Thread: the rand() function

ok y is it that when i try do do something like this:


#include <iostream>
#include <string>

using namespace std;

int rand();

int main ()
{
cout << rand();

return 0;
}

it outputs the same number every time? i dont get it. i thought it was supposed to be random.

the rand function is psuedo-random.

Which means it appears to be random but actually isn't, if you do it a few more times in your program and run two or more times the same numbers are printed.

To get a more random generation of numbers, you have to seed the rand function with a number example

Code:

#include <iostream>
#include <cstdlib>
#include <ctime>

int main() {
    srand(time(NULL));  // this line seed's the rand function
    for (int i=0; i <10 ; i++){
        cout << rand();
    }
    return 0;
}

hope this helps


PS the rand function without seeding it is normally used to test side effects of programs and there correctness.

great. i get it now. thanks buddy.

Well, you see large programs that use "random" data use the psuedo-random numbers to test that the algorithm works. Because they know what numbers to expect. And can test the outcome. So when they know the algorithm works they can use "real random" numbers in the program. And trust it's results. Hope that helps.

kwigibo

that's a good way to do it

Originally posted by Unregistered
ok y is it that when i try do do something like this:


#include <iostream>
#include <string>

using namespace std;

int rand();

int main ()
{
cout << rand();

return 0;
}

it outputs the same number every time? i dont get it. i thought it was supposed to be random.

There is no need to declare rand before main. In fact I wouldnt have expected it to compile like that, shows what I know

it's a waste of filesize, because you are basically prototyping a prototype.