Is this a "sound" way to generate a random floating point number?

This is a discussion on Is this a "sound" way to generate a random floating point number? within the C++ Programming forums, part of the General Programming Boards category; I'm basically creating an openGL program where I have to determine the size, alpha, accelleration, and color of particles, so ...

  1. #1
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    Is this a "sound" way to generate a random floating point number?

    I'm basically creating an openGL program where I have to determine the size, alpha, accelleration, and color of particles, so they range in the less than 1 range. I am doing it by generating a larger random number and dividing it by a factor to reduce it to a fraction of the larger number for example

    Code:
    part.size = static_cast<float>(rand() &#37; ((3 - 1) + 1) + 1 ) / 10;  //random size from 1.0 - 3.0
    part.dy =  static_cast<float>(rand() % ((100 - 25) + 1) + 25 ) / 100; //random acceleration from .25 - 1.0
    I was just wondering, I've never really found much information on random numbers, but is there anything flawed about this strategy, any overhead from the calculations, or will this do?
    Last edited by indigo0086; 07-01-2007 at 10:08 PM.

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    CSharpener vart's Avatar
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    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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    Code Goddess Prelude's Avatar
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    >static_cast<float>(rand() % ((3 - 1) + 1) + 1 ) / 10; //random size from 1.0 - 3.0
    Casting an integer to a float doesn't magically give you precision between the whole numbers. You'll get x.000000 in all cases. This applies to your other statement as well. Neither of them will give you the desired results.
    My best code is written with the delete key.

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    Well it came out pretty good, and I'm getting random numbers that are floats. I only cast the integer result from the rand range, then divided that by the factor to reduce it to a fraction.

    Code:
            part.blue = static_cast<float>(rand() &#37; ((10 - 3) + 1) + 3) / 10;
            part.alpha = static_cast<float>(rand() % ((10 - 5) + 1) + 5 ) / 10;  
            part.size = static_cast<float>(rand() % ((75 - 10) + 1) + 10 ) / 100;  
            part.dy =  static_cast<float>(rand() % ((300 - 25) + 1) + 25)/ 100;
    So far when I render the particles from the code I get visual results similar to what I would expect. I was just wondering if there were downfals to it.

  5. #5
    CSharpener vart's Avatar
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    you can remove the static cast and add .0 somewhere in the statement like:
    (rand() &#37; ((10 - 3) + 1) + 3) / 10.0; (this will use double for division)
    or
    (rand() % ((10 - 3) + 1) + 3) / 10.0f ; (floats)
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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    ok, that works. I don't need that much significance since when drawing a polygon in OpenGL, there is little difference between .4 and .4568482. I just needed something to give me a floating point precision of about two digits at most.

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    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by indigo0086 View Post
    I'm basically creating an openGL program where I have to determine the size, alpha, accelleration, and color of particles, so they range in the less than 1 range. I am doing it by generating a larger random number and dividing it by a factor to reduce it to a fraction of the larger number for example
    There's nothing wrong with that approach, as long as the underlying random number generator works correctly. You will always have slight quantization errors which shift the distribution away from perfect uniformity, even if the base RNG is perfectly distributed, but in most cases it is unimportant.

    However, your particular method:

    Code:
    part.size = static_cast<float>(rand() % ((3 - 1) + 1) + 1 ) / 10;
    part.dy =  static_cast<float>(rand() % ((100 - 25) + 1) + 25 ) / 100;
    Could probably be improved. Instead of coding this by hand everywhere, write a single function that returns a floating point value in the range [0.0, 1.0). You can do this easily by dividing the output of rand() by RAND_MAX + 1:

    Code:
    float random_unity(void)
    {
        return float(rand()) / (float(RAND_MAX) + 1.0);
    }
    The cast of RAND_MAX to float is important. You can probably figure out why. Using this you can easily get numbers in any given range:

    Code:
    float random_in_range(float min, float max)
    {
        return random_unity() * (max - min) + min;
    }
    Note that you'll never get exactly 1.0 as a result from random_unity(), and accordingly you'll never get exactly "max" from random_in_range(). If you want the range to be fully inclusive, i.e. [0.0, 1.0] instead of [0.0, 1.0), drop the "+ 1" from the denominator in random_unity().

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    Thanks for that solution, didn't think I would need a random utility but it makes it easier since I'm going to want to change the boundary settings and this'll make it more portable.

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