How to generate a random int in C?

The rand() function in <stdlib.h> returns a pseudo-random integer between 0 and RAND_MAX. You can use srand(unsigned int seed) to set a seed.

It's common practice to use the % operator in conjunction with rand() to get a different range (though bear in mind that this throws off the uniformity somewhat). For example:

/ random int between 0 and 19 */
int r = rand() % 20;
If you really care about uniformity you can do something like this:

/* Returns an integer in the range [0, n).
*
* Uses rand(), and so is affected-by/affects the same seed.
*/
int randint(int n) {
if ((n - 1) == RAND_MAX) {
return rand();
} else {
// Supporting larger values for n would requires an even more
// elaborate implementation that combines multiple calls to rand()
assert (n <= RAND_MAX)

// Chop off all of the values that would cause skew...
int end = RAND_MAX / n; // truncate skew
assert (end > 0);
end = n;

// ... and ignore results from rand() that fall above that limit.
// (Worst case the loop condition should succeed 50% of the time,
// so we can expect to bail out of this loop pretty quickly.)
int r;
while ((r = rand()) >= end);

return r % n;
}
}

4 years ago

You can also try this:

#include <time.h>

#include <stdlib.h>



srand(time(NULL));   // Initialization, should only be called once.

int r = rand();      // Returns a pseudo-random integer between 0 and RAND_MAX.

4 years ago