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<div class="section" id="a-simple-tutorial-on-arrays">
<h1>A simple tutorial on Arrays</h1>
<p>Let’s start with a simple tutorial to create and modify arrays.</p>
<p>Get the necessary headers for numpy components and set up necessary namespaces:</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>
</pre></div>
</div>
<p>Initialise the Python runtime, and the numpy module. Failure to call these results in segmentation errors:</p>
<div class="highlight-c++"><div class="highlight"><pre><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>Zero filled n-dimensional arrays can be created using the shape and data-type of the array as a parameter. Here, the shape is 3x3 and the datatype is the built-in float type:</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="n">p</span><span class="o">::</span><span class="n">tuple</span> <span class="n">shape</span> <span class="o">=</span> <span class="n">p</span><span class="o">::</span><span class="n">make_tuple</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">3</span><span class="p">);</span>
<span class="n">np</span><span class="o">::</span><span class="n">dtype</span> <span class="n">dtype</span> <span class="o">=</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">float</span><span class="o">></span><span class="p">();</span>
<span class="n">np</span><span class="o">::</span><span class="n">ndarray</span> <span class="n">a</span> <span class="o">=</span> <span class="n">np</span><span class="o">::</span><span class="n">zeros</span><span class="p">(</span><span class="n">shape</span><span class="p">,</span> <span class="n">dtype</span><span class="p">);</span>
</pre></div>
</div>
<p>You can also create an empty array like this</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="n">np</span><span class="o">::</span><span class="n">ndarray</span> <span class="n">b</span> <span class="o">=</span> <span class="n">np</span><span class="o">::</span><span class="n">empty</span><span class="p">(</span><span class="n">shape</span><span class="p">,</span><span class="n">dtype</span><span class="p">);</span>
</pre></div>
</div>
<p>Print the original and reshaped array. The array a which is a list is first converted to a string, and each value in the list is extracted using extract< >:</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">"Original array:</span><span class="se">\n</span><span class="s">"</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">a</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="c1">// Reshape the array into a 1D array</span>
<span class="n">a</span> <span class="o">=</span> <span class="n">a</span><span class="p">.</span><span class="n">reshape</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">9</span><span class="p">));</span>
<span class="c1">// Print it again.</span>
<span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o"><<</span> <span class="s">"Reshaped array:</span><span class="se">\n</span><span class="s">"</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">a</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>
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