Objective C - Random results is either 1 or -1

Question

I am trying randomly generate a positive or negative number and rather then worry about the bigger range I am hoping to randomly generate either 1 or -1 to just multiply by my other random number.

I know this can be done with a longer rule of generating 0 or 1 and then checking return and using that to either multiply by 1 or -1.

Hoping someone knows of an easier way to just randomly set the sign on a number. Trying to keep my code as clean as possible.

Answer 1

If I understand the question correctly, you want a pseudorandom sequence of 1 and -1:

int f(void)
{
        return random() & 1 ? 1 : -1;
    //  or...
    //  return 2 * (random() & 1) - 1;
    //  or...
    //  return ((random() & 1) << 1) - 1;
    //  or...
    //  return (random() & 2) - 1; // This one from Chris Lutz
}

Update: Ok, something has been bothering me since I wrote this. One of the frequent weaknesses of common RNGs is that the low order bits can go through short cycles. It's probably best to test a higher-order bit:   random() & 0x80000 ? 1 : -1

Answer 2

To generate either 1 or -1 directly, you could do:

int PlusOrMinusOne() {
    return (rand() % 2) * 2 - 1
}

But why are you worried about the broader range?

Answer 3

This extra step won't give you any additional "randomness". Just generate your number straight away in the range that you need (e.g. -10..10). Standard rand() will return a value from this range: 0..1 You can multiply it by a constant to increase the span of the range or you can add a constant to push it left/right on the X-Axis. E.g. to generate random values from from (-5..10) range you will have: rand()*15-5

Answer 4

rand will give you a number from 0 to RAND_MAX which will cover every bit in an int except for the sign. By shifting that result left 1 bit you turn the signed MSB into the sign, but have zeroed-out the 0^th bit, which you can repopulate with a random bit from another call to rand. The code will look something like:

int my_rand()
{
    return (rand() << 1) + (rand() & 1);
}

Answer 5

I like to use arc4random() because it doesn't require you to seed the random number generator. It also conveniently returns a uint_32_t, so you don't have to worry about the result being between 0 and 1, etc. It'll just give you a random integer.

int myRandom() {
  return (arc4random() % 2 ? 1 : -1);
}