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Thread: random vector of numbers

  1. #1
    Senior Member
    Join Date
    Apr 2002
    Posts
    417

    random vector of numbers

    hi,

    Im trying to create a vector of random integers in C++. Im trying to seed the rand()
    function w/ the time that the system have been up. but it just gives me a vector of same numbers. running again gives a different set of same numbers. Can anyone tell my how to seed the the rand function correctly

    here's my code

    #################
    #include "iostream"
    #include "vector"
    #include "time.h"
    using namespace std;


    int GiveRandInt(int N); // returns a random integer from 0 to N-1


    int main()
    {

    int NumElements = 10;
    int MaxElementSize = 100;


    vector<int> Selection(NumElements);

    for(unsigned int i = 0; i <= Selection.size() - 1; i++)
    {
    Selection[i] = GiveRandInt( MaxElementSize );
    cout << "element " << i << ": " << Selection[i] <<endl;
    }

    return 0;
    }


    int GiveRandInt(int N)
    {

    srand(time(NULL)); // Seed our random numbers w/ time


    //srand(GetTickCount()); // I saw a place use this but it doesn't work here. Anyone know if I need some special header for this to work?

    int RandNum = rand() % N;
    return RandNum;
    }
    #################

  2. #2

    Re:random vector of numbers

    The reason you are getting the same number every time is that you are reseeding it every time you want to get a random number. You only need to seed it once.

    Put this line at the beginning of main() (and remove it from GiveRandInt()):
    Code:
    srand(time(NULL));
    BTW, the way to get the clock ticks since program start is "clock()" and that is included in <ctime> (the c++ name for <time.h&gt. If you want to use clock ticks along with the time use this line instead:
    Code:
    srand(time(NULL) + clock());

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