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<h1>
<span class="m-breadcrumb"><a href="Algorithms.html">Taskflow Algorithms</a> &raquo;</span>
Parallel Sort
</h1>
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<h3>Contents</h3>
<ul>
<li><a href="#ParallelSortInclude">Include the Header</a></li>
<li><a href="#SortARangeOfItems">Sort a Range of Items</a></li>
<li><a href="#SortARangeOfItemsWithACustomComparator">Sort a Range of Items with a Custom Comparator</a></li>
<li><a href="#ParallelSortEnableStatefulDataPassing">Enable Stateful Data Passing</a></li>
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<p>Taskflow provides template functions for constructing tasks to sort ranges of items in parallel.</p><section id="ParallelSortInclude"><h2><a href="#ParallelSortInclude">Include the Header</a></h2><p>You need to include the header file, <code>taskflow/algorithm/sort.hpp</code>, for creating a parallel-sort task.</p><pre class="m-code"><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;taskflow/algorithm/sort.hpp&gt;</span></pre></section><section id="SortARangeOfItems"><h2><a href="#SortARangeOfItems">Sort a Range of Items</a></h2><p>The task created by <a href="classtf_1_1FlowBuilder.html#a7d844e9856c7c65b26ccdb83ffdab1d6" class="m-doc">tf::<wbr />Taskflow::<wbr />sort(B first, E last)</a> performs parallel sort to rank a range of elements specified by <code>[first, last)</code> in increasing order. The given iterators must be <em>random-accessible</em>. The following example creates a task to sort a data vector in increasing order.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span>
<span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="w"> </span><span class="n">data</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="mi">9</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">11</span><span class="p">,</span><span class="w"> </span><span class="mi">-8</span><span class="p">};</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Task</span><span class="w"> </span><span class="n">sort</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">sort</span><span class="p">(</span><span class="n">data</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span><span class="w"> </span><span class="n">data</span><span class="p">.</span><span class="n">end</span><span class="p">());</span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">).</span><span class="n">wait</span><span class="p">();</span>
<span class="n">assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_sorted</span><span class="p">(</span><span class="n">data</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span><span class="w"> </span><span class="n">data</span><span class="p">.</span><span class="n">end</span><span class="p">()));</span></pre><aside class="m-note m-info"><h4>Note</h4><p>Elements are compared using the operator <code>&lt;</code>.</p></aside></section><section id="SortARangeOfItemsWithACustomComparator"><h2><a href="#SortARangeOfItemsWithACustomComparator">Sort a Range of Items with a Custom Comparator</a></h2><p><a href="classtf_1_1FlowBuilder.html#a35e180eb63de6c9f28e43185e837a4fa" class="m-doc">tf::<wbr />Taskflow::<wbr />sort(B first, E last, C cmp)</a> is an overload of parallel sort that allows users to specify a custom comparator. The following example sorts a data vector in decreasing order.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span>
<span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="w"> </span><span class="n">data</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="mi">9</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">11</span><span class="p">,</span><span class="w"> </span><span class="mi">-8</span><span class="p">};</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Task</span><span class="w"> </span><span class="n">sort</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">sort</span><span class="p">(</span><span class="n">data</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span><span class="w"> </span><span class="n">data</span><span class="p">.</span><span class="n">end</span><span class="p">(),</span><span class="w"> </span>
<span class="w"> </span><span class="p">[](</span><span class="kt">int</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">b</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="o">&gt;</span><span class="w"> </span><span class="n">b</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="p">);</span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">).</span><span class="n">wait</span><span class="p">();</span>
<span class="n">assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_sorted</span><span class="p">(</span><span class="n">data</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span><span class="w"> </span><span class="n">data</span><span class="p">.</span><span class="n">end</span><span class="p">(),</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">greater</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="p">{}));</span></pre><aside class="m-note m-info"><h4>Note</h4><p><a href="classtf_1_1FlowBuilder.html#a35e180eb63de6c9f28e43185e837a4fa" class="m-doc">tf::<wbr />Taskflow::<wbr />sort</a> is not stable. That is, two or more objects with equal keys may not appear in the same order before sorting.</p></aside></section><section id="ParallelSortEnableStatefulDataPassing"><h2><a href="#ParallelSortEnableStatefulDataPassing">Enable Stateful Data Passing</a></h2><p>The iterators taken by <a href="classtf_1_1FlowBuilder.html#a35e180eb63de6c9f28e43185e837a4fa" class="m-doc">tf::<wbr />Taskflow::<wbr />sort</a> are templated. You can use <a href="http://en.cppreference.com/w/cpp/utility/functional/reference_wrapper.html" class="m-doc-external">std::<wbr />reference_wrapper</a> to enable stateful data passing between the sort task and others. The following example creates a task <code>init</code> to initialize the data vector and a task <code>sort</code> to sort the data in parallel after <code>init</code> finishes.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span>
<span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="w"> </span><span class="n">data</span><span class="p">;</span>
<span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;::</span><span class="n">iterator</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">last</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Task</span><span class="w"> </span><span class="n">init</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([</span><span class="o">&amp;</span><span class="p">](){</span><span class="w"> </span>
<span class="w"> </span><span class="n">data</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="mi">9</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">11</span><span class="p">,</span><span class="w"> </span><span class="mi">-8</span><span class="p">};</span><span class="w"> </span>
<span class="w"> </span><span class="n">first</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">data</span><span class="p">.</span><span class="n">begin</span><span class="p">();</span>
<span class="w"> </span><span class="n">last</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">data</span><span class="p">.</span><span class="n">end</span><span class="p">();</span>
<span class="p">});</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Task</span><span class="w"> </span><span class="n">sort</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">sort</span><span class="p">(</span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">ref</span><span class="p">(</span><span class="n">first</span><span class="p">),</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">ref</span><span class="p">(</span><span class="n">last</span><span class="p">),</span><span class="w"> </span><span class="p">[]</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">l</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">r</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">l</span><span class="w"> </span><span class="o">&lt;</span><span class="w"> </span><span class="n">r</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="p">);</span>
<span class="n">init</span><span class="p">.</span><span class="n">precede</span><span class="p">(</span><span class="n">sort</span><span class="p">);</span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">).</span><span class="n">wait</span><span class="p">();</span>
<span class="n">assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_sorted</span><span class="p">(</span><span class="n">data</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span><span class="w"> </span><span class="n">data</span><span class="p">.</span><span class="n">end</span><span class="p">()));</span></pre></section>
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