Problem with function that generates random numbers

This is a discussion on Problem with function that generates random numbers within the C Programming forums, part of the General Programming Boards category; Hello, everyone! I'm having a problem with the following program: I try to call the same function that generates random ...

  1. #1
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    Problem with function that generates random numbers

    Hello, everyone! I'm having a problem with the following program: I try to call the same function that generates random integers and sends it back to main() (I'm doing that because I wanna apply it to a bigger situation), but the random integers obtained are always the same, why is it happening? Thanks in advance!

    Code:
    #include <stdio.h>
    #include <time.h>
    #include <stdlib.h>
    #include <math.h>
    
    /* Function prototype: */
    int generate(void);
    
    int main(void)
    {
        printf("%d\n\n", generate());
        printf("%d\n\n", generate());
        printf("%d\n\n", generate());
    
        return 0;
    }
    
    
    
    int generate(void)
    {
        int x;
    
        srand(time(NULL));
    
        x = - 100 + rand() % 201;
    
        return x;
    }

  2. #2
    - - - - - - - - oogabooga's Avatar
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    Put srand(time(NULL)) at the beginning of main.
    The cost of software maintenance increases with the square of the programmer's creativity. - Robert D. Bliss

  3. #3
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    Thank you, it worked. Why is this necessary?

  4. #4
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    The random number generator used by rand() isn't really random. It's some complicated calculations that produce a sequence of pseudo-random numbers. It needs a "seed" or starting point for this calculation. That seed is by default always the same number, so you will always get the same sequence. This can actually be advantageous for testing purposes, but is otherwise bad. time(NULL) returns the current system time as an integral value. srand(time(NULL)) changes the seed to be the current system time, so every time you run it, you get a different starting point for the pseudo-random number sequence that rand() produces. Note that time() is only accurate to the second, so if you manage to run your program twice in the same second, you could get the same sequence.

  5. #5
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    Thank you, anduril462.

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