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ParallelFind.html

ParallelFind.html 35.99 KB

<!DOCTYPE html>
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 <title>Taskflow Algorithms &raquo; Parallel Find | Taskflow QuickStart</title>
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 <h1>
 <a href="Algorithms.html">Taskflow Algorithms</a> &raquo;
 Parallel Find
 </h1>
 <nav class="m-block m-default">
 <h3>Contents</h3>
 <ul>
 <li><a href="#ParallelFindIncludeTheHeader">Include the Header</a></li>
 <li><a href="#WhatIsAFindAlgorithm">What is a Find Algorithm?</a></li>
 <li><a href="#CreateAParallelFindIfTask">Create a Parallel Find-If Task</a></li>
 <li><a href="#ParallelFindCaptureIteratorsByReference">Capture Iterators by Reference</a></li>
 <li><a href="#CreateAParallelFindIfNotTask">Create a Parallel Find-If-Not Task</a></li>
 <li><a href="#ParallelFindMinMaxElement">Find the Smallest and the Largest Elements</a></li>
 <li><a href="#ParallelFindConfigureAPartitioner">Configure a Partitioner</a></li>
 </ul>
 </nav>
Taskflow provides template functions for constructing tasks to perform parallel iterations over ranges of items.<section id="ParallelFindIncludeTheHeader"><h2><a href="#ParallelFindIncludeTheHeader">Include the Header</a></h2>You need to include the header file, <code>taskflow/algorithm/find.hpp</code>, for using parallel-find algorithms.<pre class="m-code">#include &lt;taskflow/algorithm/find.hpp&gt;</pre></section><section id="WhatIsAFindAlgorithm"><h2><a href="#WhatIsAFindAlgorithm">What is a Find Algorithm?</a></h2>A find algorithm allows you to find an element in a range <code>[first, last)</code> that satisfies a specific criteria. The algorithm returns an iterator to the first found element in the range or returns <code>last</code> if there is no such iterator. Taskflow provides the following parallel-find algorithms:<ul><li><a href="classtf_1_1FlowBuilder.html#a94e70e5f7a459a7762cc312b6d6dec79" class="m-doc">tf::Taskflow::find_if(B first, E last, T&amp; result, UOP predicate, P&amp;&amp; part)</a></li><li><a href="classtf_1_1FlowBuilder.html#aae268c9763df9323dc349fb1b35988cf" class="m-doc">tf::Taskflow::find_if_not(B first, E last, T&amp; result, UOP predicate, P&amp;&amp; part)</a></li><li><a href="classtf_1_1FlowBuilder.html#ad6a7caf3cfc8d1887bd211be307a3eb0" class="m-doc">tf::Taskflow::min_element(B first, E last, T&amp; result, C comp, P&amp;&amp; part)</a></li><li><a href="classtf_1_1FlowBuilder.html#af0415d1fa5f36fee2405b0fcbf592fe0" class="m-doc">tf::Taskflow::max_element(B first, E last, T&amp; result, C comp, P&amp;&amp; part)</a></li></ul></section><section id="CreateAParallelFindIfTask"><h2><a href="#CreateAParallelFindIfTask">Create a Parallel Find-If Task</a></h2><a href="classtf_1_1FlowBuilder.html#a94e70e5f7a459a7762cc312b6d6dec79" class="m-doc">tf::Taskflow::find_if</a> performs parallel iterations to find the first element in the range <code>[first, last)</code> that makes the given predicate return <code>true</code>. It resembles a parallel implementation of the following loop:<pre class="m-code">template&lt;typename InputIt, typename UnaryPredicate&gt;
InputIt find_if(InputIt first, InputIt last, UnaryPredicate predicate) {
 for(; first != last; ++first) {
 if(predicate(*first)) {
 return first;
 }
 }
 return last;
}</pre>The example below creates a task to find the element that is equal to 22 from an input range of 10 elements. The result will be stored in the forth argument passed by reference:<pre class="m-code">std::vector&lt;int&gt; input = {1, 9, 22, 3, -6, 13, 12, 0, 9, 11};
std::vector&lt;int&gt;::iterator result;
taskflow.find_if(
 input.begin(), input.end(), [](int i){ return i == 22; }, result
);
executor.run(taskflow);
assert(*result == 22);</pre></section><section id="ParallelFindCaptureIteratorsByReference"><h2><a href="#ParallelFindCaptureIteratorsByReference">Capture Iterators by Reference</a></h2>You can pass iterators by reference using <a href="https://en.cppreference.com/w/cpp/utility/functional/ref">std::ref</a> to marshal parameters update between dependent tasks. This is especially useful when the range iterators are not known at the time of creating a find-if task, but need initialization from another task.<pre class="m-code">std::vector&lt;int&gt; input;
std::vector&lt;int&gt;::iterator result, first, last;

