I'm learning C and I want to generate a number between 0 and 6400. This is the code I came up with:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main()
{
srand(time(0));
int i = (rand() % 6401);
printf("Random number between 0 and 6400: %d\n", i);
return 0;
}
When I compile and run this code from the command line I get some very weird results:
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6282
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6282
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6285
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6285
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6289
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6289
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6292
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6292
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6295
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6298
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6298
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6302
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6302
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6305
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6305
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6308
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6308
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6311
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6311
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6315
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6315
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6318
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6318
K:\C\Labo\Oefeningen 2019>a
Random number between 0 and 6400: 6321
K:\C\Labo\Oefeningen 2019>
The numbers are all different but I would expect a somewhat even distribution between 0 and 6400. The weird thing is that I was using the same function without problems an hour ago? (I was using it to generate smaller numbers before.) I'm certain it's something really stupid that I'm missing but I've been stuck for an hour now.
EDIT: I know It will give the same value when you run the code within the same second. I waited multiple seconds (10-20) between executions and I still get the same result? The values are rarely the same, they are just very very very similar 100 % of the time. How do I get around this?
The numbers generated by rand aren't truly random, they're generated with a formula. That's why seeding is both possible and necessary. Depending on the formula used, there can be a high correlation between the seed and the first few random numbers.
The cures are to use a better formula (something not rand), use a more random seed, or waste a few random numbers just after seeding.