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<div class="section" id="how-to-use-dtypes">
<h1>How to use dtypes</h1>
<p>Here is a brief tutorial to show how to create ndarrays with built-in python data types, and extract the types and values of member variables</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></span><span class="cp">#include</span> <span class="cpf">&lt;boost/python/numpy.hpp&gt;</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">&lt;iostream&gt;</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>Next, we create the shape and dtype. We use the get_builtin method to get the numpy dtype corresponding to the builtin C++ dtype
Here, we will create a 3x3 array passing a tuple with (3,3) for the size, and double as the data type</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><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">&lt;</span><span class="kt">double</span><span class="o">&gt;</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>Finally, we can print the array using the extract method in the python namespace.
Here, we first convert the variable into a string, and then extract it as a C++ character array from the python string using the &lt;char const * &gt; template</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;Original array:</span><span class="se">\n</span><span class="s">&quot;</span> <span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
</pre></div>
</div>
<p>We can also print the dtypes of the data members of the ndarray by using the get_dtype method for the ndarray</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;Datatype is:</span><span class="se">\n</span><span class="s">&quot;</span> <span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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="n">get_dtype</span><span class="p">()))</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span> <span class="p">;</span>
</pre></div>
</div>
<p>We can also create custom dtypes and build ndarrays with the custom dtypes</p>
<p>We use the dtype constructor to create a custom dtype. This constructor takes a list as an argument.</p>
<p>The list should contain one or more tuples of the format (variable name, variable type)</p>
<p>So first create a tuple with a variable name and its dtype, double, to create a custom dtype</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">p</span><span class="o">::</span><span class="n">tuple</span> <span class="n">for_custom_dtype</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="s">&quot;ha&quot;</span><span class="p">,</span><span class="n">dtype</span><span class="p">)</span> <span class="p">;</span>
</pre></div>
</div>
<p>Next, create a list, and add this tuple to the list. Then use the list to create the custom dtype</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">p</span><span class="o">::</span><span class="n">list</span> <span class="n">list_for_dtype</span> <span class="p">;</span>
<span class="n">list_for_dtype</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="n">for_custom_dtype</span><span class="p">)</span> <span class="p">;</span>
<span class="n">np</span><span class="o">::</span><span class="n">dtype</span> <span class="n">custom_dtype</span> <span class="o">=</span> <span class="n">np</span><span class="o">::</span><span class="n">dtype</span><span class="p">(</span><span class="n">list_for_dtype</span><span class="p">)</span> <span class="p">;</span>
</pre></div>
</div>
<p>We are now ready to create an ndarray with dimensions specified by *shape* and of custom dtpye</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span> <span class="n">np</span><span class="o">::</span><span class="n">ndarray</span> <span class="n">new_array</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">custom_dtype</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
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<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></span><span class="cp">#include</span> <span class="cpf">&lt;boost/python/numpy.hpp&gt;</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">&lt;iostream&gt;</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></span><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">&lt;</span><span class="kt">int</span><span class="o">&gt;</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></span><span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;C++ array :&quot;</span> <span class="o">&lt;&lt;</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">&lt;</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">&lt;&lt;</span> <span class="n">arr</span><span class="p">[</span><span class="n">j</span><span class="p">]</span> <span class="o">&lt;&lt;</span> <span class="sc">&#39; &#39;</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">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span>
<span class="o">&lt;&lt;</span> <span class="s">&quot;Python ndarray :&quot;</span> <span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">&lt;&lt;</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></span><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">&lt;&lt;</span> <span class="s">&quot;Is the change reflected in the C++ array used to create the ndarray ? &quot;</span> <span class="o">&lt;&lt;</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">&lt;</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">&lt;&lt;</span> <span class="n">arr</span><span class="p">[</span><span class="n">j</span><span class="p">]</span> <span class="o">&lt;&lt;</span> <span class="sc">&#39; &#39;</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></span> <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">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span>
<span class="o">&lt;&lt;</span> <span class="s">&quot;Is the change reflected in the Python ndarray ?&quot;</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span>
<span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">&lt;&lt;</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>
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<div class="section" id="creating-ndarrays">
<h1>Creating ndarrays</h1>
<p>The Boost.Numpy library exposes quite a few methods to create ndarrays. ndarrays can be created in a variety of ways, include empty arrays and zero filled arrays.
