rand() function is not really random

This is a discussion on rand() function is not really random within the C Programming forums, part of the General Programming Boards category; I made a dice program using Code: dice=rand()%6 + 1; It gives random values from 1 to 6 but every ...

  1. #1
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    rand() function is not really random

    I made a dice program using
    Code:
    dice=rand()%6 + 1;
    It gives random values from 1 to 6 but every time I run the program, it just gives me the same set of random values.I tried using
    Code:
    random=(rand()*time(NULL))%6 + 1;if(random>0) //since random sometimes become negative{     dice=random; }
    But this one always shows 1 and sometimes the other numbers. It's like 90% chance of getting 1 and I do not know why.

  2. #2
    C++ Witch laserlight's Avatar
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    Have you used srand to seed the PRNG with a different seed on each run of the program?
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
    Registered User manasij7479's Avatar
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    I think you forgot to seed srand() with a new value each time(at the beginning).
    srand(time(NULL)) is often used, as everytime you run, the time is going to differ slightly.

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    [FONT=Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace]seed it...

    use srand function..

    Code:
    srand(time(NULL));
    [/FONT]

  5. #5
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    You could do any of these options as well:

    - Use a more cryptographically secure random generator. Arc4random() is not standard, but it's available on pretty much every Linux/Mac/*nix build I've seen. Also, a whole array of functions like this can be found here > jrand48(3) - Linux man page

    - Seed your random, using srand(time(NULL));

    - Write a "hack" quick random function:
    Code:
    #include <sys/time.h>
    
    
    uint64_t fake_rand(int clamp)
    {
    	uint64_t val;
    
    
    	asm ("rdtsc" : "=A" (val) : : );
    
    
    	return (val % clamp);
    }

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