Does rand() ever return zero on OSX?

问题

I have been running this code for almost ten hours with no love:

 while ( true ) {
     int r = rand();
     assert( r != 0 );
 }

I expect rand() to eventually roll a zero and thus trigger the assert.

Am I doing something wrong or does rand() never return zero?

Or have I not been waiting long enough to expect to see it? I'm on 2012-vintage 2GHz i7.

回答1:

If this link is the definition of Mac OS X rand(), then 0 will never be produced. It's not a very good pseudo random number generator, IMHO.

Amongst its other flaws, its state is the previously returned value, which means that it is a strict function from previous value to next value; it's cycle cannot be larger than its range, which is [1, RAND_MAX-1]. (It cannot produce 0 because 0 is a fixed-point in the algorithm.)

回答2:

The standard requires that successive calls to rand form a sequence which is a subset of the space [0; RAND_MAX]. It doesn't require that all the possible values occur in any of the possible sequences.

So to answer your question: You aren't guaranteed to get a zero; It's up to the implementation (And judging from the other answers, BSD libc's implementation does indeed never return 0).

§7.22.2.2

There are no guarantees as to the quality of the random sequence produced and some implementations are known to produce sequences with distressingly non-random low-order bits.

回答3:

Edit: what @rici said.

It looks like the FreeBSD rand never returns 0

That message chain has a patch, but it is not included in the apple open-source source.

~~It should eventually return zero, but on my computer, RAND_MAX is 2147483647~~

printf("rand_max = %d\r\n", RAND_MAX);

Wolfram alpha can't calculate that small of a probability.

Running these examples:

prob x>0 for x binomial with n=100000 and p=.00001

prob x>0 for x binomial with n=10000 and p=.0001

prob x>0 for x binomial with n=1000 and p=.001

Show that after for a probablity of 1/N, N attempts appears to approach about 0.63

It does look like 5*N attempts should get you over 99% though.

I don't know how long your computer will take to make 10 billion calls to rand though.

来源：https://stackoverflow.com/questions/29336510/does-rand-ever-return-zero-on-osx

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