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<doxygen xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="compound.xsd" version="1.9.1" xml:lang="en-US">
  <compounddef id="ParallelIterations" kind="page">
    <compoundname>ParallelIterations</compoundname>
    <title>Parallel Iterations</title>
    <tableofcontents>
      <tocsect>
        <name>Include the Header</name>
        <reference>ParallelIterations_1ParallelIterationsIncludeTheHeader</reference>
    </tocsect>
      <tocsect>
        <name>Create an Index-based Parallel-Iteration Task</name>
        <reference>ParallelIterations_1A1IndexBasedParallelFor</reference>
    </tocsect>
      <tocsect>
        <name>Capture Indices by Reference</name>
        <reference>ParallelIterations_1ParallelForEachCaptureIndicesByReference</reference>
    </tocsect>
      <tocsect>
        <name>Create an Iterator-based Parallel-Iteration Task</name>
        <reference>ParallelIterations_1A1IteratorBasedParallelFor</reference>
    </tocsect>
      <tocsect>
        <name>Capture Iterators by Reference</name>
        <reference>ParallelIterations_1ParallelForEachCaptureIteratorsByReference</reference>
    </tocsect>
      <tocsect>
        <name>Configure a Partitioner</name>
        <reference>ParallelIterations_1ParallelIterationsConfigureAPartitioner</reference>
    </tocsect>
    </tableofcontents>
    <briefdescription>
    </briefdescription>
    <detaileddescription>
<para>Taskflow provides template functions for constructing tasks to perform parallel iterations over ranges of items.</para>
<sect1 id="ParallelIterations_1ParallelIterationsIncludeTheHeader">
<title>Include the Header</title>
<para>You need to include the header file, <computeroutput>taskflow/algorithm/for_each.hpp</computeroutput>, for using parallel-iteration algorithms.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="preprocessor">#include<sp/>&lt;taskflow/algorithm/for_each.hpp&gt;</highlight></codeline>
</programlisting></para>
</sect1>
<sect1 id="ParallelIterations_1A1IndexBasedParallelFor">
<title>Create an Index-based Parallel-Iteration Task</title>
<para>Index-based parallel-for performs parallel iterations over a range <computeroutput>[first, last)</computeroutput> with the given <computeroutput>step</computeroutput> size. The task created by tf::Taskflow::for_each_index(B first, E last, S step, C callable, P&amp;&amp; part) represents parallel execution of the following loop:</para>
<para><programlisting filename=".cpp"><codeline><highlight class="comment">//<sp/>positive<sp/>step</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="keywordflow">for</highlight><highlight class="normal">(</highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>i=first;<sp/>i&lt;last;<sp/>i+=step)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>callable(i);</highlight></codeline>
<codeline><highlight class="normal">}</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>negative<sp/>step</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="keywordflow">for</highlight><highlight class="normal">(</highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>i=first;<sp/>i&gt;last;<sp/>i+=step)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>callable(i);</highlight></codeline>
<codeline><highlight class="normal">}</highlight></codeline>
</programlisting></para>
<para>We support only integer-based range. The range can go positive or negative direction.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal">taskflow.for_each_index(0,<sp/>100,<sp/><sp/>2,<sp/>[](</highlight><highlight class="keywordtype">int</highlight><highlight class="normal"><sp/>i)<sp/>{<sp/>});<sp/><sp/></highlight><highlight class="comment">//<sp/>50<sp/>loops<sp/>with<sp/>a<sp/>+<sp/>step</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">taskflow.for_each_index(100,<sp/>0,<sp/>-2,<sp/>[](</highlight><highlight class="keywordtype">int</highlight><highlight class="normal"><sp/>i)<sp/>{<sp/>});<sp/><sp/></highlight><highlight class="comment">//<sp/>50<sp/>loops<sp/>with<sp/>a<sp/>-<sp/>step</highlight></codeline>
</programlisting></para>
<para>Notice that either positive or negative direction is defined in terms of the range, <computeroutput>[first, last)</computeroutput>, where <computeroutput>end</computeroutput> is excluded. In the positive case, the 50 items are 0, 2, 4, 6, 8, ..., 96, 98. In the negative case, the 50 items are 100, 98, 96, 04, ... 4, 2. An example of the Taskflow graph for the positive case under 12 workers is depicted below:</para>
