function from C++ iterators that
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is compatible with the Python iteration protocol. For example:
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</p>
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<pre class="programlisting"><span class="identifier">object</span> <span class="identifier">get_iterator</span> <span class="special">=</span> <span class="identifier">iterator</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="special">>();</span>
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<span class="identifier">object</span> <span class="identifier">iter</span> <span class="special">=</span> <span class="identifier">get_iterator</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
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<span class="identifier">object</span> <span class="identifier">first</span> <span class="special">=</span> <span class="identifier">iter</span><span class="special">.</span><span class="identifier">next</span><span class="special">();</span>
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</pre>
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<p>
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Or for use in class_<>:
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</p>
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<pre class="programlisting"><span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"__iter__"</span><span class="special">,</span> <span class="identifier">iterator</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="special">>())</span>
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</pre>
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<p>
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<span class="bold"><strong>range</strong></span>
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</p>
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<p>
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We can create a Python savvy iterator using the range function:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" type="disc">
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<li class="listitem">
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range(start, finish)
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</li>
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<li class="listitem">
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range<Policies,Target>(start, finish)
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</li>
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</ul></div>
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<p>
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Here, start/finish may be one of:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" type="disc">
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<li class="listitem">
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member data pointers
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</li>
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<li class="listitem">
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member function pointers
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</li>
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<li class="listitem">
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adaptable function object (use Target parameter)
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</li>
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</ul></div>
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<p>
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<span class="bold"><strong>iterator</strong></span>
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
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iterator<T, Policies>()
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</li></ul></div>
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<p>
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Given a container <code class="literal">T</code>, iterator is a shortcut that simply
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calls <code class="literal">range</code> with &T::begin, &T::end.
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</p>
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<p>
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Let's put this into action... Here's an example from some hypothetical bogon
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Particle accelerator code:
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</p>
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<pre class="programlisting"><span class="identifier">f</span> <span class="special">=</span> <span class="identifier">Field</span><span class="special">()</span>
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<span class="keyword">for</span> <span class="identifier">x</span> <span class="keyword">in</span> <span class="identifier">f</span><span class="special">.</span><span class="identifier">pions</span><span class="special">:</span>
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<span class="identifier">smash</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span>
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<span class="keyword">for</span> <span class="identifier">y</span> <span class="keyword">in</span> <span class="identifier">f</span><span class="special">.</span><span class="identifier">bogons</span><span class="special">:</span>
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<span class="identifier">count</span><span class="special">(</span><span class="identifier">y</span><span class="special">)</span>
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</pre>
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<p>
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Now, our C++ Wrapper:
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</p>
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<pre class="programlisting"><span class="identifier">class_</span><span class="special"><</span><span class="identifier">F</span><span class="special">>(</span><span class="string">"Field"</span><span class="special">)</span>
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<span class="special">.</span><span class="identifier">property</span><span class="special">(</span><span class="string">"pions"</span><span class="special">,</span> <span class="identifier">range</span><span class="special">(&</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">p_begin</span><span class="special">,</span> <span class="special">&</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">p_end</span><span class="special">))</span>
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<span class="special">.</span><span class="identifier">property</span><span class="special">(</span><span class="string">"bogons"</span><span class="special">,</span> <span class="identifier">range</span><span class="special">(&</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">b_begin</span><span class="special">,</span> <span class="special">&</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">b_end</span><span class="special">));</span>
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</pre>
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<p>
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<span class="bold"><strong>stl_input_iterator</strong></span>
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</p>
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<p>
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So far, we have seen how to expose C++ iterators and ranges to Python. Sometimes
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we wish to go the other way, though: we'd like to pass a Python sequence to
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an STL algorithm or use it to initialize an STL container. We need to make
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a Python iterator look like an STL iterator. For that, we use <code class="computeroutput"><span class="identifier">stl_input_iterator</span><span class="special"><></span></code>.
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Consider how we might implement a function that exposes <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">assign</span><span class="special">()</span></code> to Python:
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</p>
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<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
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<span class="keyword">void</span> <span class="identifier">list_assign</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">l</span><span class="special">,</span> <span class="identifier">object</span> <span class="identifier">o</span><span class="special">)</span> <span class="special">{</span>
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<span class="comment">// Turn a Python sequence into an STL input range</span>
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<span class="identifier">stl_input_iterator</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">begin</span><span class="special">(</span><span class="identifier">o</span><span class="special">),</span> <span class="identifier">end</span><span class="special">;</span>
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<span class="identifier">l</span><span class="special">.</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">);</span>
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<span class="special">}</span>
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<span class="comment">// Part of the wrapper for list<int></span>
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<span class="identifier">class_</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">>(</span><span class="string">"list_int"</span><span class="special">)</span>
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<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"assign"</span><span class="special">,</span> <span class="special">&</span><span class="identifier">list_assign</span><span class="special"><</span><span class="keyword">int</span><span class="special">>)</span>
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<span class="comment">// ...</span>
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<span class="special">;</span>
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</pre>
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<p>
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Now in Python, we can assign any integer sequence to <code class="computeroutput"><span class="identifier">list_int</span></code>
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objects:
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</p>
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<pre class="programlisting"><span class="identifier">x</span> <span class="special">=</span> <span class="identifier">list_int</span><span class="special">();</span>
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<span class="identifier">x</span><span class="special">.</span><span class="identifier">assign</span><span class="special">([</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">,</span><span class="number">5</span><span class="special">])</span>
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</pre>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2002-2005 Joel
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de Guzman, David Abrahams<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">
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http://www.boost.org/LICENSE_1_0.txt </a>)
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</p>
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