// task to set up the range iterators
tf::Task init = taskflow.emplace([&amp;](){
 input = {1, 9, 22, 3, -6, 13, 12, 0, 9, 11};
 first = input.begin(),
 last = input.end();
});

// task to perform parallel find
tf::Task task = taskflow.find_if(
 std::ref(first), std::ref(last), result, [](int i){ return i == 22; }
);

init.precede(task);

executor.run(taskflow);
assert(*result == 22);</pre>In the above example, when <code>init</code> finishes, <code>input</code> has been initialized to 10 elements with <code>first</code> and <code>last</code> pointing to the data range of <code>input</code>. The find-if task will then work on this initialized range as a result of passing iterators by reference.</section><section id="CreateAParallelFindIfNotTask"><h2><a href="#CreateAParallelFindIfNotTask">Create a Parallel Find-If-Not Task</a></h2><a href="classtf_1_1FlowBuilder.html#aae268c9763df9323dc349fb1b35988cf" class="m-doc">tf::Taskflow::find_if_not</a> performs parallel iterations to find the first element in the range <code>[first, last)</code> that makes the given predicate return <code>false</code>. It resembles a parallel implementation of the following loop:<pre class="m-code">template&lt;typename InputIt, typename UnaryPredicate&gt;
InputIt find_if(InputIt first, InputIt last, UnaryPredicate predicate) {
 for(; first != last; ++first) {
 if(!predicate(*first)) {
 return first;
 }
 }
 return last;
}</pre>The example below creates a task to find the element that is NOT equal to 22 from an input range of 10 elements. The result will be stored in the forth argument passed by reference:<pre class="m-code">std::vector&lt;int&gt; input = {1, 1, 22, 1, 1, 1, 1, 1, 1, 1};
std::vector&lt;int&gt;::iterator result;
taskflow.find_if_not(
 input.begin(), input.end(), result, [](int i){ return i == 1; }
);
executor.run(taskflow);
assert(*result == 22);</pre>Similar to <a href="ParallelFind.html#ParallelFindCaptureIteratorsByReference" class="m-doc">Capture Iterators by Reference</a>, iterators of <a href="classtf_1_1FlowBuilder.html#aae268c9763df9323dc349fb1b35988cf" class="m-doc">tf::Taskflow::find_if_not</a> are templated to allow passing iterators by reference using <a href="https://en.cppreference.com/w/cpp/utility/functional/ref">std::ref</a>. This is especially useful when the range iterators are not known at the time of creating a find-if-not task, but need initialization from another task.</section><section id="ParallelFindMinMaxElement"><h2><a href="#ParallelFindMinMaxElement">Find the Smallest and the Largest Elements</a></h2><a href="classtf_1_1FlowBuilder.html#ad6a7caf3cfc8d1887bd211be307a3eb0" class="m-doc">tf::Taskflow::min_element</a> finds the smallest element in a range <code>[first, last)</code> using the given comparison function object. The example below finds the smallest element, i.e., -1, from an input range of 10 elements and stores the iterator to that smallest element in <code>result:</code><pre class="m-code">std::vector&lt;int&gt; input = {1, 1, 1, 1, 1, -1, 1, 1, 1, 1};
std::vector&lt;int&gt;::iterator result;
taskflow.min_element(
 input.begin(), input.end(), std::less&lt;int&gt;(), result
);
executor.run(taskflow).wait();
assert(*result == -1);</pre>Similarly, <a href="classtf_1_1FlowBuilder.html#af0415d1fa5f36fee2405b0fcbf592fe0" class="m-doc">tf::Taskflow::max_element</a> finds the largest element in a range <code>[first, last)</code> using the given comparison function object. The example below finds the largest element, i.e., 2, from an input range of 10 elements and stores the iterator to that largest element in <code>result:</code><pre class="m-code">std::vector&lt;int&gt; input = {1, 1, 1, 1, 1, 2, 1, 1, 1, 1};
std::vector&lt;int&gt;::iterator result;
taskflow.max_element(
 input.begin(), input.end(), std::less&lt;int&gt;(), result
);
executor.run(taskflow).wait();
assert(*result == 2);</pre><aside class="m-note m-info"><h4>Note</h4>When using <a href="classtf_1_1FlowBuilder.html#af0415d1fa5f36fee2405b0fcbf592fe0" class="m-doc">tf::Taskflow::max_element</a> to find the large element, we will still need to use <a href="http://en.cppreference.com/w/cpp/utility/functional/less.html" class="m-doc-external">std::less</a> as our comparison function. Details can be referred to <a href="https://en.cppreference.com/w/cpp/algorithm/max_element">std::max_element</a>.</aside></section><section id="ParallelFindConfigureAPartitioner"><h2><a href="#ParallelFindConfigureAPartitioner">Configure a Partitioner</a></h2>You can configure a partitioner for parallel-find tasks (<a href="classtf_1_1FlowBuilder.html#a94e70e5f7a459a7762cc312b6d6dec79" class="m-doc">tf::Taskflow::find_if</a>, <a href="classtf_1_1FlowBuilder.html#aae268c9763df9323dc349fb1b35988cf" class="m-doc">tf::Taskflow::find_if_not</a>, <a href="classtf_1_1FlowBuilder.html#ad6a7caf3cfc8d1887bd211be307a3eb0" class="m-doc">tf::Taskflow::min_element</a>, <a href="classtf_1_1FlowBuilder.html#af0415d1fa5f36fee2405b0fcbf592fe0" class="m-doc">tf::Taskflow::max_element</a>) to run with different scheduling methods, such as guided partitioning, dynamic partitioning, and static partitioning. The following example creates two parallel-find tasks using two different partitioners, one with the static partitioning algorithm and another one with the guided partitioning algorithm:<pre class="m-code">std::vector&lt;int&gt; vec(1024, -1);
std::vector&lt;int&gt;::iterator result;