ndarrays can also be created from arbitrary python sequences as well as from data and dtypes.</p>
<p>This tutorial will introduce you to some of the ways in which you can create ndarrays. The methods covered here include creating ndarrays from arbitrary Python sequences, as well as from C++ containers, using both unit and non-unit strides</p>
<p>First, as before, initialise the necessary namepaces and runtimes</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="cp">#include</span> <span class="cpf">&lt;boost/python/numpy.hpp&gt;</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">&lt;iostream&gt;</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>Let&#8217;s now create an ndarray from a simple tuple. We first create a tuple object, and then pass it to the array method, to generate the necessary tuple</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">p</span><span class="o">::</span><span class="n">object</span> <span class="n">tu</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="sc">&#39;a&#39;</span><span class="p">,</span><span class="sc">&#39;b&#39;</span><span class="p">,</span><span class="sc">&#39;c&#39;</span><span class="p">);</span>
<span class="n">np</span><span class="o">::</span><span class="n">ndarray</span> <span class="n">example_tuple</span> <span class="o">=</span> <span class="n">np</span><span class="o">::</span><span class="n">array</span><span class="p">(</span><span class="n">tu</span><span class="p">);</span>
</pre></div>
</div>
<p>Let&#8217;s now try the same with a list. We create an empty list, add an element using the append method, and as before, call the array method</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">p</span><span class="o">::</span><span class="n">list</span> <span class="n">l</span><span class="p">;</span>
<span class="n">l</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="sc">&#39;a&#39;</span><span class="p">);</span>
<span class="n">np</span><span class="o">::</span><span class="n">ndarray</span> <span class="n">example_list</span> <span class="o">=</span> <span class="n">np</span><span class="o">::</span><span class="n">array</span> <span class="p">(</span><span class="n">l</span><span class="p">);</span>
</pre></div>
</div>
<p>Optionally, we can also specify a dtype for the array</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">np</span><span class="o">::</span><span class="n">dtype</span> <span class="n">dt</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">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="p">();</span>
<span class="n">np</span><span class="o">::</span><span class="n">ndarray</span> <span class="n">example_list1</span> <span class="o">=</span> <span class="n">np</span><span class="o">::</span><span class="n">array</span> <span class="p">(</span><span class="n">l</span><span class="p">,</span><span class="n">dt</span><span class="p">);</span>
</pre></div>
</div>
<p>We can also create an array by supplying data arrays and a few other parameters.</p>
<p>First,create an integer array</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="kt">int</span> <span class="n">data</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>
</pre></div>
</div>
<p>Create a shape, and strides, needed by the function</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><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">5</span><span class="p">);</span>
<span class="n">p</span><span class="o">::</span><span class="n">tuple</span> <span class="n">stride</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="k">sizeof</span><span class="p">(</span><span class="kt">int</span><span class="p">));</span>
</pre></div>
</div>
<p>Here, shape is (4,) , and the stride is <cite>sizeof(int)`</cite>.
