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<div><div class="header">
<div class="headertitle"><div class="title">Parallel Sort</div></div>
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<div class="toc"><h3>Table of Contents</h3>
<ul>
<li class="level1">
<a href="#ParallelSortInclude">Include the Header</a>
</li>
<li class="level1">
<a href="#ParallelSortCreate">Sort a Range of Items</a>
<ul>
<li class="level2">
<a href="#ParallelSortCaptureIteratorsByReference">Capture Iterators by Reference</a>
</li>
</ul>
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<li class="level1">
<a href="#ParallelSortCustomComparator">Sort with a Custom Comparator</a>
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<div class="textblock"><p>Taskflow provides template functions for constructing tasks to sort ranges of items in parallel.</p>
<h1><a class="anchor" id="ParallelSortInclude"></a>
Include the Header</h1>
<p>You need to include the header file, <code>taskflow/algorithm/sort.hpp</code>, for creating a parallel-sort task.</p>
<div class="fragment"><div class="line"><span class="preprocessor">#include <taskflow/algorithm/sort.hpp></span></div>
</div><!-- fragment --><h1><a class="anchor" id="ParallelSortCreate"></a>
Sort a Range of Items</h1>
<p>The task created by <a class="el" href="classtf_1_1FlowBuilder.html#a7d844e9856c7c65b26ccdb83ffdab1d6" title="constructs a dynamic task to perform STL-styled parallel sort using the std::less<T> comparator,...">tf::Taskflow::sort(B first, E last)</a> sorts the range <code>[first, last)</code> in ascending order using a parallel divide-and-conquer algorithm built on top of introductory sort (introsort). Taskflow recursively splits the range into sub-ranges, sorts each sub-range in parallel across available workers, and merges the sorted sub-ranges back into a fully sorted result. The following diagram illustrates this process on eight elements:</p>
<div class="dotgraph">
<iframe scrolling="no" frameborder="0" src="dot_parallel_sort.svg" width="688" height="500"><p><b>This browser is not able to show SVG: try Firefox, Chrome, Safari, or Opera instead.</b></p></iframe></div>
<p>The dashed edges show the recursive splitting phase; the green edges show per-subrange sequential sort at the base case; the orange edges show the parallel merge phase that reconstructs the fully sorted sequence. To correctly use <a class="el" href="classtf_1_1FlowBuilder.html#a35e180eb63de6c9f28e43185e837a4fa" title="constructs a dynamic task to perform STL-styled parallel sort">tf::Taskflow::sort</a>, the given iterators must be <em>random-accessible</em>. The following example creates a task that sorts a vector in ascending order:</p>
<div class="fragment"><div class="line"><a class="code hl_class" href="classtf_1_1Taskflow.html">tf::Taskflow</a> taskflow;</div>
<div class="line"><a class="code hl_class" href="classtf_1_1Executor.html">tf::Executor</a> executor;</div>
<div class="line"> </div>
<div class="line">std::vector<int> data = {1, 4, 9, 2, 3, 11, -8};</div>
<div class="line"> </div>
<div class="line"><a class="code hl_class" href="classtf_1_1Task.html">tf::Task</a> sort = taskflow.<a class="code hl_function" href="classtf_1_1FlowBuilder.html#a35e180eb63de6c9f28e43185e837a4fa">sort</a>(data.begin(), data.end());</div>
<div class="line"> </div>
<div class="line">executor.<a class="code hl_function" href="classtf_1_1Executor.html#a519777f5783981d534e9e53b99712069">run</a>(taskflow).wait();</div>
<div class="line"> </div>
<div class="line">assert(std::is_sorted(data.begin(), data.end()));</div>
<div class="ttc" id="aclasstf_1_1Executor_html"><div class="ttname"><a href="classtf_1_1Executor.html">tf::Executor</a></div><div class="ttdoc">class to create an executor</div><div class="ttdef"><b>Definition</b> executor.hpp:62</div></div>
<div class="ttc" id="aclasstf_1_1Executor_html_a519777f5783981d534e9e53b99712069"><div class="ttname"><a href="classtf_1_1Executor.html#a519777f5783981d534e9e53b99712069">tf::Executor::run</a></div><div class="ttdeci">tf::Future< void > run(Taskflow &taskflow)</div><div class="ttdoc">runs a taskflow once</div></div>
