How to use rand() and srand() in C. Ranges, seeds, and applications illustrated.
srand((unsigned)time(NULL));rand()rand() % N for 0 to N-1RAND_MAX and modulo biasrand() function in stdlib.h. They are called "pseudo" because internally they are produced by a mathematical formula and are not truly random — but for ordinary use they are good enough.
#include <stdlib.h> // rand(), srand() #include <time.h> // time() - for the seed
rand() returns an integer in the range 0 to RAND_MAX (typically 2147483647). But if you use it as-is, you get the same sequence every time.
srand(seed) sets the random number "seed" and produces a different sequence from that seed. Call it once, at the start of the program.
#include <stdio.h> #include <stdlib.h> #include <time.h> int main(void) { srand(time(NULL)); // seed from current time (different every run) for (int i = 0; i < 5; i++) { printf("%d\n", rand()); } return 0; }
rand() returns very large numbers, so use the modulo operator % to narrow the range.
// 0 to 9 int r = rand() % 10; // 1 to 6 (a die) int dice = rand() % 6 + 1; // min to max int val = rand() % (max - min + 1) + min;
| Goal | Code |
|---|---|
| 0 to N-1 | rand() % N |
| 1 to N | rand() % N + 1 |
| min to max | rand() % (max - min + 1) + min |
#include <stdio.h> #include <stdlib.h> #include <time.h> int main(void) { char *hands[] = {"Rock", "Scissors", "Paper"}; srand(time(NULL)); int cpu = rand() % 3; // 0:Rock 1:Scissors 2:Paper printf("CPU: %s\n", hands[cpu]); return 0; }
// Fisher-Yates shuffle for (int i = n - 1; i > 0; i--) { int j = rand() % (i + 1); // swap a[i] and a[j] int tmp = a[i]; a[i] = a[j]; a[j] = tmp; }
rand() % N really uniformly distributed? Check it with a histogram. The more trials you run, the more even it becomes.
rand(), srand(), and time().#include <stdio.h> #include <stdlib.h> #include <time.h> int main(void) { srand(time(NULL)); // Roll a die 10 times for (int i = 0; i < 10; i++) { int dice = rand() % 6 + 1; printf("Roll %d: %d\n", i + 1, dice); } return 0; }
srand(time(NULL)) and see identical runsCheck your understanding!
rand() returns an integer between 0 and RAND_MAX (usually at least 32767). Use the % operator to restrict it to a specific range.
srand() sets the seed of the random number generator. The same seed produces the same sequence, so time(NULL) is typically used.
rand() % 6 returns 0 to 5, so add 1 to get 1 to 6. rand() % 7 would return 0 to 6.