A stride is the number of bytes that must be traveled to get to the next desired element while constructing the ndarray.</p>
<p>The function also needs an owner, to keep track of the data array passed. Passing none is dangerous</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">p</span><span class="o">::</span><span class="n">object</span> <span class="n">own</span><span class="p">;</span>
</pre></div>
</div>
<p>The from_data function takes the data array, datatype,shape,stride and owner as arguments and returns an ndarray</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">np</span><span class="o">::</span><span class="n">ndarray</span> <span class="n">data_ex1</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">data</span><span class="p">,</span><span class="n">dt</span><span class="p">,</span> <span class="n">shape</span><span class="p">,</span><span class="n">stride</span><span class="p">,</span><span class="n">own</span><span class="p">);</span>
</pre></div>
</div>
<p>Now let&#8217;s print the ndarray we created</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;Single dimensional array ::&quot;</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span>
<span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">data_ex</span><span class="p">))</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
</pre></div>
</div>
<p>Let&#8217;s make it a little more interesting. Lets make an 3x2 ndarray from a multi-dimensional array using non-unit strides</p>
<p>First lets create a 3x4 array of 8-bit integers</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="kt">uint8_t</span> <span class="n">mul_data</span><span class="p">[][</span><span class="mi">4</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="mi">6</span><span class="p">,</span><span class="mi">7</span><span class="p">,</span><span class="mi">8</span><span class="p">},{</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span><span class="mi">7</span><span class="p">}};</span>
</pre></div>
</div>
<p>Now let&#8217;s create an array of 3x2 elements, picking the first and third elements from each row . For that, the shape will be 3x2.
The strides will be 4x2 i.e. 4 bytes to go to the next desired row, and 2 bytes to go to the next desired column</p>
<div class="highlight-c++"><div class="highlight"><pre><span></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">2</span><span class="p">);</span>
<span class="n">stride</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="k">sizeof</span><span class="p">(</span><span class="kt">uint8_t</span><span class="p">)</span><span class="o">*</span><span class="mi">2</span><span class="p">,</span><span class="k">sizeof</span><span class="p">(</span><span class="kt">uint8_t</span><span class="p">));</span>
</pre></div>
</div>
<p>Get the numpy dtype for the built-in 8-bit integer data type</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">np</span><span class="o">::</span><span class="n">dtype</span> <span class="n">dt1</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">&lt;</span><span class="kt">uint8_t</span><span class="o">&gt;</span><span class="p">();</span>
</pre></div>
</div>
<p>Now lets first create and print out the ndarray as is.
Notice how we can pass the shape and strides in the function directly, as well as the owner. The last part can be done because we don&#8217;t have any use to
manipulate the &#8220;owner&#8221; object</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">np</span><span class="o">::</span><span class="n">ndarray</span> <span class="n">mul_data_ex</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">mul_data</span><span class="p">,</span> <span class="n">dt1</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">3</span><span class="p">,</span><span class="mi">4</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">4</span><span class="p">,</span><span class="mi">1</span><span class="p">),</span>
<span class="n">p</span><span class="o">::</span><span class="n">object</span><span class="p">());</span>
<span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;Original multi dimensional array :: &quot;</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span>
<span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">mul_data_ex</span><span class="p">))</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
</pre></div>
</div>
<p>Now create the new ndarray using the shape and strides and print out the array we created using non-unit strides</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span> <span class="n">mul_data_ex</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">mul_data</span><span class="p">,</span> <span class="n">dt1</span><span class="p">,</span> <span class="n">shape</span><span class="p">,</span> <span class="n">stride</span><span class="p">,</span> <span class="n">p</span><span class="o">::</span><span class="n">object</span><span class="p">());</span>
<span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;Selective multidimensional array :: &quot;</span><span class="o">&lt;&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">endl</span>
<span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">mul_data_ex</span><span class="p">))</span> <span class="o">&lt;&lt;</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>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The from_data method will throw <code class="docutils literal"><span class="pre">error_already_set</span></code> if the number of elements dictated by the shape and the corresponding strides don&#8217;t match.</p>
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<div class="section" id="a-simple-tutorial-on-arrays">
<h1>A simple tutorial on Arrays</h1>
