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copied!<p>You must seed the random number generator. <a href="http://www.cplusplus.com/reference/cstdlib/srand/" rel="noreferrer">see here for an example</a>.</p> <h1>not-seeded.c</h1> <pre><code>#include <stdio.h> #include <stdlib.h> int main () { printf ("Random not seeded: %d\n", rand()%10); return 0; } </code></pre> <h1>not-seeded output</h1> <pre><code>Random not seeded: 3 Random not seeded: 3 Random not seeded: 3 Random not seeded: 3 Random not seeded: 3 </code></pre> <h1>seeded.c</h1> <pre><code>#include <stdio.h> #include <stdlib.h> #include <time.h> int main () { srand(time(NULL)); printf ("Random seeded: %d\n", rand()%10); return 0; } </code></pre> <h1>seeded output</h1> <pre><code>Random fast seeded: 7 Random fast seeded: 7 Random fast seeded: 7 Random fast seeded: 1 Random fast seeded: 1 Random fast seeded: 1 Random fast seeded: 5 Random fast seeded: 5 Random fast seeded: 5 Random fast seeded: 5 </code></pre> <h1>fast-seeded.c</h1> <p>If you want to be able to call your utility more than once a second, you will have to use a different source for you seed, otherwise you will still end up with some repeated random numbers.</p> <p>Here is an example that uses nanoseconds instead of <code>time()</code> which only returns seconds.</p> <pre><code>#include <stdio.h> #include <stdlib.h> #include <time.h> int main () { struct timespec ts; clock_gettime(CLOCK_MONOTONIC, &ts); /* using nano-seconds instead of seconds */ srand((time_t)ts.tv_nsec); printf ("Random fast seeded: %d\n", rand()%10); return 0; } </code></pre> <h1>fast-seeded output</h1> <p>You can see here that the numbers aren't grouped as much as the previous example.</p> <pre><code>Random fast seeded: 9 Random fast seeded: 6 Random fast seeded: 5 Random fast seeded: 6 Random fast seeded: 1 Random fast seeded: 1 Random fast seeded: 9 Random fast seeded: 4 Random fast seeded: 3 Random fast seeded: 2 </code></pre> <h1>uniformly distributed random numbers</h1> <p>If you are interested in uniformly distributed random numbers, you should see <strong>user3003631</strong>'s answer below. If you are in fact using C++, that's how I would recommend doing random numbers. <a href="http://www.cplusplus.com/reference/random/uniform_int_distribution/" rel="noreferrer">More information here too on this.</a></p>

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