How to shuffle a std::vector?

Question

I am looking for a generic, reusable way to shuffle a std::vector in C++. This is how I currently do it, but I think it's not very efficient because it needs an intermediate array and it needs to know the item type (DeckCard in this example):

srand(time(NULL));  cards_.clear();  while (temp.size() > 0) {     int idx = rand() % temp.size();     DeckCard* card = temp[idx];     cards_.push_back(card);     temp.erase(temp.begin() + idx); } 

Answer 1

From C++11 onwards, you should prefer:

#include <algorithm> #include <random>  auto rng = std::default_random_engine {}; std::shuffle(std::begin(cards_), std::end(cards_), rng); 

Live example on Coliru

Make sure to reuse the same instance of rng throughout multiple calls to std::shuffle if you intend to generate different permutations every time!

Moreover, if you want your program to create different sequences of shuffles each time it is run, you can seed the constructor of the random engine with the output of std::random_device:

auto rd = std::random_device {};  auto rng = std::default_random_engine { rd() }; std::shuffle(std::begin(cards_), std::end(cards_), rng); 

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For C++98 you may use:

#include <algorithm>  std::random_shuffle(cards_.begin(), cards_.end()); 

Answer 2

http://www.cplusplus.com/reference/algorithm/shuffle/

// shuffle algorithm example #include <iostream>     // std::cout #include <algorithm>    // std::shuffle #include <vector>       // std::vector #include <random>       // std::default_random_engine #include <chrono>       // std::chrono::system_clock  int main ()  {     // obtain a time-based seed:     unsigned seed = std::chrono::system_clock::now().time_since_epoch().count();     std::default_random_engine e(seed);      while(true)     {       std::vector<int> foo{1,2,3,4,5};        std::shuffle(foo.begin(), foo.end(), e);        std::cout << "shuffled elements:";       for (int& x: foo) std::cout << ' ' << x;       std::cout << '\n';     }      return 0; }