<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Iterators</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. python 2.0"> <link rel="up" href="../index.html" title="Chapter 1. python 2.0"> <link rel="prev" href="embedding.html" title="Embedding"> <link rel="next" href="exception.html" title="Exception Translation"> </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="embedding.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="exception.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="python.iterators"></a>Iterators</h2></div></div></div> <p> In C++, and STL in particular, we see iterators everywhere. Python also has iterators, but these are two very different beasts. </p> <p> <span class="bold"><strong>C++ iterators:</strong></span> </p> <div class="itemizedlist"><ul class="itemizedlist" type="disc"> <li class="listitem"> C++ has 5 type categories (random-access, bidirectional, forward, input, output) </li> <li class="listitem"> There are 2 Operation categories: reposition, access </li> <li class="listitem"> A pair of iterators is needed to represent a (first/last) range. </li> </ul></div> <p> <span class="bold"><strong>Python Iterators:</strong></span> </p> <div class="itemizedlist"><ul class="itemizedlist" type="disc"> <li class="listitem"> 1 category (forward) </li> <li class="listitem"> 1 operation category (next()) </li> <li class="listitem"> Raises StopIteration exception at end </li> </ul></div> <p> The typical Python iteration protocol: <code class="literal"><span class="bold"><strong>for y in x...</strong></span></code> is as follows: </p> <p> </p> <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><span class="keyword">try</span><span class="special">:</span> <span class="keyword">while</span> <span class="number">1</span><span class="special">:</span> <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> <span class="special">...</span> <span class="comment"># process y </span><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></pre> <p> Boost.Python provides some mechanisms to make C++ iterators play along nicely 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 is compatible with the Python iteration protocol. For example: </p> <p> </p> <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> <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> <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> </pre> <p> Or for use in class_<>: </p> <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> </pre> <p> <span class="bold"><strong>range</strong></span> </p> <p> We can create a Python savvy iterator using the range function: </p> <div class="itemizedlist"><ul class="itemizedlist" type="disc"> <li class="listitem"> range(start, finish) </li> <li class="listitem"> range<Policies,Target>(start, finish) </li> </ul></div> <p> Here, start/finish may be one of: </p> <div class="itemizedlist"><ul class="itemizedlist" type="disc"> <li class="listitem"> member data pointers </li> <li class="listitem"> member function pointers </li> <li class="listitem"> adaptable function object (use Target parameter) </li> </ul></div> <p> <span class="bold"><strong>iterator</strong></span> </p> <div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"> iterator<T, Policies>() </li></ul></div> <p> Given a container <code class="literal">T</code>, iterator is a shortcut that simply calls <code class="literal">range</code> with &T::begin, &T::end. </p> <p> Let's put this into action... Here's an example from some hypothetical bogon Particle accelerator code: </p> <p> </p> <pre class="programlisting"><span class="identifier">f</span> <span class="special">=</span> <span class="identifier">Field</span><span class="special">()</span> <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> <span class="identifier">smash</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <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> <span class="identifier">count</span><span class="special">(</span><span class="identifier">y</span><span class="special">)</span> </pre> <p> Now, our C++ Wrapper: </p> <p> </p> <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> <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> <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> </pre> <p> <span class="bold"><strong>stl_input_iterator</strong></span> </p> <p> So far, we have seen how to expose C++ iterators and ranges to Python. Sometimes we wish to go the other way, though: we'd like to pass a Python sequence to an STL algorithm or use it to initialize an STL container. We need to make 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>. 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: </p> <p> </p> <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> <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> <span class="comment">// Turn a Python sequence into an STL input range </span> <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> <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> <span class="special">}</span> <span class="comment">// Part of the wrapper for list<int> </span><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> <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> <span class="comment">// ... </span> <span class="special">;</span> </pre> <p> Now in Python, we can assign any integer sequence to <code class="computeroutput"><span class="identifier">list_int</span></code> objects: </p> <p> </p> <pre class="programlisting"><span class="identifier">x</span> <span class="special">=</span> <span class="identifier">list_int</span><span class="special">();</span> <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> </pre> </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 © 2002-2005 Joel de Guzman, David Abrahams<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="embedding.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="exception.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>