<para><dotfile name="/home/thuang295/Code/taskflow/doxygen/images/parallel_for_1.dot"></dotfile>
</para>
</sect1>
<sect1 id="ParallelIterations_1ParallelForEachCaptureIndicesByReference">
<title>Capture Indices by Reference</title>
<para>You can pass indices by reference using <ulink url="https://en.cppreference.com/w/cpp/utility/functional/ref">std::ref</ulink> to marshal parameter update between dependent tasks. This is especially useful when the range indices are unknown at the time of creating a for-each-index task, but is initialized from another task.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="keywordtype">int</highlight><highlight class="normal">*<sp/>vec;</highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="keywordtype">int</highlight><highlight class="normal"><sp/>first,<sp/>last;</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>init<sp/>=<sp/>taskflow.emplace([&amp;](){</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>first<sp/>=<sp/>0;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>last<sp/><sp/>=<sp/>1000;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>vec<sp/>=<sp/></highlight><highlight class="keyword">new</highlight><highlight class="normal"><sp/></highlight><highlight class="keywordtype">int</highlight><highlight class="normal">[1000];<sp/><sp/></highlight></codeline>
<codeline><highlight class="normal">});</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>pf<sp/>=<sp/>taskflow.for_each_index(std::ref(first),<sp/>std::ref(last),<sp/>1,<sp/></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>[&amp;]<sp/>(</highlight><highlight class="keywordtype">int</highlight><highlight class="normal"><sp/>i)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/home/thuang295/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;parallel<sp/>iteration<sp/>on<sp/>index<sp/>&quot;</highlight><highlight class="normal"><sp/>&lt;&lt;<sp/>vec[i]<sp/>&lt;&lt;<sp/></highlight><highlight class="charliteral">&apos;\n&apos;</highlight><highlight class="normal">;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>wrong!<sp/>must<sp/>use<sp/>std::ref,<sp/>or<sp/>first<sp/>and<sp/>last<sp/>are<sp/>captured<sp/>by<sp/>copy</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>auto<sp/>pf<sp/>=<sp/>taskflow.for_each_index(first,<sp/>last,<sp/>1,<sp/>[&amp;](int<sp/>i)<sp/>{</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/><sp/><sp/>std::cout<sp/>&lt;&lt;<sp/>&quot;parallel<sp/>iteration<sp/>on<sp/>index<sp/>&quot;<sp/>&lt;&lt;<sp/>vec[i]<sp/>&lt;&lt;<sp/>&apos;\n&apos;;</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>});</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">init.precede(pf);</highlight></codeline>
</programlisting></para>
<para>When <computeroutput>init</computeroutput> finishes, the parallel-for task <computeroutput>pf</computeroutput> will see <computeroutput>first</computeroutput> as 0 and <computeroutput>last</computeroutput> as 1000 and performs parallel iterations over the 1000 items.</para>
</sect1>
<sect1 id="ParallelIterations_1A1IteratorBasedParallelFor">
<title>Create an Iterator-based Parallel-Iteration Task</title>
<para>Iterator-based parallel-for performs parallel iterations over a range specified by two <ulink url="https://en.cppreference.com/w/cpp/iterator/iterator">STL-styled iterators</ulink>, <computeroutput>first</computeroutput> and <computeroutput>last</computeroutput>. The task created by tf::Taskflow::for_each(B first, E last, C callable, P&amp;&amp; part) represents a parallel execution of the following loop:</para>
<para><programlisting filename=".cpp"><codeline><highlight class="keywordflow">for</highlight><highlight class="normal">(</highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>i=first;<sp/>i&lt;last;<sp/>i++)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>callable(*i);</highlight></codeline>
<codeline><highlight class="normal">}</highlight></codeline>
</programlisting></para>
<para>tf::Taskflow::for_each(B first, E last, C callable, P&amp;&amp; part) simultaneously applies the callable to the object obtained by dereferencing every iterator in the range <computeroutput>[first, last)</computeroutput>. It is user&apos;s responsibility for ensuring the range is valid within the execution of the parallel-for task. Iterators must have the post-increment operator ++ defined.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal"><ref refid="cpp/container/vector" kindref="compound" external="/home/thuang295/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::vector&lt;int&gt;</ref><sp/>vec<sp/>=<sp/>{1,<sp/>2,<sp/>3,<sp/>4,<sp/>5};</highlight></codeline>
