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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">9.62 <code>CO_REDUCE</code> — 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(*,*) "Product value", 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> |
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