C - Any way to initialize srand multiple times? - c

Question

Is there any way to initialize srand multiple times? I have an specific function to generate random numbers but every time i call it gives the same number. Any way to fix this?
int whoatk2() {
srand(time(NULL));
int a;
a = rand() % 50;
return a;
}

Answer 1

Unless you want a specific sequence, you should be calling srand once, and with a effectively arbitrary value like time(NULL). This would usually be done when your program starts.
If you call it every time you want a random number, you run the very real risk of initialising the generator with the same value each time (if you're calling it many times per second) and therefore getting the same "random" value each time.
This is the sort of thing you should be doing:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(void) {
srand(time(NULL)); // do this once
for (int i = 0; i < 10; ++i) { // do this many times
printf("%d\n", rand());
}
return 0;
}
Sample output is shown below, with the left column containing the output for the correct code above, and the right column containing the result of moving the srand call to inside the loop as you currently have (swapping the two lines that have comments above):
966271109 74846356
1059160369 74846356
1868029595 74846356
758240870 74846356
1795677958 74846356
806788680 74846356
1374271653 74846356
1658543317 74846356
1843517305 74846356
1568992484 74846356

Answer 2

srand is the function to initialize the pseudo-random generator with a seed, and rand is the function to get a random number from the generator...

Answer 3

You get the same random number repeatedly because you reinitialize the random number generator with the same seed every time you call whoatk2, which happens many times during the same second. The sequence of pseudo-random numbers returned by rand() is completely determined by the value passed to srand().
You should call srand() just once at the beginning of the program, preferably with a value that varies faster than time(NULL). You can use timespec_get(), gettimeofday() or similar:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#ifdef TIME_UTC
void random_initialize(void) {
struct timespec ts;
timespec_get(&ts, TIME_UTC);
srand(ts.tv_nsec);
}
#else
#include <sys/time.h>
void random_initialize(void) {
struct timeval tv;
gettimeofday(&tv, NULL);
srand(tv.tv_usec);
}
#endif
int whoatk2(void) {
return rand() % 50;
}
int main() {
random_initialize();
for (int i = 0; i < 20; i++) {
printf("%d ", whoatk2());
}
printf("\n");
return 0;
}
Running 10 times:
36 33 35 31 8 32 1 5 30 23 20 8 5 35 47 10 14 26 34 40
32 49 33 16 28 31 22 6 32 33 45 44 17 32 18 38 8 34 28 38
48 38 35 49 25 18 40 3 38 3 36 46 12 26 21 16 15 48 34 39
49 33 9 28 24 39 4 32 33 9 29 23 10 29 41 24 12 33 35 33
5 40 49 16 22 47 27 15 29 39 35 32 28 23 39 35 19 24 10 0
5 45 33 24 4 28 48 40 32 22 27 45 32 11 35 28 0 37 32 9
36 3 38 16 12 25 14 27 30 31 0 49 41 40 39 5 23 26 40 8
11 9 21 15 34 0 13 47 49 47 33 21 3 42 16 19 32 32 33 15
14 2 27 26 19 15 5 41 32 49 48 31 20 12 39 16 15 19 32 20
3 2 34 32 47 24 34 21 0 28 6 48 44 16 6 46 40 48 20 13