<codeline><highlight class="normal">taskflow.for_each(vec.begin(),<sp/>vec.end(),<sp/>[](</highlight><highlight class="keywordtype">int</highlight><highlight class="normal"><sp/>i){<sp/></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>std::cout<sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;parallel<sp/>for<sp/>on<sp/>item<sp/>&quot;</highlight><highlight class="normal"><sp/>&lt;&lt;<sp/>i<sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&apos;\n&apos;</highlight><highlight class="normal">;<sp/><sp/></highlight></codeline>
<codeline><highlight class="normal">});</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><ref refid="cpp/container/list" kindref="compound" external="/home/thuang295/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::list&lt;std::string&gt;</ref><sp/>list<sp/>=<sp/>{</highlight><highlight class="stringliteral">&quot;hi&quot;</highlight><highlight class="normal">,<sp/></highlight><highlight class="stringliteral">&quot;from&quot;</highlight><highlight class="normal">,<sp/></highlight><highlight class="stringliteral">&quot;t&quot;</highlight><highlight class="normal">,<sp/></highlight><highlight class="stringliteral">&quot;a&quot;</highlight><highlight class="normal">,<sp/></highlight><highlight class="stringliteral">&quot;s&quot;</highlight><highlight class="normal">,<sp/></highlight><highlight class="stringliteral">&quot;k&quot;</highlight><highlight class="normal">,<sp/></highlight><highlight class="stringliteral">&quot;f&quot;</highlight><highlight class="normal">,<sp/></highlight><highlight class="stringliteral">&quot;low&quot;</highlight><highlight class="normal">};</highlight></codeline>
<codeline><highlight class="normal">taskflow.for_each(list.begin(),<sp/>list.end(),<sp/>[](</highlight><highlight class="keyword">const</highlight><highlight class="normal"><sp/><ref refid="cpp/string/basic_string" kindref="compound" external="/home/thuang295/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::string</ref>&amp;<sp/>str){<sp/></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>std::cout<sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;parallel<sp/>for<sp/>on<sp/>item<sp/>&quot;</highlight><highlight class="normal"><sp/>&lt;&lt;<sp/>str<sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&apos;\n&apos;</highlight><highlight class="normal">;<sp/><sp/></highlight></codeline>
<codeline><highlight class="normal">});</highlight></codeline>
</programlisting></para>
</sect1>
<sect1 id="ParallelIterations_1ParallelForEachCaptureIteratorsByReference">
<title>Capture Iterators by Reference</title>
<para>Similar to <ref refid="classtf_1_1FlowBuilder_1a3b132bd902331a11b04b4ad66cf8bf77" kindref="member">tf::Taskflow::for_each_index</ref>, iterators of <ref refid="classtf_1_1FlowBuilder_1aae3edfa278baa75b08414e083c14c836" kindref="member">tf::Taskflow::for_each</ref> are templated to allow capturing range parameters by reference, such that one task can set up the range before another task performs the parallel-for algorithm. For example:</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal"><ref refid="cpp/container/vector" kindref="compound" external="/home/thuang295/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::vector&lt;int&gt;</ref><sp/>vec;</highlight></codeline>
<codeline><highlight class="normal"><ref refid="cpp/container/vector" kindref="compound" external="/home/thuang295/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::vector&lt;int&gt;::iterator</ref><sp/>first,<sp/>last;;</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>init<sp/>=<sp/>taskflow.emplace([&amp;](){</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>vec.resize(1000);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>first<sp/>=<sp/>vec.begin();</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>last<sp/><sp/>=<sp/>vec.end();</highlight></codeline>
<codeline><highlight class="normal">});</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>pf<sp/>=<sp/>taskflow.for_each(std::ref(first),<sp/>std::ref(last),<sp/>[&amp;](</highlight><highlight class="keywordtype">int</highlight><highlight class="normal"><sp/>i)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/home/thuang295/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;parallel<sp/>iteration<sp/>on<sp/>item<sp/>&quot;</highlight><highlight class="normal"><sp/>&lt;&lt;<sp/>i<sp/>&lt;&lt;<sp/></highlight><highlight class="charliteral">&apos;\n&apos;</highlight><highlight class="normal">;</highlight></codeline>