tf::ExecutionPolicy&lt;tf::StaticPartitioner&gt; static_partitioner;
tf::ExecutionPolicy&lt;tf::GuidedPartitioner&gt; guided_partitioner;

// create a parallel-find task with a static partitioner
taskflow.find_if(
 vec.begin(), vec.end(), result, [&amp;](int i) { return i == -1; }, static_partitioner
);

// create a parallel-find task with a guided partitioner
taskflow.find_if(
 vec.begin(), vec.end(), result, [&amp;](int i) { return i == -1; }, guided_partitioner
);</pre><aside class="m-note m-info"><h4>Note</h4>By default, parallel-find tasks use <a href="namespacetf.html#a77d193108ae375fdfc554e7f10bea23e" class="m-doc">tf::DefaultPartitioner</a> if no partitioner is specified.</aside></section>
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一键复制编辑原始数据按行查看历史

twhuang 提交于 2023年05月10日 03:18 +08:00 . fixed old async behavior in documentation

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 Parallel Find
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 <li><a href="#ParallelFindIncludeTheHeader">Include the Header</a></li>
 <li><a href="#WhatIsAFindAlgorithm">What is a Find Algorithm?</a></li>
 <li><a href="#CreateAParallelFindIfTask">Create a Parallel Find-If Task</a></li>
 <li><a href="#ParallelFindCaptureIteratorsByReference">Capture Iterators by Reference</a></li>
 <li><a href="#CreateAParallelFindIfNotTask">Create a Parallel Find-If-Not Task</a></li>
 <li><a href="#ParallelFindMinMaxElement">Find the Smallest and the Largest Elements</a></li>
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 </ul>
 </nav>
<p>Taskflow provides template functions for constructing tasks to perform parallel iterations over ranges of items.</p><section id="ParallelFindIncludeTheHeader"><h2><a href="#ParallelFindIncludeTheHeader">Include the Header</a></h2><p>You need to include the header file, <code>taskflow/algorithm/find.hpp</code>, for using parallel-find algorithms.</p><pre class="m-code"><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;taskflow/algorithm/find.hpp&gt;</span><span class="cp"></span></pre></section><section id="WhatIsAFindAlgorithm"><h2><a href="#WhatIsAFindAlgorithm">What is a Find Algorithm?</a></h2><p>A find algorithm allows you to find an element in a range <code>[first, last)</code> that satisfies a specific criteria. The algorithm returns an iterator to the first found element in the range or returns <code>last</code> if there is no such iterator. Taskflow provides the following parallel-find algorithms:</p><ul><li><a href="classtf_1_1FlowBuilder.html#a94e70e5f7a459a7762cc312b6d6dec79" class="m-doc">tf::<wbr />Taskflow::<wbr />find_if(B first, E last, T&amp; result, UOP predicate, P&amp;&amp; part)</a></li><li><a href="classtf_1_1FlowBuilder.html#aae268c9763df9323dc349fb1b35988cf" class="m-doc">tf::<wbr />Taskflow::<wbr />find_if_not(B first, E last, T&amp; result, UOP predicate, P&amp;&amp; part)</a></li><li><a href="classtf_1_1FlowBuilder.html#ad6a7caf3cfc8d1887bd211be307a3eb0" class="m-doc">tf::<wbr />Taskflow::<wbr />min_element(B first, E last, T&amp; result, C comp, P&amp;&amp; part)</a></li><li><a href="classtf_1_1FlowBuilder.html#af0415d1fa5f36fee2405b0fcbf592fe0" class="m-doc">tf::<wbr />Taskflow::<wbr />max_element(B first, E last, T&amp; result, C comp, P&amp;&amp; part)</a></li></ul></section><section id="CreateAParallelFindIfTask"><h2><a href="#CreateAParallelFindIfTask">Create a Parallel Find-If Task</a></h2><p><a href="classtf_1_1FlowBuilder.html#a94e70e5f7a459a7762cc312b6d6dec79" class="m-doc">tf::<wbr />Taskflow::<wbr />find_if</a> performs parallel iterations to find the first element in the range <code>[first, last)</code> that makes the given predicate return <code>true</code>. It resembles a parallel implementation of the following loop:</p><pre class="m-code"><span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span><span class="w"> </span><span class="nc">InputIt</span><span class="p">,</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">UnaryPredicate</span><span class="o">&gt;</span><span class="w"></span>
<span class="n">InputIt</span><span class="w"> </span><span class="n">find_if</span><span class="p">(</span><span class="n">InputIt</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">InputIt</span><span class="w"> </span><span class="n">last</span><span class="p">,</span><span class="w"> </span><span class="n">UnaryPredicate</span><span class="w"> </span><span class="n">predicate</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">for</span><span class="p">(;</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">last</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">first</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="p">(</span><span class="n">predicate</span><span class="p">(</span><span class="o">*</span><span class="n">first</span><span class="p">))</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">first</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">last</span><span class="p">;</span><span class="w"></span>
