1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Iterators</title> 5<link rel="stylesheet" href="../../boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../index.html" title="Boost.Python Tutorial"> 8<link rel="up" href="../index.html" title="Boost.Python Tutorial"> 9<link rel="prev" href="embedding.html" title="Embedding"> 10<link rel="next" href="exception.html" title="Exception Translation"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="" width="" height="" src="../../images/boost.png"></td></tr></table> 14<hr> 15<div class="spirit-nav"> 16<a accesskey="p" href="embedding.html"><img src="../../images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="exception.html"><img src="../../images/next.png" alt="Next"></a> 17</div> 18<div class="section"> 19<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 20<a name="tutorial.iterators"></a><a class="link" href="iterators.html" title="Iterators">Iterators</a> 21</h2></div></div></div> 22<p> 23 In C++, and STL in particular, we see iterators everywhere. Python also has 24 iterators, but these are two very different beasts. 25 </p> 26<p> 27 <span class="bold"><strong>C++ iterators:</strong></span> 28 </p> 29<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 30<li class="listitem"> 31 C++ has 5 type categories (random-access, bidirectional, forward, input, 32 output) 33 </li> 34<li class="listitem"> 35 There are 2 Operation categories: reposition, access 36 </li> 37<li class="listitem"> 38 A pair of iterators is needed to represent a (first/last) range. 39 </li> 40</ul></div> 41<p> 42 <span class="bold"><strong>Python Iterators:</strong></span> 43 </p> 44<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 45<li class="listitem"> 46 1 category (forward) 47 </li> 48<li class="listitem"> 49 1 operation category (next()) 50 </li> 51<li class="listitem"> 52 Raises StopIteration exception at end 53 </li> 54</ul></div> 55<p> 56 The typical Python iteration protocol: <code class="literal"><span class="bold"><strong>for y 57 in x...</strong></span></code> is as follows: 58 </p> 59<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> 60<span class="keyword">try</span><span class="special">:</span> 61 <span class="keyword">while</span> <span class="number">1</span><span class="special">:</span> 62 <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> 63 <span class="special">...</span> <span class="comment"># process y</span> 64<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> 65</pre> 66<p> 67 Boost.Python provides some mechanisms to make C++ iterators play along nicely 68 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 69 is compatible with the Python iteration protocol. For example: 70 </p> 71<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> 72<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> 73<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> 74</pre> 75<p> 76 Or for use in class_<>: 77 </p> 78<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> 79</pre> 80<p> 81 <span class="bold"><strong>range</strong></span> 82 </p> 83<p> 84 We can create a Python savvy iterator using the range function: 85 </p> 86<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 87<li class="listitem"> 88 range(start, finish) 89 </li> 90<li class="listitem"> 91 range<Policies,Target>(start, finish) 92 </li> 93</ul></div> 94<p> 95 Here, start/finish may be one of: 96 </p> 97<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 98<li class="listitem"> 99 member data pointers 100 </li> 101<li class="listitem"> 102 member function pointers 103 </li> 104<li class="listitem"> 105 adaptable function object (use Target parameter) 106 </li> 107</ul></div> 108<p> 109 <span class="bold"><strong>iterator</strong></span> 110 </p> 111<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> 112 iterator<T, Policies>() 113 </li></ul></div> 114<p> 115 Given a container <code class="literal">T</code>, iterator is a shortcut that simply 116 calls <code class="literal">range</code> with &T::begin, &T::end. 117 </p> 118<p> 119 Let's put this into action... Here's an example from some hypothetical bogon 120 Particle accelerator code: 121 </p> 122<pre class="programlisting"><span class="identifier">f</span> <span class="special">=</span> <span class="identifier">Field</span><span class="special">()</span> 123<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> 124 <span class="identifier">smash</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> 125<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> 126 <span class="identifier">count</span><span class="special">(</span><span class="identifier">y</span><span class="special">)</span> 127</pre> 128<p> 129 Now, our C++ Wrapper: 130 </p> 131<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> 132 <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> 133 <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> 134</pre> 135<p> 136 <span class="bold"><strong>stl_input_iterator</strong></span> 137 </p> 138<p> 139 So far, we have seen how to expose C++ iterators and ranges to Python. Sometimes 140 we wish to go the other way, though: we'd like to pass a Python sequence to 141 an STL algorithm or use it to initialize an STL container. We need to make 142 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>. 143 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: 144 </p> 145<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> 146<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> 147 <span class="comment">// Turn a Python sequence into an STL input range</span> 148 <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> 149 <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> 150<span class="special">}</span> 151 152<span class="comment">// Part of the wrapper for list<int></span> 153<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> 154 <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> 155 <span class="comment">// ...</span> 156 <span class="special">;</span> 157</pre> 158<p> 159 Now in Python, we can assign any integer sequence to <code class="computeroutput"><span class="identifier">list_int</span></code> 160 objects: 161 </p> 162<pre class="programlisting"><span class="identifier">x</span> <span class="special">=</span> <span class="identifier">list_int</span><span class="special">();</span> 163<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> 164</pre> 165</div> 166<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 167<td align="left"></td> 168<td align="right"><div class="copyright-footer">Copyright © 2002-2005 Joel 169 de Guzman, David Abrahams<p> 170 Distributed under the Boost Software License, Version 1.0. (See accompanying 171 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> 172 </p> 173</div></td> 174</tr></table> 175<hr> 176<div class="spirit-nav"> 177<a accesskey="p" href="embedding.html"><img src="../../images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="exception.html"><img src="../../images/next.png" alt="Next"></a> 178</div> 179</body> 180</html> 181