<div class="ttc" id="aclasstf_1_1FlowBuilder_html_a35e180eb63de6c9f28e43185e837a4fa"><div class="ttname"><a href="classtf_1_1FlowBuilder.html#a35e180eb63de6c9f28e43185e837a4fa">tf::FlowBuilder::sort</a></div><div class="ttdeci">Task sort(B first, E last, C cmp)</div><div class="ttdoc">constructs a dynamic task to perform STL-styled parallel sort</div></div>
<div class="ttc" id="aclasstf_1_1Task_html"><div class="ttname"><a href="classtf_1_1Task.html">tf::Task</a></div><div class="ttdoc">class to create a task handle over a taskflow node</div><div class="ttdef"><b>Definition</b> task.hpp:569</div></div>
<div class="ttc" id="aclasstf_1_1Taskflow_html"><div class="ttname"><a href="classtf_1_1Taskflow.html">tf::Taskflow</a></div><div class="ttdoc">class to create a taskflow object</div><div class="ttdef"><b>Definition</b> taskflow.hpp:64</div></div>
</div><!-- fragment --><p>By default, elements are compared using the operator "<" during the sorting process.</p>
<dl class="section note"><dt>Note</dt><dd><a class="el" href="classtf_1_1FlowBuilder.html#a35e180eb63de6c9f28e43185e837a4fa" title="constructs a dynamic task to perform STL-styled parallel sort">tf::Taskflow::sort</a> is <em>not</em> a stable sorter. Elements that compare equal may appear in any order in the sorted output.</dd></dl>
<h2><a class="anchor" id="ParallelSortCaptureIteratorsByReference"></a>
Capture Iterators by Reference</h2>
<p>You can pass iterators by reference using <a href="https://en.cppreference.com/w/cpp/utility/functional/ref">std::ref</a> to marshal parameter updates between dependent tasks. This is useful when the range is not known at task-graph construction time but is initialized by an upstream task.</p>
<div class="fragment"><div class="line"><a class="code hl_class" href="classtf_1_1Taskflow.html">tf::Taskflow</a> taskflow;</div>
<div class="line"><a class="code hl_class" href="classtf_1_1Executor.html">tf::Executor</a> executor;</div>
<div class="line"> </div>
<div class="line">std::vector<int> data;</div>
<div class="line">std::vector<int>::iterator first, last;</div>
<div class="line"> </div>
<div class="line"><a class="code hl_class" href="classtf_1_1Task.html">tf::Task</a> init = taskflow.<a class="code hl_function" href="classtf_1_1FlowBuilder.html#a4d52a7fe2814b264846a2085e931652c">emplace</a>([&]() {</div>
<div class="line"> data = {1, 4, 9, 2, 3, 11, -8};</div>
<div class="line"> first = data.begin();</div>
<div class="line"> last = data.end();</div>
<div class="line">});</div>
<div class="line"> </div>
<div class="line"><a class="code hl_class" href="classtf_1_1Task.html">tf::Task</a> sort = taskflow.<a class="code hl_function" href="classtf_1_1FlowBuilder.html#a35e180eb63de6c9f28e43185e837a4fa">sort</a>(std::ref(first), std::ref(last));</div>
<div class="line"> </div>
<div class="line"><span class="comment">// wrong! first and last are captured by copy at construction time</span></div>
<div class="line"><span class="comment">// tf::Task sort = taskflow.sort(first, last);</span></div>
<div class="line"> </div>
<div class="line">init.<a class="code hl_function" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(sort);</div>
<div class="line"> </div>
<div class="line">executor.<a class="code hl_function" href="classtf_1_1Executor.html#a519777f5783981d534e9e53b99712069">run</a>(taskflow).wait();</div>
<div class="line"> </div>
<div class="line">assert(std::is_sorted(data.begin(), data.end()));</div>
<div class="ttc" id="aclasstf_1_1FlowBuilder_html_a4d52a7fe2814b264846a2085e931652c"><div class="ttname"><a href="classtf_1_1FlowBuilder.html#a4d52a7fe2814b264846a2085e931652c">tf::FlowBuilder::emplace</a></div><div class="ttdeci">Task emplace(C &&callable)</div><div class="ttdoc">creates a static task</div><div class="ttdef"><b>Definition</b> flow_builder.hpp:1781</div></div>
<div class="ttc" id="aclasstf_1_1Task_html_a8c78c453295a553c1c016e4062da8588"><div class="ttname"><a href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">tf::Task::precede</a></div><div class="ttdeci">Task & precede(Ts &&... tasks)</div><div class="ttdoc">adds precedence links from this to other tasks</div><div class="ttdef"><b>Definition</b> task.hpp:1305</div></div>
