mode and map ? ? ?

[xXBurnt]

I am struggling with a project right now and I was wondering if there was a quick way to find multiple modes in a map. So far I have a function that returns a vector of one or many modes depending on the map. I can't seem to wrap my head around how to test it->second against other seconds to find the one that occurs most often.

vector<double> mode(vector<double>& input)
{
map<double, double> tester;
vector<double> mode;

for (map<double, double>::size_type i = 0; i < input.size(); ++i)
{
++tester[input[i]];
}

for (map<double, double>::iterator it = tester.begin(); it != teseter.end(); ++it)
//if it->second is greater than or equal to other seconds, add to vector

return modes;
}

[LB]

Flip your map into a std::multimap - that is, make the values the keys and vice versa, and the map will sort it out for you (literally).

[JLBorges]

Brute force.

Values of type int instead of double to side-step the complications involved in meeting C++ order-relation requirements with floating point values.

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

std::vector<int> mode( const std::vector<int>& input )
{
    if( input.empty() ) return {} ;

    std::map< int, int > frequency_map ;
    for( int i : input ) ++frequency_map[i] ;

    std::map< int, std::vector<int>, std::greater<int> > reverse_frequency_map ;
    for( const auto& pair : frequency_map )
        reverse_frequency_map[pair.second].push_back(pair.first) ;

    return reverse_frequency_map.begin()->second ;
}

int main()
{
   for( int v : mode( { 7, 1, 2, 6, 3, 2, 7, 7, 6, 3, 4, 5, 2, 4, 3, 1, 6, 7, 2, 3 } ) )
       std::cout << v << ' ' ;
   std::cout << '\n' ;
}

http://coliru.stacked-crooked.com/a/186d02a6381a5a70