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Proper use of srand()
The exercise was to generate 1000 random numbers between 0-9 and to print how many of each number were generated - and to repeat the task with ten different seeds. I used srand() with time() and yet I get the same distribution of numbers each time. Why is this?
Code:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(void)
{
int nums[10] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
int i,j;
printf("1 2 3 4 5 6 7 8 9 10\n");
for (i = 0; i < 10; i++)
{
srand((unsigned int) time(0));
for (j = 0; j < 1000; j++)
nums[(rand() % 10)]++;
for (j = 0; j < 10; j++)
{
printf("%-6d", nums[j]);
nums[j] = 0;
}
printf("\n");
}
getch();
return 0;
}
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time() only has second precision. Chances are your program finishes well within 1 second, so all calls to srand have the same seed.
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Only probable explanation is that the random number seed isn't changing and that's why you are getting a uniform distribution.
Perhaps sleep() for a while and see if that changes it to a random distribution. Just my 2c.
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If the exercise simply calls for the use of ten seeds, you can do something simple like:
Code:
for(int i = 0; i < 10; i++)
{
srand(i);
/* ... */
}
It sounds like the point is to see how the PRNG works, so it would hardly matter which seeds are used.
My C library treats srand(0) and srand(1) identically; perhaps it would make sense to use srand(i + 1).
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Aha! Using i for the argument to srand() does the trick. I didn't know time() was only to the second! I thought it was like the timer on my old CBM-64....
