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<title>Taskflow Algorithms » Parallel Iterations | Taskflow QuickStart</title>
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<h1>
<span class="m-breadcrumb"><a href="Algorithms.html">Taskflow Algorithms</a> »</span>
Parallel Iterations
</h1>
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<h3>Contents</h3>
<ul>
<li><a href="#A1IndexBasedParallelFor">Create an Index-based Parallel-Iteration Task</a></li>
<li><a href="#A1IteratorBasedParallelFor">Create an Iterator-based Parallel-Iteration Task</a></li>
</ul>
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<p>Taskflow provides template functions for constructing tasks to perform parallel iterations over ranges of items.</p><section id="A1IndexBasedParallelFor"><h2><a href="#A1IndexBasedParallelFor">Create an Index-based Parallel-Iteration Task</a></h2><p>Index-based parallel-for performs parallel iterations over a range <code>[first, last)</code> with the given <code>step</code> size. The task created by <a href="classtf_1_1FlowBuilder.html#a38ffe176bc3ae1827b9964322e3769d4" class="m-doc">tf::<wbr />Taskflow::<wbr />for_each_index(B first, E last, S step, C callable)</a> represents parallel execution of the following loop:</p><pre class="m-code"><span class="c1">// positive step</span>
<span class="k">for</span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">i</span><span class="o">=</span><span class="n">first</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o"><</span><span class="n">last</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">+=</span><span class="n">step</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">callable</span><span class="p">(</span><span class="n">i</span><span class="p">);</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
<span class="c1">// negative step</span>
<span class="k">for</span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">i</span><span class="o">=</span><span class="n">first</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">></span><span class="n">last</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">+=</span><span class="n">step</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">callable</span><span class="p">(</span><span class="n">i</span><span class="p">);</span><span class="w"></span>
<span class="p">}</span><span class="w"></span></pre><p>We support only integer-based range. The range can go positive or negative direction.</p><pre class="m-code"><span class="n">taskflow</span><span class="p">.</span><span class="n">for_each_index</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="mi">100</span><span class="p">,</span><span class="w"> </span><span class="mi">2</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">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="p">});</span><span class="w"> </span><span class="c1">// 50 loops with a + step</span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">for_each_index</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="mi">-2</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">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="p">});</span><span class="w"> </span><span class="c1">// 50 loops with a - step</span></pre><p>Notice that either positive or negative direction is defined in terms of the range, <code>[first, last)</code>, where <code>end</code> 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:</p><div class="m-graph"><svg style="width: 77.062rem; height: 7.375rem;" viewBox="0.00 0.00 1232.51 117.54">
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</div><p>The index types, <code>B</code>, <code>E</code>, and <code>S</code>, are templates to preserve the variable types and their underlying types must be of the same <em>integral</em> type (e.g., <code>int</code>, <code>size_t</code>, <code>unsigned</code>). By default, <a href="classtf_1_1FlowBuilder.html#a38ffe176bc3ae1827b9964322e3769d4" class="m-doc">tf::<wbr />Taskflow::<wbr />for_each_index</a> creates a task that spawns a subflow (see <a href="DynamicTasking.html" class="m-doc">Dynamic Tasking</a>) to run iterations in parallel. The subflow closure captures all input arguments through perfect forwarding to form a stateful closure such that any changes on the arguments will be visible to the execution context of the subflow. For example:</p><pre class="m-code"><span class="kt">int</span><span class="o">*</span><span class="w"> </span><span class="n">vec</span><span class="p">;</span><span class="w"></span>
<span class="kt">int</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="w"></span>
<span class="k">auto</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">&</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="mi">0</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="n">last</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1000</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="n">vec</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="kt">int</span><span class="p">[</span><span class="mi">1000</span><span class="p">];</span><span class="w"> </span>
<span class="p">});</span><span class="w"></span>
<span class="k">auto</span><span class="w"> </span><span class="n">pf</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">for_each_index</span><span class="p">(</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="mi">1</span><span class="p">,</span><span class="w"> </span>
<span class="w"> </span><span class="p">[</span><span class="o">&</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">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"parallel iteration on index "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">vec</span><span class="p">[</span><span class="n">i</span><span class="p">]</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">'\n'</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="p">);</span><span class="w"></span>
<span class="c1">// wrong! must use std::ref, or first and last are captured by copy</span>
<span class="c1">// auto pf = taskflow.for_each_index(first, last, 1, [&](int i) {</span>
<span class="c1">// std::cout << "parallel iteration on index " << vec[i] << '\n';</span>
<span class="c1">// });</span>
