Random element in a map

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面向向阳花 2021-01-07 16:14

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

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遥遥无期 (楼主)

2021-01-07 16:41

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

Copy the map to a vector.
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 
#include 
#include 
#include 

#include 
#include 
#include 

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

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

  // copy map to vector
#ifndef OPERATE_ON_KEY
  typedef vector > Vector;
  Vector v(m.begin(), m.end());
#else
  typedef vector 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 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;
}

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