std::random_shuffle produce the same result even though srand(time(0)) called once

Question

In a function, I want to generate a list of numbers in range: (This function will be called only once when executing the program.)

void DataSet::finalize(double trainPercent, bool genValidData)
{
    srand(time(0));
    printf("%d\n", rand());

    // indices = {0, 1, 2, 3, 4, ..., m_train.size()-1}
    vector<size_t> indices(m_train.size());
    for (size_t i = 0; i < indices.size(); i++)
        indices[i] = i;

    random_shuffle(indices.begin(), indices.end());
// Output
    for (size_t i = 0; i < 10; i++)
        printf("%ld ", indices[i]);
    puts("");

}

The results are like:

850577673
246 239 7 102 41 201 288 23 1 237 

After a few seconds:

856981140
246 239 7 102 41 201 288 23 1 237 

And more:

857552578
246 239 7 102 41 201 288 23 1 237

Why the function rand() works properly but `random_shuffle' does not?

Adallard

possible duplicate of How to make sure that std::random_shuffle always produces a different result? – Ternary

You should only call srand() once, at the beginning of your program. Also, see this, and this – Ternary

@JonathonReinhart, not a duplicate because he called srand once. Right? – Lindsy

@PaulDraper Once every time he ran it. Also, there's lots of other advice on that question and the others I've linked. I can only mark one duplicate. – Ternary

@JonathonReinhart Thanks. I've read the advice. But I cannot figure out why rand() works and random_shuffle() doesn't seem to have any associations with srand(). – Adallard

I move srand() call into main function, but the results are similar. – Adallard

Can you see the example in here: cplusplus.com/reference/algorithm/random_shuffle I tried it and it gives me a different vector each time. And it uses srand(). – Whitelivered

@Whitelivered Yes, it works. The lambda-expr version: random_shuffle(begin(indices), end(indices), [](int n) { return rand() % n; }); produces different result, but if I change my random generator to default_random_engine(), then the problems come. So the question becomes that why default_random_engine() not works. – Adallard

@YangDawei: I modified the sample I cited to print the vector after the call to random_shuffle() with built-in random number generator. It also prints different vector each time. I'm using MinGW g++ v4.8.1 to compile the code on a Windows machine. – Whitelivered

Answer 1

random_shuffle() isn't actually specified to use rand() and so srand() may not have any impact. If you want to be sure, you should use one of the C++11 forms, random_shuffle(b, e, RNG) or shuffle(b, e, uRNG).

An alternative would be to use random_shuffle(indices.begin(), indices.end(), rand()); because apparently your implementation of random_shuffle() is not using rand().

Engle

Yes, I force the random_shuffle to use rand() so that it works properly. Instead of random_shuffle(indices.begin(), indices.end(), rand()), I use random_shuffle(begin(indices), end(indices), [](int n) { return rand() % n; }) because the previous one produce errors. I use cmake installed by macports, with CXX flags -std=c++11. My g++ is clang-500.2.79, but I don't know what compiler is used exactly by cmake. – Adallard

@YangDawei: you can, for example, message("${CMAKE_CXX_COMPILER}") in your CMakeLists.txt to find out which compiler is used. – Offset