<span class="n">init</span><span class="p">.</span><span class="n">precede</span><span class="p">(</span><span class="n">pf</span><span class="p">);</span><span class="w"></span></pre><p>When <code>init</code> finishes, the parallel-for task <code>pf</code> will see <code>first</code> as 0 and <code>last</code> as 1000 and performs parallel iterations over the 1000 items. This property is especially important for task graph parallelism, because users can define end-to-end parallelism through stateful closures that marshal parameter exchange between dependent tasks.</p></section><section id="A1IteratorBasedParallelFor"><h2><a href="#A1IteratorBasedParallelFor">Create an Iterator-based Parallel-Iteration Task</a></h2><p>Iterator-based parallel-for performs parallel iterations over a range specified by two <a href="https://en.cppreference.com/w/cpp/iterator/iterator">STL-styled iterators</a>, <code>first</code> and <code>last</code>. The task created by <a href="classtf_1_1FlowBuilder.html#ab405d7b10040530d8b04c11767b4960d" class="m-doc">tf::<wbr />Taskflow::<wbr />for_each(B first, E last, C callable)</a> represents a parallel execution of the following loop:</p><pre class="m-code"><span class="k">for</span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">i</span><span class="o">=</span><span class="n">first</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o"><</span><span class="n">last</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">callable</span><span class="p">(</span><span class="o">*</span><span class="n">i</span><span class="p">);</span><span class="w"></span>
<span class="p">}</span><span class="w"></span></pre><p>By default, <a href="classtf_1_1FlowBuilder.html#ab405d7b10040530d8b04c11767b4960d" class="m-doc">tf::<wbr />Taskflow::<wbr />for_each(B first, E last, C callable)</a> creates a task that spawns a subflow (see <a href="DynamicTasking.html" class="m-doc">Dynamic Tasking</a>) that applies the callable to the object obtained by dereferencing every iterator in the range <code>[first, last)</code>. It is user's responsibility for ensuring the range is valid within the execution of the parallel-for task. Iterators must have the post-increment operator ++ defined. This version of parallel-for applies to all iterable STL containers.</p><pre class="m-code"><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">vec</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">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">4</span><span class="p">,</span><span class="w"> </span><span class="mi">5</span><span class="p">};</span><span class="w"></span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">for_each</span><span class="p">(</span><span class="n">vec</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span><span class="w"> </span><span class="n">vec</span><span class="p">.</span><span class="n">end</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">i</span><span class="p">){</span><span class="w"> </span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"parallel for on item "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">'\n'</span><span class="p">;</span><span class="w"> </span>
<span class="p">});</span><span class="w"></span>
<span class="n">std</span><span class="o">::</span><span class="n">list</span><span class="o"><</span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">></span><span class="w"> </span><span class="n">list</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="s">"hi"</span><span class="p">,</span><span class="w"> </span><span class="s">"from"</span><span class="p">,</span><span class="w"> </span><span class="s">"t"</span><span class="p">,</span><span class="w"> </span><span class="s">"a"</span><span class="p">,</span><span class="w"> </span><span class="s">"s"</span><span class="p">,</span><span class="w"> </span><span class="s">"k"</span><span class="p">,</span><span class="w"> </span><span class="s">"f"</span><span class="p">,</span><span class="w"> </span><span class="s">"low"</span><span class="p">};</span><span class="w"></span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">for_each</span><span class="p">(</span><span class="n">list</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span><span class="w"> </span><span class="n">list</span><span class="p">.</span><span class="n">end</span><span class="p">(),</span><span class="w"> </span><span class="p">[](</span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&</span><span class="w"> </span><span class="n">str</span><span class="p">){</span><span class="w"> </span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"parallel for on item "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">str</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">'\n'</span><span class="p">;</span><span class="w"> </span>
<span class="p">});</span><span class="w"></span></pre><p>Similar to index-based parallel-for, the iterator types are templates to enable users to leverage the property of stateful closure. For example:</p><pre class="m-code"><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">vec</span><span class="p">;</span><span class="w"></span>
<span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o"><</span><span class="kt">int</span><span class="o">>::</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="w"></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">&</span><span class="p">](){</span><span class="w"></span>
<span class="w"> </span><span class="n">vec</span><span class="p">.</span><span class="n">resize</span><span class="p">(</span><span class="mi">1000</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">vec</span><span class="p">.</span><span class="n">begin</span><span class="p">();</span><span class="w"></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">vec</span><span class="p">.</span><span class="n">end</span><span class="p">();</span><span class="w"></span>
<span class="p">});</span><span class="w"></span>
<span class="n">tf</span><span class="o">::</span><span class="n">Task</span><span class="w"> </span><span class="n">pf</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">for_each</span><span class="p">(</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="o">&</span><span class="p">](</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"parallel iteration on item "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">'\n'</span><span class="p">;</span><span class="w"></span>
<span class="p">});</span><span class="w"></span>
<span class="c1">// wrong! must use std::ref, or first and last are captured by copy</span>
<span class="c1">// tf::Task pf = taskflow.for_each(first, last, [&](int i) {</span>
<span class="c1">// std::cout << "parallel iteration on item " << i << '\n';</span>
<span class="c1">// });</span>
<span class="n">init</span><span class="p">.</span><span class="n">precede</span><span class="p">(</span><span class="n">pf</span><span class="p">);</span><span class="w"></span></pre><p>When <code>init</code> finishes, the parallel-for task <code>pf</code> will see <code>first</code> pointing to the beginning of <code>vec</code> and <code>last</code> pointing to the end of <code>vec</code> 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.</p></section>
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