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| <a name="RANDOM_005fNUMBER-_002d_002d_002d-Pseudo_002drandom-number"></a> |
| <h3 class="section">8.200 <code>RANDOM_NUMBER</code> — Pseudo-random number</h3> |
| <a name="index-RANDOM_005fNUMBER"></a> |
| <a name="index-random-number-generation-3"></a> |
| |
| <dl compact="compact"> |
| <dt><em>Description</em>:</dt> |
| <dd><p>Returns a single pseudorandom number or an array of pseudorandom numbers |
| from the uniform distribution over the range <em>0 \leq x < 1</em>. |
| </p> |
| <p>The runtime-library implements George Marsaglia’s KISS (Keep It Simple |
| Stupid) random number generator (RNG). This RNG combines: |
| </p><ol> |
| <li> The congruential generator <em>x(n) = 69069 \cdot x(n-1) + 1327217885</em> |
| with a period of <em>2^{32}</em>, |
| </li><li> A 3-shift shift-register generator with a period of <em>2^{32} - 1</em>, |
| </li><li> Two 16-bit multiply-with-carry generators with a period of |
| <em>597273182964842497 > 2^{59}</em>. |
| </li></ol> |
| <p>The overall period exceeds <em>2^{123}</em>. |
| </p> |
| <p>Please note, this RNG is thread safe if used within OpenMP directives, |
| i.e., its state will be consistent while called from multiple threads. |
| However, the KISS generator does not create random numbers in parallel |
| from multiple sources, but in sequence from a single source. If an |
| OpenMP-enabled application heavily relies on random numbers, one should |
| consider employing a dedicated parallel random number generator instead. |
| </p> |
| </dd> |
| <dt><em>Standard</em>:</dt> |
| <dd><p>Fortran 95 and later |
| </p> |
| </dd> |
| <dt><em>Class</em>:</dt> |
| <dd><p>Subroutine |
| </p> |
| </dd> |
| <dt><em>Syntax</em>:</dt> |
| <dd><p><code>RANDOM_NUMBER(HARVEST)</code> |
| </p> |
| </dd> |
| <dt><em>Arguments</em>:</dt> |
| <dd><table> |
| <tr><td width="15%"><var>HARVEST</var></td><td width="70%">Shall be a scalar or an array of type <code>REAL</code>.</td></tr> |
| </table> |
| |
| </dd> |
| <dt><em>Example</em>:</dt> |
| <dd><div class="smallexample"> |
| <pre class="smallexample">program test_random_number |
| REAL :: r(5,5) |
| CALL init_random_seed() ! see example of RANDOM_SEED |
| CALL RANDOM_NUMBER(r) |
| end program |
| </pre></div> |
| |
| </dd> |
| <dt><em>See also</em>:</dt> |
| <dd><p><a href="RANDOM_005fSEED.html#RANDOM_005fSEED">RANDOM_SEED</a> |
| </p></dd> |
| </dl> |
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