C++ Cookbook

7.3. Randomly Shuffling Data

Problem

You have a sequence of data, and you need to jumble it into some random order.

Solution

Use the random_shuffle standard algorithm, defined in <algorithm>. random_shuffle takes two random-access iterators, and (optionally) a random-number generation functor, and rearranges the elements in the range at random. Example 7-3 shows how to do this.

Example 7-3. Shuffling a sequence at random

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
#include "utils.h" // For printContainer(): see 7.10

using namespace std;

int main() {

   vector<int> v;
   back_insert_iterator<std::vector<int> > p =
      back_inserter(v);

   for (int i = 0; i < 10; ++i)
     *p = i;

   printContainer(v, true);

   random_shuffle(v.begin(), v.end());

   printContainer(v, true);
}

Your output might look like this:

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

Discussion

random_shuffle is intuitive to use. Give it a range, and it will shuffle the range at random. There are two versions, and their prototypes look like this:

void random_shuffle(RndIter first, RndIter last);
void random_shuffle(RndIter first, RndIter last, RandFunc& rand);

In the first version, the “random” is using an implementation-specific random-number generation function, which should be sufficient for most of your needs. If it isn’t—perhaps you want a nonuniform distribution, e.g., Gaussian—you can write your own and supply that instead using the second version.

Your random-number generator must be a functor that ...