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<title>The GNU Fortran Compiler: CO_REDUCE</title>

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<a name="CO_005fREDUCE-_002d_002d_002d-Reduction-of-values-on-the-current-set-of-images"></a>
<h3 class="section">9.66 <code>CO_REDUCE</code> &mdash; Reduction of values on the current set of images</h3>
<a name="index-CO_005fREDUCE"></a>
<a name="index-Collectives_002c-generic-reduction"></a>

<dl compact="compact">
<dt><em>Description</em>:</dt>
<dd><p><code>CO_REDUCE</code> determines element-wise the reduction of the value of <var>A</var>
on all images of the current team.  The pure function passed as <var>OPERATOR</var>
is used to pairwise reduce the values of <var>A</var> by passing either the value
of <var>A</var> of different images or the result values of such a reduction as
argument.  If <var>A</var> is an array, the deduction is done element wise. If
<var>RESULT_IMAGE</var> is present, the result values are returned in <var>A</var> on
the specified image only and the value of <var>A</var> on the other images become
undefined.  If <var>RESULT_IMAGE</var> is not present, the value is returned on all
images.  If the execution was successful and <var>STAT</var> is present, it is
assigned the value zero.  If the execution failed, <var>STAT</var> gets assigned
a nonzero value and, if present, <var>ERRMSG</var> gets assigned a value describing
the occurred error.
</p>
</dd>
<dt><em>Standard</em>:</dt>
<dd><p>Technical Specification (TS) 18508 or later
</p>
</dd>
<dt><em>Class</em>:</dt>
<dd><p>Collective subroutine
</p>
</dd>
<dt><em>Syntax</em>:</dt>
<dd><p><code>CALL CO_REDUCE(A, OPERATOR, [, RESULT_IMAGE, STAT, ERRMSG])</code>
</p>
</dd>
<dt><em>Arguments</em>:</dt>
<dd><table>
<tr><td width="15%"><var>A</var></td><td width="70%">is an <code>INTENT(INOUT)</code> argument and shall be
nonpolymorphic. If it is allocatable, it shall be allocated; if it is a pointer,
it shall be associated.  <var>A</var> shall have the same type and type parameters on
all images of the team; if it is an array, it shall have the same shape on all
images.</td></tr>
<tr><td width="15%"><var>OPERATOR</var></td><td width="70%">pure function with two scalar nonallocatable
arguments, which shall be nonpolymorphic and have the same type and type
parameters as <var>A</var>.  The function shall return a nonallocatable scalar of
the same type and type parameters as <var>A</var>.  The function shall be the same on
all images and with regards to the arguments mathematically commutative and
associative.  Note that <var>OPERATOR</var> may not be an elemental function, unless
it is an intrisic function.</td></tr>
<tr><td width="15%"><var>RESULT_IMAGE</var></td><td width="70%">(optional) a scalar integer expression; if
present, it shall have the same the same value on all images and refer to an
image of the current team.</td></tr>
<tr><td width="15%"><var>STAT</var></td><td width="70%">(optional) a scalar integer variable</td></tr>
<tr><td width="15%"><var>ERRMSG</var></td><td width="70%">(optional) a scalar character variable</td></tr>
</table>

</dd>
<dt><em>Example</em>:</dt>
<dd><div class="smallexample">
<pre class="smallexample">program test
  integer :: val
  val = this_image ()
  call co_reduce (val, result_image=1, operator=myprod)
  if (this_image() == 1) then
    write(*,*) &quot;Product value&quot;, val  ! prints num_images() factorial
  end if
contains
  pure function myprod(a, b)
    integer, value :: a, b
    integer :: myprod
    myprod = a * b
  end function myprod
end program test
</pre></div>

</dd>
<dt><em>Note</em>:</dt>
<dd><p>While the rules permit in principle an intrinsic function, none of the
intrinsics in the standard fulfill the criteria of having a specific
function, which takes two arguments of the same type and returning that
type as result.
</p>
</dd>
<dt><em>See also</em>:</dt>
<dd><p><a href="CO_005fMIN.html#CO_005fMIN">CO_MIN</a>, <a href="CO_005fMAX.html#CO_005fMAX">CO_MAX</a>, <a href="CO_005fSUM.html#CO_005fSUM">CO_SUM</a>, <a href="CO_005fBROADCAST.html#CO_005fBROADCAST">CO_BROADCAST</a>
</p></dd>
</dl>


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