Random Decimals

This is a discussion on Random Decimals within the C++ Programming forums, part of the General Programming Boards category; Hi Ive found lots tutorials detailing how to create random integer values, but I was wondering how to create random ...

  1. #1
    TJJ
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    Random Decimals

    Hi

    Ive found lots tutorials detailing how to create random integer values, but I was wondering how to create random decimal values between a range?

    Thanks for any help

    TJJ

  2. #2
    Registered User major_small's Avatar
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    do something like this:
    Code:
    float number=(1+rand()%10000)/100.0;
    the only thing random about that code is the rand() part... take notice that in the formula 1+rand()%100;, the 1+ just adds one to whatever comes out, and the %100 is just performing a modulo on whatever rand() comes out to be...

    don't forget to put this somewhere near the beginning of your program:
    Code:
    srand(time(0));
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  3. #3
    C++ Developer XSquared's Avatar
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    >float number=(1+rand()%10000)/100.0;
    float number=(float)(1+rand()%10000)/100.0;
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    No you don't need that float casting there because the number he's dividing (100.0) is already a float. Well, technically its a double so you'd have to make it 100.0F.

  5. #5
    Carnivore ('-'v) Hunter2's Avatar
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    (1+rand()%10000) isn't a floating point number. Usually, when performing operations on mixed types, the result is an integer value; so yes, the typecast is necessary if only to eliminate ambiguity.
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    No: only one of the operands has to be a floating point value for the result to be floating point. The (1+rand()%10000) part yields an integer but the 100.0 is a floating point value. So when you divide, the result will be a floating point value.

  7. #7
    & the hat of GPL slaying Thantos's Avatar
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    Or heck why not use the better approach of :
    Code:
    float floatrandom(float lowerbound, float upperbound)
    {
      return ((float)rand() / RAND_MAX) * (upperbound - lowerbound) + lowerbound;
    }
    The mod operator shouldn't be used for getting random numbers. For example if you wanted to get a number between 0 and 65535 but your RAND_MAX was only 32767 then using the mod operator will make us SOL. However by using the rand()/RAND_MAX method you have now extended your range to handle any number you want.

  8. #8
    Carnivore ('-'v) Hunter2's Avatar
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    >>No: only one of the operands has to be a floating point value for the result to be floating point.
    Since when? I'm pretty sure I recall encountering a bug caused by this, where I had one integer and one floating-point (I think I was trying to convert degrees to radians or vice versa), and the whole thing was converted to an integer and rounded down, causing some very funked out results.

    I'll run a test though, maybe you're right.

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    Last edited by Hunter2; 04-26-2004 at 03:49 PM.
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