<p>Let&#8217;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></span><span class="cp">#include</span> <span class="cpf">&lt;boost/python/numpy.hpp&gt;</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">&lt;iostream&gt;</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></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>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></span><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">&lt;</span><span class="kt">float</span><span class="o">&gt;</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></span><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&lt; &gt;:</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;Original array:</span><span class="se">\n</span><span class="s">&quot;</span> <span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">&lt;&lt;</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">&lt;&lt;</span> <span class="s">&quot;Reshaped array:</span><span class="se">\n</span><span class="s">&quot;</span> <span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">&lt;&lt;</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>
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<div class="section" id="ufuncs">
<h1>Ufuncs</h1>
<p>Ufuncs or universal functions operate on ndarrays element by element, and support array broadcasting, type casting, and other features.</p>
<p>Lets try and see how we can use the binary and unary ufunc methods</p>
<p>After the neccessary includes</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="cp">#include</span> <span class="cpf">&lt;boost/python/numpy.hpp&gt;</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">&lt;iostream&gt;</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>Now we create the structs necessary to implement the ufuncs. The typedefs <em>must</em> be made as the ufunc generators take these typedefs as inputs and return an error otherwise</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="k">struct</span> <span class="n">UnarySquare</span>
<span class="p">{</span>
<span class="k">typedef</span> <span class="kt">double</span> <span class="n">argument_type</span><span class="p">;</span>
<span class="k">typedef</span> <span class="kt">double</span> <span class="n">result_type</span><span class="p">;</span>
<span class="kt">double</span> <span class="nf">operator</span><span class="p">()(</span><span class="kt">double</span> <span class="n">r</span><span class="p">)</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> <span class="n">r</span> <span class="o">*</span> <span class="n">r</span><span class="p">;}</span>
<span class="p">};</span>
<span class="k">struct</span> <span class="n">BinarySquare</span>
<span class="p">{</span>
<span class="k">typedef</span> <span class="kt">double</span> <span class="n">first_argument_type</span><span class="p">;</span>
<span class="k">typedef</span> <span class="kt">double</span> <span class="n">second_argument_type</span><span class="p">;</span>
<span class="k">typedef</span> <span class="kt">double</span> <span class="n">result_type</span><span class="p">;</span>
<span class="kt">double</span> <span class="nf">operator</span><span class="p">()(</span><span class="kt">double</span> <span class="n">a</span><span class="p">,</span><span class="kt">double</span> <span class="n">b</span><span class="p">)</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> <span class="p">(</span><span class="n">a</span><span class="o">*</span><span class="n">a</span> <span class="o">+</span> <span class="n">b</span><span class="o">*</span><span class="n">b</span><span class="p">)</span> <span class="p">;</span> <span class="p">}</span>
<span class="p">};</span>
</pre></div>
</div>
<p>Initialise the Python runtime and the numpy module</p>
<div class="highlight-c++"><div class="highlight"><pre><span></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>Now expose the struct UnarySquare to Python as a class, and let ud be the class object.</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">p</span><span class="o">::</span><span class="n">object</span> <span class="n">ud</span> <span class="o">=</span> <span class="n">p</span><span class="o">::</span><span class="n">class_</span><span class="o">&lt;</span><span class="n">UnarySquare</span><span class="p">,</span> <span class="n">boost</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">UnarySquare</span><span class="o">&gt;</span> <span class="o">&gt;</span><span class="p">(</span><span class="s">&quot;UnarySquare&quot;</span><span class="p">);</span>
<span class="n">ud</span><span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">&quot;__call__&quot;</span><span class="p">,</span> <span class="n">np</span><span class="o">::</span><span class="n">unary_ufunc</span><span class="o">&lt;</span><span class="n">UnarySquare</span><span class="o">&gt;::</span><span class="n">make</span><span class="p">());</span>
</pre></div>
</div>
<p>Let inst be an instance of the class ud</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">p</span><span class="o">::</span><span class="n">object</span> <span class="n">inst</span> <span class="o">=</span> <span class="n">ud</span><span class="p">();</span>
</pre></div>
</div>
<p>Use the &#8220;__call__&#8221; method to call the overloaded () operator and print the value</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;Square of unary scalar 1.0 is &quot;</span> <span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">inst</span><span class="p">.</span><span class="n">attr</span><span class="p">(</span><span class="s">&quot;__call__&quot;</span><span class="p">)(</span><span class="mf">1.0</span><span class="p">)))</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
</pre></div>
</div>
<p>Create an array in C++</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><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="p">;</span>
