rand() generating same number upon compilation

Question

Possible Duplicate:
What's the Right Way to use the rand() Function in C++?

I've been learning how to use the rand() function and I wrote a small guessing game in C++ as seen bellow, but the problem is, no matter how many times I compile the programs the generated number is the same -> 41

#include <iostream>
#include <cstdlib>
#include <conio.h>
using namespace std;

int main()
{
    int x = rand()%100;
    int y=0;
    cout << "Ghiceste numarul!" << endl;
    cin >> y;

    while(y != x) {

         if(y > x) {
            cout << "Numarul tau este prea mare! Incearca un numar mai mic!" << endl;
            cin >> y;
          }

             if(y < x) {
                 cout << "Numarul tau este prea mic!" << endl;
                 cin >> y;
               }

      if (y == x) {
      cout << "FELICITARI, AI GHICIT NUMARUL!\n";
      return 0;
      }
    }
}

I also tried to change the max value of rand() and it did change as long as I put it < 41.

Any ideas? I don't have a clue as to why this is happening. I am using CodeBlocks IDE and I tried rebuilding (CTRL+F11)

Solution 1

Try adding

srand(time(0));

at the beginning of main.

Solution 2

You should try first to initialize a seed for the rand() function as follows:

srand (time(NULL))

at the beginning of main. Make sure to include the time.h in the header

#include <time.h>

or

#include <ctime>

Solution 3

it's probably using the same seed each time for the random number generator. if you set the seed of the random number generator each time to a different value, you will get different numbers. according to the docs:

In order to generate random-like numbers, srand is usually initialized to some distinctive value, like those related with the execution time. For example, the value returned by the function time (declared in header ) is different each second, which is distinctive enough for most randoming needs.

Comments

[spencercw]

Do this, but be aware you only need to do it once at the beginning of your program. Don't do it every time you call rand().

[Brendan Long]

And be aware of the caveat: If you run your program more than once per second, it will re-use random seeds (and give you the same numbers every time you run the program until a second passes).

[Adam S]

Both good points. Be sure to check out the SO post referenced in tenfour's comment to your question.