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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"><head>	<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />	<style type="text/css">	body { font-family: Verdana, Arial, Helvetica, sans-serif;}	a.at-term {	font-style: italic; }	</style>	<title>Reduction Operations</title>	<meta name="Generator" content="ATutor">	<meta name="Keywords" content=""></head><body> <p>A <em><a href="../glossary.html#reduction+operation" target="body" class="at-term">reduction operation</a></em> is a global calculation or comparison, involving data (e.g., partial sums) from several different threads. The reduction clause allows you to automatically perform such operations in a safe way. </p>

<p>Basic syntax: </p>

<pre><code>reduction(operator|intrinsic:var1[, var2...])</code></pre>
  

<p>The operator or intrinsic indicates the operation or comparison to be performed. The listed variables indicate the data on which the operations act. Each thread gets a private copy of each listed variable, which is initialized in a standard way depending on the operation or intrinsic specified. For example, for a sum 
the local copies are initialized to zero. </p>

<p>Example: </p>

<pre><code>c$omp  do shared(x) private(i)
c$omp& <em>reduction(+:sum)</em>
     do i = 1, N
        sum = sum + x(i)
     end do</code></pre>


<p> In this example, each thread automatically gets a private copy of the variable <var>sum</var>, initialized to zero. The threads compute the partial sums corresponding to their share of the loop iterations. Finally, the partial sums are combined from each thread to obtain the global sum. </p>

<p>Global minimum example: </p>

<pre><code>c$omp  do shared(x) private(i)
c$omp& <em>reduction(min:globalmin)</em>
     do i = 1, N
        globalmin = min(globalmin, x(i))
     end do</code></pre>


<p>In this example, the local minima computed by individual threads are compared to form the global minimum. Note that the variables listed in the reduction 
clause must be shared in the enclosing parallel context.</p> 

<p>In Fortran, allowed operators are <code>+, *, -, .AND., .OR., .EQV., .NEQV.</code> 
  Allowed intrinsics are <code>MAX, MIN, IAND, IOR, IEOR.</code>
<p> 
  
<p>In C/C++, allowed operators are <code>+, *, -, &, ^, |, &&, ||. </code> </p>
<p>Pointers and reference variables are not allowed in reductions. </p></body></html>
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