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My random numbers aren't so random

by Daniel Buckmaster · in Torque Game Engine · 11/02/2007 (7:28 am) · 5 replies

I'm writing a method to make an AIPlayer fire a shot after a random interval of time. However, my random number always turns out to be the equivalent of about 4/10! Here's the method-
void AIPlayer::singleShot(int minTO,int maxTO)
{
	srand(time(NULL));
	S32 interval = abs(maxTO - minTO);
	S32 time = rand() * interval / (RAND_MAX + 1);
	time += minTO;
	mFireCountdown = true;
	mCountdownTime = time;
}
If I put in 10 and 20, I get 14. 0 and 1000, I get 400. Am I right in thinking it might be a mistake to seed the generator every time?
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Torque Owner Daniel Buckmaster
T3D Steering Committee

About the author

Studying mechatronic engineering and computer science at the University of Sydney. Game development is probably my most time-consuming hobby!

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    Associate Ross Pawley
    #1
    11/02/2007 (8:17 am)
    Yeah you shouldn't seed it every time.

    Torque Owner Daniel Buckmaster
    T3D Steering Committee
    #2
    11/02/2007 (12:57 pm)
    When should I seed it then? AIPlayer::onAdd?

    Associate Fyodor -bank- Osokin
    Dedicated Logic
    #3
    11/02/2007 (1:23 pm)
    I thought the "seed" generation is done on when the engine is started...

    Torque Owner Michael Bacon
    Default Studio Name
    #4
    11/02/2007 (4:40 pm)
    Looking at mathTypes.cc maybe you should use these functions instead:
    [b]S32 mRandI(S32 i1, S32 i2)[/b]
{
   return gRandGen.randI(i1, i2);
}

[b]F32 mRandF(F32 f1, F32 f2)[/b]
{
   return gRandGen.randF(f1, f2);
}

ConsoleFunction(getRandom, F32, 1, 3, "(int a=1, int b=0)"
                "Get a random number between a and b.")
{
   if (argc == 2)
      return F32(gRandGen.randI(0,dAtoi(argv[1])));
   else
   {
      if (argc == 3) 
      {
         S32 min = dAtoi(argv[1]);
         S32 max = dAtoi(argv[2]);
         if (min > max) 
         {
            S32 t = min;
            min = max;
            max = t;
         }
         return F32(gRandGen.randI(min,max));
      }
   }
   return gRandGen.randF();
}

    and I dont think this line is quite right:
    S32 time = rand() * interval / (RAND_MAX + 1);

    I'm not sure why you are adding 1 to RAND_MAX before dividing.
    S32 time = rand() * interval / RAND_MAX;

    Should work just fine.

    I.e. if interval is 10 then 0 *10 / RAND_MAX = 0 and 32767 * 10 / 32767 = 1

    Torque Owner Daniel Buckmaster
    T3D Steering Committee
    #5
    11/03/2007 (2:56 am)
    Ah. Thank you very much for that :)
    About adding 1, I saw that in another forum thread on random numbers. Don't know where it came from...

    EDIT: New method works perfectly. Thanks :)