</pre></div>
</div>
<p>..and use it to create the ndarray in Python</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">np</span><span class="o">::</span><span class="n">ndarray</span> <span class="n">demo_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">&lt;</span><span class="kt">int</span><span class="o">&gt;</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">4</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">4</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 out the demo array</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;Demo array is &quot;</span> <span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">demo_array</span><span class="p">))</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
</pre></div>
</div>
<p>Call the &#8220;__call__&#8221; method to perform the operation and assign the value to result_array</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">p</span><span class="o">::</span><span class="n">object</span> <span class="n">result_array</span> <span class="o">=</span> <span class="n">inst</span><span class="p">.</span><span class="n">attr</span><span class="p">(</span><span class="s">&quot;__call__&quot;</span><span class="p">)(</span><span class="n">demo_array</span><span class="p">);</span>
</pre></div>
</div>
<p>Print the resultant array</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;Square of demo array is &quot;</span> <span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">result_array</span><span class="p">))</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
</pre></div>
</div>
<p>Lets try the same with a list</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">p</span><span class="o">::</span><span class="n">list</span> <span class="n">li</span><span class="p">;</span>
<span class="n">li</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="mi">3</span><span class="p">);</span>
<span class="n">li</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="mi">7</span><span class="p">);</span>
</pre></div>
</div>
<p>Print out the demo list</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;Demo list is &quot;</span> <span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">li</span><span class="p">))</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
</pre></div>
</div>
<p>Call the ufunc for the list</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">result_array</span> <span class="o">=</span> <span class="n">inst</span><span class="p">.</span><span class="n">attr</span><span class="p">(</span><span class="s">&quot;__call__&quot;</span><span class="p">)(</span><span class="n">li</span><span class="p">);</span>
</pre></div>
</div>
<p>And print the list out</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;Square of demo list is &quot;</span> <span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">result_array</span><span class="p">))</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
</pre></div>
</div>
<p>Now lets try Binary ufuncs. Again, expose the struct BinarySquare to Python as a class, and let ud be the class object</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">ud</span> <span class="o">=</span> <span class="n">p</span><span class="o">::</span><span class="n">class_</span><span class="o">&lt;</span><span class="n">BinarySquare</span><span class="p">,</span> <span class="n">boost</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">BinarySquare</span><span class="o">&gt;</span> <span class="o">&gt;</span><span class="p">(</span><span class="s">&quot;BinarySquare&quot;</span><span class="p">);</span>
<span class="n">ud</span><span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">&quot;__call__&quot;</span><span class="p">,</span> <span class="n">np</span><span class="o">::</span><span class="n">binary_ufunc</span><span class="o">&lt;</span><span class="n">BinarySquare</span><span class="o">&gt;::</span><span class="n">make</span><span class="p">());</span>
</pre></div>
</div>
<p>And initialise ud</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">inst</span> <span class="o">=</span> <span class="n">ud</span><span class="p">();</span>
</pre></div>
</div>
<p>Print the two input lists</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;The two input list for binary ufunc are &quot;</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span>
<span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">demo_array</span><span class="p">))</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span>
<span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">demo_array</span><span class="p">))</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
</pre></div>
</div>
<p>Call the binary ufunc taking demo_array as both inputs</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">result_array</span> <span class="o">=</span> <span class="n">inst</span><span class="p">.</span><span class="n">attr</span><span class="p">(</span><span class="s">&quot;__call__&quot;</span><span class="p">)(</span><span class="n">demo_array</span><span class="p">,</span><span class="n">demo_array</span><span class="p">);</span>
</pre></div>
</div>
<p>And print the output</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span> <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="s">&quot;Square of list with binary ufunc is &quot;</span> <span class="o">&lt;&lt;</span> <span class="n">p</span><span class="o">::</span><span class="n">extract</span><span class="o">&lt;</span><span class="kt">char</span> <span class="k">const</span> <span class="o">*&gt;</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">result_array</span><span class="p">))</span> <span class="o">&lt;&lt;</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>
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