</div><!-- fragment --><p>When <code>init</code> finishes, <code>first</code> and <code>last</code> point to the initialized data range of <code>data</code>, and the sort task performs parallel sort over all elements.</p>
<h1><a class="anchor" id="ParallelSortCustomComparator"></a>
Sort with a Custom Comparator</h1>
<p><a class="el" href="classtf_1_1FlowBuilder.html#a35e180eb63de6c9f28e43185e837a4fa" title="constructs a dynamic task to perform STL-styled parallel sort">tf::Taskflow::sort(B first, E last, C cmp)</a> is an overload of parallel sort that accepts a custom comparator <code>cmp</code>. The following example sorts a vector of integers in <em>descending</em> order:</p>
<div class="fragment"><div class="line"><a class="code hl_class" href="classtf_1_1Taskflow.html">tf::Taskflow</a> taskflow;</div>
<div class="line"><a class="code hl_class" href="classtf_1_1Executor.html">tf::Executor</a> executor;</div>
<div class="line"> </div>
<div class="line">std::vector<int> data = {1, 4, 9, 2, 3, 11, -8};</div>
<div class="line"> </div>
<div class="line"><a class="code hl_class" href="classtf_1_1Task.html">tf::Task</a> sort = taskflow.<a class="code hl_function" href="classtf_1_1FlowBuilder.html#a35e180eb63de6c9f28e43185e837a4fa">sort</a>(data.begin(), data.end(),</div>
<div class="line"> [](<span class="keywordtype">int</span> a, <span class="keywordtype">int</span> b) { return a > b; }</div>
<div class="line">);</div>
<div class="line"> </div>
<div class="line">executor.<a class="code hl_function" href="classtf_1_1Executor.html#a519777f5783981d534e9e53b99712069">run</a>(taskflow).wait();</div>
<div class="line"> </div>
<div class="line">assert(std::is_sorted(data.begin(), data.end(), std::greater<int>{}));</div>
</div><!-- fragment --><p>A custom comparator is especially useful when sorting a range of objects whose natural ordering is not defined by the operator "<". For example, user-defined structs or classes that must be ordered by a specific field. The following example defines a <code>Student</code> struct and sorts a vector of students by GPA in descending order:</p>
<div class="fragment"><div class="line"><span class="keyword">struct </span>Student {</div>
<div class="line"> std::string name;</div>
<div class="line"> <span class="keywordtype">double</span> gpa;</div>
<div class="line">};</div>
<div class="line"> </div>
<div class="line"><a class="code hl_class" href="classtf_1_1Taskflow.html">tf::Taskflow</a> taskflow;</div>
<div class="line"><a class="code hl_class" href="classtf_1_1Executor.html">tf::Executor</a> executor;</div>
<div class="line"> </div>
<div class="line">std::vector<Student> students = {</div>
<div class="line"> {<span class="stringliteral">"Alice"</span>, 3.8},</div>
<div class="line"> {<span class="stringliteral">"Bob"</span>, 3.5},</div>
<div class="line"> {<span class="stringliteral">"Charlie"</span>, 3.9},</div>
<div class="line"> {<span class="stringliteral">"Diana"</span>, 3.7}</div>
<div class="line">};</div>
<div class="line"> </div>
<div class="line">taskflow.<a class="code hl_function" href="classtf_1_1FlowBuilder.html#a35e180eb63de6c9f28e43185e837a4fa">sort</a>(students.begin(), students.end(),</div>
<div class="line"> [](<span class="keyword">const</span> Student& a, <span class="keyword">const</span> Student& b) {</div>
<div class="line"> return a.gpa > b.gpa; <span class="comment">// descending GPA</span></div>
<div class="line"> }</div>
<div class="line">);</div>
<div class="line"> </div>
<div class="line">executor.<a class="code hl_function" href="classtf_1_1Executor.html#a519777f5783981d534e9e53b99712069">run</a>(taskflow).wait();</div>
<div class="line"> </div>
<div class="line"><span class="comment">// students are now ordered by descending GPA: Charlie, Alice, Diana, Bob</span></div>
<div class="line">assert(students[0].name == <span class="stringliteral">"Charlie"</span>);</div>
<div class="line">assert(students[1].name == <span class="stringliteral">"Alice"</span>);</div>
<div class="line">assert(std::is_sorted(students.begin(), students.end(),</div>
<div class="line"> [](<span class="keyword">const</span> Student& a, <span class="keyword">const</span> Student& b) { return a.gpa > b.gpa; }</div>
<div class="line">));</div>
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