Question

I would like to make random numbers in a specific range, like “pick a random number between 18 and 35”? How can I do that with the rand() function?

Answer 1

If this is written in C, then you are pretty close. Compiling this code:

#include <stdio.h>
#include <stdlib.h>

int main() {
    int i;
    for (i = 0; i < 1000000; i++) {
        printf("%d\n", rand()%(35-18+1)+18);
    }
}

And running it in a pipeline produces this output:

chris@zack:~$ gcc -o test test.c
chris@zack:~$ ./test | sort | uniq -c
  55470 18
  55334 19
  55663 20
  55463 21
  55818 22
  55564 23
  55322 24
  55886 25
  55947 26
  55554 27
  55342 28
  55526 29
  55719 30
  55435 31
  55669 32
  55818 33
  55205 34
  55265 35

The key is you forgot to add 1 — the fencepost error.

You can generalize this into a function:

int random_between(int min, int max) {
    return rand() % (max - min + 1) + min;
}