<span class="p">}</span><span class="w"></span></pre><p>The example below creates a task to find the element that is equal to 22 from an input range of 10 elements. The result will be stored in the forth argument passed by reference:</p><pre class="m-code"><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">input</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">9</span><span class="p">,</span><span class="w"> </span><span class="mi">22</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">-6</span><span class="p">,</span><span class="w"> </span><span class="mi">13</span><span class="p">,</span><span class="w"> </span><span class="mi">12</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">9</span><span class="p">,</span><span class="w"> </span><span class="mi">11</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">&lt;</span><span class="kt">int</span><span class="o">&gt;::</span><span class="n">iterator</span><span class="w"> </span><span class="n">result</span><span class="p">;</span><span class="w"></span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">find_if</span><span class="p">(</span><span class="w"></span>
<span class="w"> </span><span class="n">input</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span><span class="w"> </span><span class="n">input</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="k">return</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">22</span><span class="p">;</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">result</span><span class="w"></span>
<span class="p">);</span><span class="w"></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="w"></span>
<span class="n">assert</span><span class="p">(</span><span class="o">*</span><span class="n">result</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">22</span><span class="p">);</span><span class="w"></span></pre></section><section id="ParallelFindCaptureIteratorsByReference"><h2><a href="#ParallelFindCaptureIteratorsByReference">Capture Iterators by Reference</a></h2><p>You can pass iterators by reference using <a href="https://en.cppreference.com/w/cpp/utility/functional/ref">std::<wbr />ref</a> to marshal parameters update between dependent tasks. This is especially useful when the range iterators are not known at the time of creating a find-if task, but need initialization from another task.</p><pre class="m-code"><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">input</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">&lt;</span><span class="kt">int</span><span class="o">&gt;::</span><span class="n">iterator</span><span class="w"> </span><span class="n">result</span><span class="p">,</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="c1">// task to set up the range iterators</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">input</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">9</span><span class="p">,</span><span class="w"> </span><span class="mi">22</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">-6</span><span class="p">,</span><span class="w"> </span><span class="mi">13</span><span class="p">,</span><span class="w"> </span><span class="mi">12</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">9</span><span class="p">,</span><span class="w"> </span><span class="mi">11</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">input</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">input</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="c1">// task to perform parallel find</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Task</span><span class="w"> </span><span class="n">task</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">find_if</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">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="n">result</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="k">return</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">22</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="n">init</span><span class="p">.</span><span class="n">precede</span><span class="p">(</span><span class="n">task</span><span class="p">);</span><span class="w"></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="w"></span>
