CODEWITHSUNDEEP
objective-ccrandom
0xdead10cc
0xdead10cc

Reputation: 521

Why does rand() % 7 always return 0?

This seems to be a really strange issue:

This is my code:

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[])
{
    @autoreleasepool {
        srand((unsigned int)time(NULL));
        int newRandomNumber = 0;
        newRandomNumber = rand() % 7;
        NSLog(@"%d", rand() % 7); //This prints out what I expected
        NSLog(@"newRandomNumber = %d", newRandomNumber); // This always prints out 0!
    }
    return 0;
}

If I replace that one line that says

newRandomNumber = rand() % 7

with

newRandomNumber = rand() % 8

everything works perfectly. Why is that the case?

Upvotes: 39

Views: 8386

Answers (2)

Grady Player
Grady Player

Reputation: 14549

It seems unlikely but running some tests, after an srand the first rand seems always to be divisible by 7, at least in an int sized variable.

On several runs I got 1303562743, 2119476443, and 2120232758, all of which mod 7 to 0.

The second rand() works, because it is the second rand(). Throw a rand() before your first rand()... or better yet, use a better random number generator random or arc4rand if available.

Also see Stack Overflow question Why is (rand() % anything) always 0 in C++?.

Upvotes: 9

georg
georg

Reputation: 214959

Well, this

int seed;
for(seed = 1; seed < 10; seed++) {
    srand(seed);
    printf("%4d %16d\n", seed, rand());
}

prints 

   1            16807
   2            33614
   3            50421
   4            67228
   5            84035
   6           100842
   7           117649
   8           134456
   9           151263

which makes me think that rand() = seed * 16807

Wikipedia article Linear congruential generator confirms that CarbonLib indeed uses Xn+1 = Xn * 16807 to generate random numbers.

Upvotes: 41

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