Random number generator

This is a discussion on Random number generator within the C Programming forums, part of the General Programming Boards category; Ok I seem to have a further problem, with the rand function. It produces the same set of random numbers ...

  1. #1
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    Random number generator

    Ok I seem to have a further problem, with the rand function. It produces the same set of random numbers in the same order every time it get's called. Now if I use the srand(time(NULL)) to create a different seed every time. The time function seems to return the same number every time and therefore produce the same random number every time.

    Code:
    #include <stdlib.h>
    char* LB_Random(char *maxR,char *dummy)
    	{	
    	int i;
    	static char n[10];
    	int rMax = atoi(maxR);	
    	rMax++;
    	i = rand() % rMax;
    	sprintf(n,"%d",i);
    	return n;	
    	}
    The above produces: 9,3,14,4,1,12,6,14 everytime

    Code:
    #include <stdlib.h>
    char* LB_Random(char *maxR,char *dummy)
    	{	
    	int i;
    	static char n[10];
    	int rMax = atoi(maxR);	
    	rMax++;
                    srand(time(NULL));
    	i = rand() % rMax;
    	sprintf(n,"%d",i);
    	return n;	
    	}
    Produces the number 5 everytime, due to the time function returning the same number.

  2. #2
    and the hat of wrongness Salem's Avatar
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    Well here - C Warning message
    You first got it wrong, how to do it right was explained and then you got it right yourself.

    Now you've regressed back into doing it wrong again.

    > srand(time(NULL));
    This line goes at the start of main.
    You do it exactly once in the program.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  3. #3
    Mad OnionKnight's Avatar
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    srand() should only be called once. Do it outside the function or keep a static boolean variable that knows whether or not srand() has been called.

  4. #4
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    Ok let me try and explain this a bit better. I'm using a piece of software which will take a C header file and an obj file, import these as extra functionality and then it can call the functions these have defined so it's something like this.

    Code:
    INCLUDE(random)
    while(not_eof)
    {
        call(random,"15")
        go to next line
    }
    The 15 represents the maximum random number, now my header file looks like this

    Code:
    extern char* LB_Random(char*,char*);
    And as you know my main code looks like this
    Code:
    char* LB_Random(char *maxR,char *dummy)
    	{	
    	int i;
    	static char n[10];
    	int rMax = atoi(maxR);	
    	rMax++;	
    	i = rand() % rMax;
    	sprintf(n,"%d",i);
    	return n;	
    	}
    Seeing as you say I can only call srand once ever, what should I do. The only thing i can think of is defining another function in the above code that seeds during initialisation, and then calling the random function over and over. So something like

    Code:
    //Init
    INCLUDE(random)
    call(seed,"")
    while(not_eof)
    {
       call(random,"15")
       go to next line
    }
    How's that sound.

  5. #5
    and the hat of wrongness Salem's Avatar
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    Looks good.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  6. #6
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    Yay it works

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