main.c

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

int pick(int *rate, int types) {
  int i = 0, max = 0, sum = 0, rnd;

  for (i = 0; i < types; ++i) {
    max += rate[i];
  }

  rnd = rand() % max;

  for (i = 0; i < types; ++i) {
    sum += rate[i];
    if (sum > rnd) { return i; }
  }
  
  return -1;
}


int main() {
  char* types[] = {
    "大吉",
    "中吉",
    "小吉",
    "吉",
    "末吉",
    "凶"
  };
  int rate[] = {
    5, /* 大吉 */
    3, /* 中吉 */
    1, /* 小吉 */
    1, /* 吉 */
    1, /* 末吉 */
    7  /* 凶 */
  };

  srand((unsigned) time(NULL));

  printf("%s\n", types[pick(rate, 6)]);

  return 0;
}