rand() always returns the same sequence on application restart

Question

I have the following method which generates a random number:

int random_number() //Random number generator
{
    int x = rand() % 1000000 + 1; //Generate an integer between 1 and 1000000
    return x;
}

The call to this method is used in a loop which iterates five times. The problem with this method is that it always seems to generate the same numbers when running the program several times. How can this be solved?

Answer 1

You need to seed the random number generator, such as:

srand ( time(NULL) );
int x = rand() % 1000000 + 1;

Seeding the pseudorandom number generator essentially decides on the random number set that it will iterate through. Using the time is the standard method of achieving adequately random results.

EDIT:

To clarify, you should seed only once and get many random numbers, something like this:

srand ( time(NULL) );
loop {
    int x = rand() % 1000000 + 1;
}

Rather than something like:

loop {
    //Particularly bad if this line is hit multiple times in one second
    srand ( time(NULL) ); 
    int x = rand() % 1000000 + 1;
}

Answer 2

make a call to srand(time(NULL)); when your program launches.

srand sets a seed to the rand function. Giving it the return value of time(NULL) helps getting a different seed at each program run.

As you tagged your question as c++, you could either use c++11 feature to handle random number generation.