Why does rand() zlways return 0?

问题:

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?

回答1:

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.

回答2:

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++?.