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Library/boost/libs/python/doc/html/numpy/tutorial/fromdata.html
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19 дек 2024, 19:11
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>How to access data using raw pointers - Boost.Python NumPy extension 1.0 documentation</title> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <link rel="stylesheet" href="../_static/style.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../', VERSION: '1.0', COLLAPSE_MODINDEX: false, FILE_SUFFIX: '.html' }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="top" title="Boost.Python NumPy extension 1.0 documentation" href="../index.html" /> <link rel="up" title="Boost.Python NumPy extension Tutorial" href="index.html" /> <link rel="next" title="Boost.Python NumPy extension Reference" href="../reference/index.html" /> <link rel="prev" title="Ufuncs" href="ufunc.html" /> </head> <body> <div class="header"> <table border="0" cellpadding="7" cellspacing="0" width="100%" summary= "header"> <tr> <td valign="top" width="300"> <h3><a href="../index.html"><img alt="C++ Boost" src="../_static/bpl.png" border="0"></a></h3> </td> <td > <h1 align="center"><a href="../index.html">(NumPy)</a></h1> <!-- <h2 align="center">CallPolicies Concept</h2>--> </td> <td> <div id="searchbox" style="display: none"> <form class="search" action="../search.html" method="get"> <input type="text" name="q" size="18" /> <input type="submit" value="Search" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </td> </tr> </table> </div> <hr/> <div class="content"> <div class="navbar" style="text-align:right;"> <a class="prev" title="Ufuncs" href="ufunc.html"><img src="../_static/prev.png" alt="prev"/></a> <a class="up" title="Boost.Python NumPy extension Tutorial" href="index.html"><img src="../_static/up.png" alt="up"/></a> <a class="next" title="Boost.Python NumPy extension Reference" href="../reference/index.html"><img src="../_static/next.png" alt="next"/></a> </div> <div class="section" id="how-to-access-data-using-raw-pointers"> <h1>How to access data using raw pointers</h1> <p>One of the advantages of the ndarray wrapper is that the same data can be used in both Python and C++ and changes can be made to reflect at both ends. The from_data method makes this possible.</p> <p>Like before, first get the necessary headers, setup the namespaces and initialize the Python runtime and numpy module:</p> <div class="highlight-c++"><div class="highlight"><pre><span class="cp">#include</span> <span class="cpf"><boost/python/numpy.hpp></span><span class="cp"></span> <span class="cp">#include</span> <span class="cpf"><iostream></span><span class="cp"></span> <span class="k">namespace</span> <span class="n">p</span> <span class="o">=</span> <span class="n">boost</span><span class="o">::</span><span class="n">python</span><span class="p">;</span> <span class="k">namespace</span> <span class="n">np</span> <span class="o">=</span> <span class="n">boost</span><span class="o">::</span><span class="n">python</span><span class="o">::</span><span class="n">numpy</span><span class="p">;</span> <span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">**</span><span class="n">argv</span><span class="p">)</span> <span class="p">{</span> <span class="n">Py_Initialize</span><span class="p">();</span> <span class="n">np</span><span class="o">::</span><span class="n">initialize</span><span class="p">();</span> </pre></div> </div> <p>Create an array in C++ , and pass the pointer to it to the from_data method to create an ndarray:</p> <div class="highlight-c++"><div class="highlight"><pre><span class="kt">int</span> <span class="n">arr</span><span class="p">[]</span> <span class="o">=</span> <span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">5</span><span class="p">};</span> <span class="n">np</span><span class="o">::</span><span class="n">ndarray</span> <span class="n">py_array</span> <span class="o">=</span> <span class="n">np</span><span class="o">::</span><span class="n">from_data</span><span class="p">(</span><span class="n">arr</span><span class="p">,</span> <span class="n">np</span><span class="o">::</span><span class="n">dtype</span><span class="o">::</span><span class="n">get_builtin</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">(),</span> <span class="n">p</span><span class="o">::</span><span class="n">make_tuple</span><span class="p">(</span><span class="mi">5</span><span class="p">),</span> <span class="n">p</span><span class="o">::</span><span class="n">make_tuple</span><span class="p">(</span><span class="k">sizeof</span><span class="p">(</span><span class="kt">int</span><span class="p">)),</span> <span class="n">p</span><span class="o">::</span><span class="n">object</span><span class="p">());</span> </pre></div> </div> <p>Print the source C++ array, as well as the ndarray, to check if they are the same:</p> <div class="highlight-c++"><div class="highlight"><pre><span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o"><<</span> <span class="s">"C++ array :"</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> <span class="k">for</span> <span class="p">(</span><span class="kt">int</span> <span class="n">j</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span><span class="n">j</span><span class="o"><</span><span class="mi">4</span><span class="p">;</span><span class="n">j</span><span class="o">++</span><span class="p">)</span> <span class="p">{</span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o"><<</span> <span class="n">arr</span><span class="p">[</span><span class="n">j</span><span class="p">]</span> <span class="o"><<</span> <span class="sc">' '</span><span class="p">;</span> <span class="p">}</span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span> <span class="o"><<</span> <span class="s">"Python ndarray :"</span> <span class="o"><<</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o"><</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*></span><span class="p">(</span><span class="n">p</span><span class="o">::</span><span class="n">str</span><span class="p">(</span><span class="n">py_array</span><span class="p">))</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> </pre></div> </div> <p>Now, change an element in the Python ndarray, and check if the value changed correspondingly in the source C++ array:</p> <div class="highlight-c++"><div class="highlight"><pre><span class="n">py_array</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="mi">5</span> <span class="p">;</span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o"><<</span> <span class="s">"Is the change reflected in the C++ array used to create the ndarray ? "</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> <span class="k">for</span> <span class="p">(</span><span class="kt">int</span> <span class="n">j</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">j</span> <span class="o"><</span> <span class="mi">5</span><span class="p">;</span> <span class="n">j</span><span class="o">++</span><span class="p">)</span> <span class="p">{</span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o"><<</span> <span class="n">arr</span><span class="p">[</span><span class="n">j</span><span class="p">]</span> <span class="o"><<</span> <span class="sc">' '</span><span class="p">;</span> <span class="p">}</span> </pre></div> </div> <p>Next, change an element of the source C++ array and see if it is reflected in the Python ndarray:</p> <div class="highlight-c++"><div class="highlight"><pre> <span class="n">arr</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="o">=</span> <span class="mi">8</span><span class="p">;</span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span> <span class="o"><<</span> <span class="s">"Is the change reflected in the Python ndarray ?"</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span> <span class="o"><<</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o"><</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*></span><span class="p">(</span><span class="n">p</span><span class="o">::</span><span class="n">str</span><span class="p">(</span><span class="n">py_array</span><span class="p">))</span> <span class="o"><<</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> <span class="p">}</span> </pre></div> </div> <p>As we can see, the changes are reflected across the ends. This happens because the from_data method passes the C++ array by reference to create the ndarray, and thus uses the same locations for storing data.</p> </div> <div class="navbar" style="text-align:right;"> <a class="prev" title="Ufuncs" href="ufunc.html"><img src="../_static/prev.png" alt="prev"/></a> <a class="up" title="Boost.Python NumPy extension Tutorial" href="index.html"><img src="../_static/up.png" alt="up"/></a> <a class="next" title="Boost.Python NumPy extension Reference" href="../reference/index.html"><img src="../_static/next.png" alt="next"/></a> </div> </div> </body> </html>