CODEWITHSUNDEEP
c++randomsrand
Ritwik Biswas
Ritwik Biswas

Reputation: 1419

Why is srand(time(NULL)) working smoothly even though I repeatedly reset it?

I have a function that creates a vector of size N, and shuffles it:

void rand_vector_generator(int N) {
   srand(time(NULL));
   vector <int> perm(N);
   for (unsigned k=0; k<N; k++) {
      perm[k] = k;
   }
   random_shuffle(perm.begin(),perm.end());
}

I'm calling this from my main function with the loop:

for(int i=0; i<20; i++)
    rand_vector_generator(10);

I expected this to not give me sufficient randomness in my shuffling because I'm calling srand(time(NULL)); with every function call and the seed is not too different from successive call to call. My understanding is that I call srand(time(NULL)); once and not multiple times so the seed doesn't "reset".

This thread somewhat affirms what I was expecting the result to be.

Instead, I get:

6 0 3 5 7 8 4 1 2 9 
0 8 6 4 2 3 7 9 1 5 
8 2 4 9 5 0 6 7 1 3 
0 6 1 8 7 4 5 2 3 9 
2 5 1 0 3 7 6 4 8 9 
4 5 3 0 1 7 2 9 6 8 
8 5 2 9 7 0 6 3 4 1 
8 4 9 3 1 5 7 0 6 2 
3 7 6 0 9 8 2 4 1 5 
8 5 2 3 7 4 6 9 1 0 
5 4 0 1 2 6 8 7 3 9 
2 5 7 9 6 0 4 3 1 8 
5 8 3 7 0 2 1 6 9 4 
7 4 9 5 1 8 2 3 0 6 
1 9 2 3 8 6 0 7 5 4 
0 6 4 3 1 2 9 7 8 5 
9 3 8 4 7 5 1 6 0 2 
1 9 6 5 3 0 2 4 8 7 
7 5 1 8 9 3 4 0 2 6 
2 9 6 5 4 0 3 7 8 1

These vectors seem pretty randomly shuffled to me. What am I missing? Does the srand call somehow exist on a different scope than the function call so it doesn't get reset every call? Or am I misunderstanding something more fundamental here?

Upvotes: 0

Views: 369

Answers (1)

YePhIcK
YePhIcK

Reputation: 5866

According to standard the use of std::rand in both std::random_shuffle and std::shuffle is implementation-defined (though it is often the case that an std::rand is used this is not guaranteed). Try it on another compiler? Another platform?

If you want to make sure the std::rand is used you should let your code use it explicitly (for example, using lambda expression):

random_shuffle(perm.begin(), perm.end(), []{return std::rand();});

On a somewhat unrelated note, the time()'s precision is one whole second, your code runs way faster than that (I would hope) so those multiple calls to srand() result in resetting to the same-ish seed

Upvotes: 3

Related Questions