how to make a random int value in c for coin flipping

Question

How do I make a random integer value in C? I need it to act like a coin flipping. Here is what I've tried:

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

int main(void)
{
    int i;

    srand(time(NULL));

    for (i=0; i<10; i++)
        if ((int)(((float) rand() / ((float) SHRT_MAX)) * 2) > 0)
            puts("etz");
        else
            puts("pali");
    return EXIT_SUCCESS;
}

Too intricate for my taste. You could just do `if (rand() % 2)` — HolyBlackCat, Dec 31 '17 at 19:58
The marked duplicate is not really a duplicate - that question is about _biasing_ the coin, not a fair coin. — Clifford, Dec 31 '17 at 20:20
Don't use [Tab] on Stack Overflow. It won't format your post correctly. Use 4-space indentation instead. — iBug, Jan 01 '18 at 11:06

score 4 · Answer 1 · edited Dec 31 '17 at 20:30

The problem with your solution is rand() always returns a positive integer so the expression (int)(((float) rand() / ((float) SHRT_MAX)) * 2) > 0 is always true except on platforms where short and int are the same width.

Another problem is that rand() returns a value from zero to RAND_MAX not SHRT_MAX - it is unlikely in a modern library that RAND_MAX == SHRT_MAX

Your solution may be adapted thus:

(float)rand() / RAND_MAX > 0.5f

but the casting and floating point comparison is unnecessary and over complex, it can be tested thus:

rand() > RAND_MAX / 2

It is likely to be suggested that you could just test for odd/even with:

rand() % 2 == 0

but that is problematic in some poorly implemented algorithms because it relies on the randomness of a single LSB - it is likely to work in a modern library, but best avoided to ensure code portability.

how to make a random int value in c for coin flipping

1 Answers1