Random number basics

This is a discussion on Random number basics within the C Programming forums, part of the General Programming Boards category; Hi, everyone. I am trying to generate random numbers with the code below, but every time I run it, I ...

  1. #1
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    Random number basics

    Hi, everyone. I am trying to generate random numbers with the code below, but every time I run it, I get the number 41. What am I doing wrong? Thanks in advance!

    Code:
    #include <stdlib.h>
    #include <stdio.h>
    
    int main (void)
    {
    
        int value;
    
        value = rand();
    
        printf("%d", value);
    
        return 0;
    
    }

  2. #2
    Registered User claudiu's Avatar
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    You need to seed in the current time to the random number generator. Look up srand().
    1. Get rid of gets(). Never ever ever use it again. Replace it with fgets() and use that instead.
    2. Get rid of void main and replace it with int main(void) and return 0 at the end of the function.
    3. Get rid of conio.h and other antiquated DOS crap headers.
    4. Don't cast the return value of malloc, even if you always always always make sure that stdlib.h is included.

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    Thanks!

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    Registered User Alexander.'s Avatar
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    The rand function should use an LCG and may always produce the same number sequences, hense the need of a seed which is fairly random (time in seconds) to cause the algorithm to produce different sequences for its period.

    If you require this for secure uses, or games, or where random numbers are important than the rand function is very unreliable.

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    Quote Originally Posted by stdq View Post
    Hi, everyone. I am trying to generate random numbers with the code below, but every time I run it, I get the number 41. What am I doing wrong? Thanks in advance!

    Code:
    #include <stdlib.h>
    #include <stdio.h>
    
    int main (void)
    {
    
        int value;
    
        value = rand();
    
        printf("%d", value);
    
        return 0;
    
    }
    But the correct answer to life, love, the universe and everything else is ... 42...

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    Quote Originally Posted by Alexander. View Post
    If you require this for secure uses, or games, or where random numbers are important than the rand function is very unreliable.
    Only if you don't know how to use it.

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    Registered User Alexander.'s Avatar
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    Quote Originally Posted by CommonTater View Post
    Only if you don't know how to use it.
    Edit: bad at forming words right now.

    LCGs are fast and can work for fairly equal distribution, although there are much better algorithms out there. Just the fact that every major compiler times platform has an LCG with such parameter diversity would just turn you off wanting to use it for anything predictably secure!
    Last edited by Alexander.; 05-14-2011 at 10:58 PM.

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    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by Alexander. View Post
    Edit: bad at forming words right now.

    LCGs are fast and can work for fairly equal distribution, although there are much better algorithms out there. Just the fact that every major compiler times platform has an LCG with such parameter diversity would just turn you off wanting to use it for anything predictably secure!
    The only issue with it not being that secure (unable to be predicted) is that the size of the internal state is too small and it isn't influenced by external factors such as the time the user last pressed a key or moved the mouse etc. Those issues can be overcome whilst still primarily using an LCG.

    Anyway that's all moot as the OP will possibly not be back seeing as they have an answer.
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