Legacy Document

Inside Macintosh: QuickDraw GX Environment and Utilities /

Chapter 8 - QuickDraw GX Mathematics / Using QuickDraw GX Mathematics

Generating Random Numbers

You can use the QuickDraw GX random number functions to return a sequence of random numbers. The RandomBits function generates random integers in the range of 0 to 2count - 1, where count is the number of bits in the integer to be generated by the random number generator.

The SetRandomSeed function allows you to use a seed other than the default seed. If the SetRandomSeed function is not used, the initial seed will always be 0. You can use the GetRandomSeed function to return the value of the current seed.

Listing 8-8 is a sample function that generates an unsigned random number between zero and the value passed in the limit parameter. It uses the RandomBits function, and it also uses the WideMultiply function, correcting for the fact that WideMultiply works with signed long integers whereas this random generator uses unsigned longs.

Listing 8-8 A random number generator

unsigned long RandomLong(unsigned long limit)
{
   wide temp;
   unsigned long random = RandomBits(32, 0);

   /* This treats random and limit as signed */
   WideMultiply(random, limit, &temp);
   if ((long)limit < 0)
      temp.hi += random;   /* correct for the "sign" of limit */
   if ((long)random < 0)
      temp.hi += limit;    /* correct for the "sign" of random */
   return temp.hi;
}

The general topic of random numbers and the functions you use to generate them generation are discussed in the section "Random Number Generation" beginning on page 8-58.