<codeline><highlight class="normal">});</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>wrong!<sp/>must<sp/>use<sp/>std::ref,<sp/>or<sp/>first<sp/>and<sp/>last<sp/>are<sp/>captured<sp/>by<sp/>copy</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>tf::Task<sp/>pf<sp/>=<sp/>taskflow.for_each(first,<sp/>last,<sp/>[&amp;](int<sp/>i)<sp/>{</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/><sp/><sp/>std::cout<sp/>&lt;&lt;<sp/>&quot;parallel<sp/>iteration<sp/>on<sp/>item<sp/>&quot;<sp/>&lt;&lt;<sp/>i<sp/>&lt;&lt;<sp/>&apos;\n&apos;;</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>});</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">init.<ref refid="classtf_1_1Task_1a8c78c453295a553c1c016e4062da8588" kindref="member">precede</ref>(pf);</highlight></codeline>
</programlisting></para>
<para>When <computeroutput>init</computeroutput> finishes, the parallel-for task <computeroutput>pf</computeroutput> will see <computeroutput>first</computeroutput> pointing to the beginning of <computeroutput>vec</computeroutput> and <computeroutput>last</computeroutput> pointing to the end of <computeroutput>vec</computeroutput> and performs parallel iterations over the 1000 items. The two tasks form an end-to-end task graph where the parameters of parallel-for are computed on the fly.</para>
</sect1>
<sect1 id="ParallelIterations_1ParallelIterationsConfigureAPartitioner">
<title>Configure a Partitioner</title>
<para>You can configure a partitioner for parallel-iteration tasks to run with different scheduling methods, such as guided partitioning, dynamic partitioning, and static partitioning. The following example creates two parallel-iteration tasks using two different partitioners, one with the static partitioning algorithm and another one with the guided partitioning algorithm:</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal"><ref refid="cpp/container/vector" kindref="compound" external="/home/thuang295/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::vector&lt;int&gt;</ref><sp/>vec(1024,<sp/>0);</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">tf::ExecutionPolicy&lt;tf::StaticPartitioner&gt;<sp/>static_partitioner;</highlight></codeline>
<codeline><highlight class="normal">tf::ExecutionPolicy&lt;tf::GuidedPartitioner&gt;<sp/>guided_partitioner;</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>create<sp/>a<sp/>parallel-iteration<sp/>task<sp/>with<sp/>static<sp/>partitioner</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">taskflow.for_each(</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>vec.begin(),<sp/>vec.end(),<sp/>[&amp;](</highlight><highlight class="keywordtype">int</highlight><highlight class="normal"><sp/>i)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>std::cout<sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;parallel<sp/>iteration<sp/>on<sp/>item<sp/>&quot;</highlight><highlight class="normal"><sp/>&lt;&lt;<sp/>i<sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&apos;\n&apos;</highlight><highlight class="normal">;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>},</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>static_partitioner</highlight></codeline>
<codeline><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>create<sp/>a<sp/>parallel-iteration<sp/>task<sp/>with<sp/>guided<sp/>partitioner</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">taskflow.for_each(</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>vec.begin(),<sp/>vec.end(),<sp/>[&amp;](</highlight><highlight class="keywordtype">int</highlight><highlight class="normal"><sp/>i)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>std::cout<sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;parallel<sp/>iteration<sp/>on<sp/>item<sp/>&quot;</highlight><highlight class="normal"><sp/>&lt;&lt;<sp/>i<sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&apos;\n&apos;</highlight><highlight class="normal">;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>},</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>guided_partitioner</highlight></codeline>
<codeline><highlight class="normal">);</highlight></codeline>
</programlisting></para>
<para><simplesect kind="attention"><para>By default, parallel-iteration tasks use <ref refid="namespacetf_1a66b72776c788898aee9e132b0ea9b405" kindref="member">tf::DefaultPartitioner</ref> if no partitioner is specified. </para>
</simplesect>
</para>
</sect1>
    </detaileddescription>
    <location file="doxygen/algorithms/for_each.dox"/>
  </compounddef>
</doxygen>
