CODEWITHSUNDEEP
c++randomrandom-seed
Akiiino
Akiiino

Reputation: 1090

std::random_shuffle not being seeded

#include <iostream>
#include <vector>
#include <algorithm>
#include <cstdlib>


int main() {
    std::vector<short> a(256);
    for (short x = 0; x != 256; ++x) {
        a[x] = x;
    }
    for (auto x : a) { std::cout << x << ' '; } std::cout << std::endl;
    std::cout << std::endl;

    std::srand(11);

    std::random_shuffle(a.begin(), a.end());
    for (auto x : a) { std::cout << x << ' '; } std::cout << std::endl;
    std::cout << std::endl;

    for (short x = 0; x != 256; ++x) {
        a[x] = x;
    }
    for (auto x : a) { std::cout << x << ' '; } std::cout << std::endl;
    std::cout << std::endl;

    std::srand(11);

    std::random_shuffle(a.begin(), a.end());
    for (auto x : a) { std::cout << x << ' '; } std::cout << std::endl;
}

So, here is my code. What I expect is, obviously, the same shuffle both times. What I get is, while the shuffles are consistent between launches, they are different and seem to ignore srand! What am I doing wrong here?

Upvotes: 2

Views: 493

Answers (2)

songyuanyao
songyuanyao

Reputation: 172894

Note that for std::random_shuffle what random number generator is used is implementation-defined, it's not guaranteed to use std::rand.

You could use std::shuffle instead and pass it a random number generator:

std::random_device rd;
std::mt19937 g(rd());
std::shuffle(a.begin(), a.end(), g);

LIVE

Upvotes: 2

Some programmer dude
Some programmer dude

Reputation: 409166

First note that the version of std::random_shuffle you use has been deprecated.

Also note (from the previous reference link)

...the function std::rand is often used.

The keyword here being often and not always.

If you want to make sure the same sequence is always created, then you should use one of the alternatives, passing either a specific random-number function, or use the std::shuffle function passing a generator (from the C++11 "new" PRNG classes).

Upvotes: 2

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