Identical Random Numbers Generated in C++ - c++

Question

I've been banging my head against this for half an hour now, and have NO idea what's wrong.
I'm trying to generate a list of 10 random numbers, 1-100. But when I run it they all come out the same number. It is very frustrating! I thought that it was because the number was still stored in the RAM, but after re-randomizing the random number and the variable three times, it still comes out with the same number. What am I doing wrong?
Code:
main() {
int i;
int randnum;
srand(time(NULL));
randnum = rand() % 2;
for (i = 0; i < 10; i++) {
srand(time(NULL));
randnum = rand() % 100 + 1;
srand(time(NULL));
rand();
list[i] = randnum;
srand(time(NULL));
randnum = rand() % 100 + 1;
srand(time(NULL));
rand();
}
srand(time(NULL));
randnum = rand() % 100 + 1;
}

Answer 1

Don't call srand() more than once. This code likely takes less than a second to execute, so every time you call srand(time(NULL)) when time is measured in seconds on your implementation, you just reset the pseudo random number generator to the same seed, so all your numbers come out the same.

Answer 2

Don't reinitialize the generator with srand(time(NULL)). Use it only once, at the beginning of your code.

Answer 3

What you're doing wrong is that you're resetting the random number generator's state.
The reason its not obvious is because you're using time. time returns time_t, which, according to the standard is "the implementation’s best approximation to the current calendar time". This generally represents the number of seconds since 00:00 hours, Jan 1, 1970 UTC. Now, your code will likely execute within a millisecond, so all your time calls return the same value.
So your code is equivalent to:
int const somenum = time(NULL);
srand(somenum); //reset state using some seed.
//rand() will always produce the same value after an
// srand call of the same seed.
randnum = rand() % 100 + 1;
srand(somenum); //reset state using some seed.
randnum = rand() % 100 + 1;
srand(somenum); //reset state using some seed.
randnum = rand() % 100 + 1;
srand(somenum); //reset state using some seed.
randnum = rand() % 100 + 1;
To test this, wait for a keypress between each call to rand and you will see they are different.
The way to fix this is to only call srand(time(NULL)) once, at the start.
Now, in C++11, there is another way:
#include <iostream>
#include <random>
int main()
{
const int rand_max = 20;
std::default_random_engine rng(std::random_device{}());
std::uniform_int_distribution<> dist(0, rand_max);
std::cout<<"This will always be as random a number as your hardware can give you: "<<dist(rng)<<std::endl;
return 0;
}
std::random_device makes use of a built in hardware random number generator if available, so you don't have to worry about seeding with time. If you really do want a pseudo random number, then just use a different random number generator.
You can also control the random number distribution in C++11.