Random element in a map

Question

what is a good way to select a random element from a map? C++. It is my understanding that maps don't have random access iterators. The key is a long long and the map is sparsely populated.

Answer 1

map<...> MyMap;
iterator item = MyMap.begin();
std::advance( item, random_0_to_n(MyMap.size()) );

Answer 2

I like James' answer if the map is small or if you don't need a random value very often. If it is large and you do this often enough to make speed important you might be able to keep a separate vector of key values to select a random value from.

map<...> MyMap;
vector<...> MyVecOfKeys; // <-- add keys to this when added to the map.

map<...>::key_type key = MyVecOfKeys[ random_0_to_n(MyVecOfKeys.size()) ];
map<...>::data_type value = MyMap[ key ];

Of course if the map is really huge you might not be able to store a copy of all the keys like this. If you can afford it though you get the advantage of lookups in logarithmic time.

Answer 3

Maybe you should consider Boost.MultiIndex, although note that it's a little too heavy-weighted.

Answer 4

Maybe draw up a random key, then use lower_bound to find the closest key actually contained.

Answer 5

Here is the case when all map items must be access in random order.

1. Copy the map to a vector.
2. Shuffle vector.

In pseudo-code (It closely reflects the following C++ implementation):

import random
import time

# populate map by some stuff for testing
m = dict((i*i, i) for i in range(3))
# copy map to vector
v = m.items()
# seed PRNG   
#   NOTE: this part is present only to reflect C++
r = random.Random(time.clock()) 
# shuffle vector      
random.shuffle(v, r.random)
# print randomized map elements
for e in v:
    print "%s:%s" % e, 
print

In C++:

#include <algorithm>
#include <iostream>
#include <map>
#include <vector>

#include <boost/date_time/posix_time/posix_time_types.hpp>
#include <boost/foreach.hpp>
#include <boost/random.hpp>

int main()
{
  using namespace std;
  using namespace boost;
  using namespace boost::posix_time;

  // populate map by some stuff for testing
  typedef map<long long, int> Map;
  Map m;
  for (int i = 0; i < 3; ++i)
    m[i * i] = i;

  // copy map to vector
#ifndef OPERATE_ON_KEY
  typedef vector<pair<Map::key_type, Map::mapped_type> > Vector;
  Vector v(m.begin(), m.end());
#else
  typedef vector<Map::key_type> Vector;
  Vector v;
  v.reserve(m.size());
  BOOST_FOREACH( Map::value_type p, m )
    v.push_back(p.first);
#endif // OPERATE_ON_KEY

  // make PRNG
  ptime now(microsec_clock::local_time());
  ptime midnight(now.date());
  time_duration td = now - midnight;
  mt19937 gen(td.ticks()); // seed the generator with raw number of ticks
  random_number_generator<mt19937, 
    Vector::iterator::difference_type> rng(gen);

  // shuffle vector
  //   rng(n) must return a uniformly distributed integer in the range [0, n)
  random_shuffle(v.begin(), v.end(), rng);

  // print randomized map elements
  BOOST_FOREACH( Vector::value_type e, v )
#ifndef OPERATE_ON_KEY
    cout << e.first << ":" << e.second << " ";
#else
    cout << e << " ";
#endif // OPERATE_ON_KEY
  cout << endl;
}

Answer 6

Continuing ryan_s theme of preconstructed maps and fast random lookup: instead of vector we can use a parallel map of iterators, which should speed up random lookup a bit.

map<K, V> const original;
...

// construct index-keyed lookup map   
map<unsigned, map<K, V>::const_iterator> fast_random_lookup;
map<K, V>::const_iterator it = original.begin(), itEnd = original.end();
for (unsigned i = 0; it != itEnd; ++it, ++i) {
  fast_random_lookup[i] = it;
}

// lookup random value
V v = *fast_random_lookup[random_0_to_n(original.size())];

Answer 7

If your map is static, then instead of a map, use a vector to store your key/value pairs in key order, binary search to look up values in log(n) time, and the vector index to get random pairs in constant time. You can wrap the vector/binary search to look like a map with a random access feature.