<span class="n">assert</span><span class="p">(</span><span class="o">*</span><span class="n">result</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">22</span><span class="p">);</span><span class="w"></span></pre><p>In the above example, when <code>init</code> finishes, <code>input</code> has been initialized to 10 elements with <code>first</code> and <code>last</code> pointing to the data range of <code>input</code>. The find-if task will then work on this initialized range as a result of passing iterators by reference.</p></section><section id="CreateAParallelFindIfNotTask"><h2><a href="#CreateAParallelFindIfNotTask">Create a Parallel Find-If-Not Task</a></h2><p><a href="classtf_1_1FlowBuilder.html#aae268c9763df9323dc349fb1b35988cf" class="m-doc">tf::<wbr />Taskflow::<wbr />find_if_not</a> performs parallel iterations to find the first element in the range <code>[first, last)</code> that makes the given predicate return <code>false</code>. It resembles a parallel implementation of the following loop:</p><pre class="m-code"><span class="k">template</span><span class="o">&lt;</span><span class="k">typename</span><span class="w"> </span><span class="nc">InputIt</span><span class="p">,</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">UnaryPredicate</span><span class="o">&gt;</span><span class="w"></span>
<span class="n">InputIt</span><span class="w"> </span><span class="n">find_if</span><span class="p">(</span><span class="n">InputIt</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">InputIt</span><span class="w"> </span><span class="n">last</span><span class="p">,</span><span class="w"> </span><span class="n">UnaryPredicate</span><span class="w"> </span><span class="n">predicate</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">for</span><span class="p">(;</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">last</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">first</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="p">(</span><span class="o">!</span><span class="n">predicate</span><span class="p">(</span><span class="o">*</span><span class="n">first</span><span class="p">))</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">first</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">last</span><span class="p">;</span><span class="w"></span>
<span class="p">}</span><span class="w"></span></pre><p>The example below creates a task to find the element that is <em>NOT</em> equal to 22 from an input range of 10 elements. The result will be stored in the forth argument passed by reference:</p><pre class="m-code"><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">input</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">1</span><span class="p">,</span><span class="w"> </span><span class="mi">22</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</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">&lt;</span><span class="kt">int</span><span class="o">&gt;::</span><span class="n">iterator</span><span class="w"> </span><span class="n">result</span><span class="p">;</span><span class="w"></span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">find_if_not</span><span class="p">(</span><span class="w"></span>
<span class="w"> </span><span class="n">input</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span><span class="w"> </span><span class="n">input</span><span class="p">.</span><span class="n">end</span><span class="p">(),</span><span class="w"> </span><span class="n">result</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="k">return</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">1</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="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="w"></span>
<span class="n">assert</span><span class="p">(</span><span class="o">*</span><span class="n">result</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">22</span><span class="p">);</span><span class="w"></span></pre><p>Similar to <a href="ParallelFind.html#ParallelFindCaptureIteratorsByReference" class="m-doc">Capture Iterators by Reference</a>, iterators of <a href="classtf_1_1FlowBuilder.html#aae268c9763df9323dc349fb1b35988cf" class="m-doc">tf::<wbr />Taskflow::<wbr />find_if_not</a> are templated to allow passing iterators by reference using <a href="https://en.cppreference.com/w/cpp/utility/functional/ref">std::<wbr />ref</a>. This is especially useful when the range iterators are not known at the time of creating a find-if-not task, but need initialization from another task.</p></section><section id="ParallelFindMinMaxElement"><h2><a href="#ParallelFindMinMaxElement">Find the Smallest and the Largest Elements</a></h2><p><a href="classtf_1_1FlowBuilder.html#ad6a7caf3cfc8d1887bd211be307a3eb0" class="m-doc">tf::<wbr />Taskflow::<wbr />min_element</a> finds the smallest element in a range <code>[first, last)</code> using the given comparison function object. The example below finds the smallest element, i.e., -1, from an input range of 10 elements and stores the iterator to that smallest element in <code>result:</code></p><pre class="m-code"><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">input</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">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">-1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</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">&lt;</span><span class="kt">int</span><span class="o">&gt;::</span><span class="n">iterator</span><span class="w"> </span><span class="n">result</span><span class="p">;</span><span class="w"></span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">min_element</span><span class="p">(</span><span class="w"></span>
