C Random Numbers

Games, quizzes and simulations need numbers that look random. C's standard library makes them with rand from stdlib.h.

rand and srand

rand() returns a number from 0 to RAND_MAX (at least 32767). The numbers come from a formula that starts from a seed. With the same seed, the same sequence appears every time. srand(seed) sets the seed; seeding with the current time gives a different sequence on each run.

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

int main(void) {
    srand((unsigned)time(NULL));   // once, at the start
    for (int i = 0; i < 5; i++) {
        printf("%d ", rand());
    }
    printf("\n");
    return 0;
}

Call srand once at the start of the program, not before every rand(): seeding again within the same second would repeat the same number.

A number in a range

rand() % n gives a number from 0 to n−1. Add the lowest value to shift the range: a dice roll from 1 to 6 is rand() % 6 + 1.

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

int rollDice(void) {
    return rand() % 6 + 1;
}

int main(void) {
    srand((unsigned)time(NULL));
    int counts[7] = {0};
    for (int i = 0; i < 6000; i++) {
        counts[rollDice()]++;
    }
    for (int face = 1; face <= 6; face++) {
        printf("%d: %d times\n", face, counts[face]);
    }
    return 0;
}

Each face comes up about 1000 times. For a decimal between 0 and 1, use rand() / (RAND_MAX + 1.0).

The same sequence for testing

While testing a program, a fixed seed makes the "random" numbers repeat exactly, so a bug can be reproduced. Here the seed is 42, so this program prints the same result every time:

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

int main(void) {
    srand(42);
    int a = rand() % 100, b = rand() % 100;
    srand(42);
    int c = rand() % 100, d = rand() % 100;
    printf("%s\n", (a == c && b == d) ? "same numbers again" : "different");
    return 0;
}

same numbers again

Shuffling

To shuffle an array fairly, go from the last element down and swap each one with a randomly chosen element at or before it (the Fisher–Yates shuffle):

for (int i = n - 1; i > 0; i--) {
    int j = rand() % (i + 1);
    int t = a[i]; a[i] = a[j]; a[j] = t;
}

rand is fine for games and exercises, but not for passwords or security: its numbers can be predicted.

Practice exercises

1. Write a number guessing game: the program picks 1 to 100, the user guesses, and the program says higher or lower.
2. Simulate 10000 coin tosses and print how many were heads.
3. Shuffle the numbers 1 to 10 with the Fisher–Yates loop and print them.