<span class="w"> </span><span class="n">input</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span><span class="w"> </span><span class="n">input</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">less</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="p">(),</span><span class="w"> </span><span class="n">result</span><span class="w"></span>
<span class="p">);</span><span class="w"></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="w"></span>
<span class="n">assert</span><span class="p">(</span><span class="o">*</span><span class="n">result</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">-1</span><span class="p">);</span><span class="w"></span></pre><p>Similarly, <a href="classtf_1_1FlowBuilder.html#af0415d1fa5f36fee2405b0fcbf592fe0" class="m-doc">tf::<wbr />Taskflow::<wbr />max_element</a> finds the largest element in a range <code>[first, last)</code> using the given comparison function object. The example below finds the largest element, i.e., 2, from an input range of 10 elements and stores the iterator to that largest element in <code>result:</code></p><pre class="m-code"><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">input</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">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </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">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">1</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">&lt;</span><span class="kt">int</span><span class="o">&gt;::</span><span class="n">iterator</span><span class="w"> </span><span class="n">result</span><span class="p">;</span><span class="w"></span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">max_element</span><span class="p">(</span><span class="w"></span>
<span class="w"> </span><span class="n">input</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span><span class="w"> </span><span class="n">input</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">less</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="p">(),</span><span class="w"> </span><span class="n">result</span><span class="w"></span>
<span class="p">);</span><span class="w"></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="w"></span>
<span class="n">assert</span><span class="p">(</span><span class="o">*</span><span class="n">result</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">2</span><span class="p">);</span><span class="w"></span></pre><aside class="m-note m-info"><h4>Note</h4><p>When using <a href="classtf_1_1FlowBuilder.html#af0415d1fa5f36fee2405b0fcbf592fe0" class="m-doc">tf::<wbr />Taskflow::<wbr />max_element</a> to find the large element, we will still need to use <a href="http://en.cppreference.com/w/cpp/utility/functional/less.html" class="m-doc-external">std::<wbr />less</a> as our comparison function. Details can be referred to <a href="https://en.cppreference.com/w/cpp/algorithm/max_element">std::<wbr />max_element</a>.</p></aside></section><section id="ParallelFindConfigureAPartitioner"><h2><a href="#ParallelFindConfigureAPartitioner">Configure a Partitioner</a></h2><p>You can configure a partitioner for parallel-find tasks (<a href="classtf_1_1FlowBuilder.html#a94e70e5f7a459a7762cc312b6d6dec79" class="m-doc">tf::<wbr />Taskflow::<wbr />find_if</a>, <a href="classtf_1_1FlowBuilder.html#aae268c9763df9323dc349fb1b35988cf" class="m-doc">tf::<wbr />Taskflow::<wbr />find_if_not</a>, <a href="classtf_1_1FlowBuilder.html#ad6a7caf3cfc8d1887bd211be307a3eb0" class="m-doc">tf::<wbr />Taskflow::<wbr />min_element</a>, <a href="classtf_1_1FlowBuilder.html#af0415d1fa5f36fee2405b0fcbf592fe0" class="m-doc">tf::<wbr />Taskflow::<wbr />max_element</a>) to run with different scheduling methods, such as guided partitioning, dynamic partitioning, and static partitioning. The following example creates two parallel-find tasks using two different partitioners, one with the static partitioning algorithm and another one with the guided partitioning algorithm:</p><pre class="m-code"><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">vec</span><span class="p">(</span><span class="mi">1024</span><span class="p">,</span><span class="w"> </span><span class="mi">-1</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">&lt;</span><span class="kt">int</span><span class="o">&gt;::</span><span class="n">iterator</span><span class="w"> </span><span class="n">result</span><span class="p">;</span><span class="w"></span>

<span class="n">tf</span><span class="o">::</span><span class="n">ExecutionPolicy</span><span class="o">&lt;</span><span class="n">tf</span><span class="o">::</span><span class="n">StaticPartitioner</span><span class="o">&gt;</span><span class="w"> </span><span class="n">static_partitioner</span><span class="p">;</span><span class="w"></span>
<span class="n">tf</span><span class="o">::</span><span class="n">ExecutionPolicy</span><span class="o">&lt;</span><span class="n">tf</span><span class="o">::</span><span class="n">GuidedPartitioner</span><span class="o">&gt;</span><span class="w"> </span><span class="n">guided_partitioner</span><span class="p">;</span><span class="w"></span>

<span class="c1">// create a parallel-find task with a static partitioner</span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">find_if</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">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="n">result</span><span class="p">,</span><span class="w"> </span><span class="p">[</span><span class="o">&amp;</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="k">return</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">-1</span><span class="p">;</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">static_partitioner</span><span class="w"></span>
<span class="p">);</span><span class="w"></span>

<span class="c1">// create a parallel-find task with a guided partitioner</span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">find_if</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">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="n">result</span><span class="p">,</span><span class="w"> </span><span class="p">[</span><span class="o">&amp;</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="k">return</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">-1</span><span class="p">;</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">guided_partitioner</span><span class="w"></span>
<span class="p">);</span><span class="w"></span></pre><aside class="m-note m-info"><h4>Note</h4><p>By default, parallel-find tasks use <a href="namespacetf.html#a77d193108ae375fdfc554e7f10bea23e" class="m-doc">tf::<wbr />DefaultPartitioner</a> if no partitioner is specified.</p></aside></section>
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