Question

I have this small program that reads a line of input & prints the words in it, with their respective number of occurrences. I want to sort the elements in the map that stores these values according to their occurrences. I mean, the words that only appear once, will be ordered to be at the beginning, then the words that appeared twice 7 so on. I know that the predicate should return a bool value, but I don’t know what the parameters should be. Should it be two iterators to the map? If some one could explain this, it would be greatly appreciated. Thank you in advance.

#include<iostream>
#include<map>

using std::cout;
using std::cin;
using std::endl;
using std::string;
using std::map;

int main()
{
    string s;
    map<string,int> counters;   //store each word & an associated counter

    //read the input, keeping track of each word & how often we see it
    while(cin>>s)
    {
        ++counters[s];
    }

    //write the words & associated counts
    for(map<string,int>::const_iterator iter = counters.begin();iter != counters.end();iter++)
    {
        cout<<iter->first<<"\t"<<iter->second<<endl;
    }

    return 0;
}

Answer 1

std::map is always sorted according to its key. You cannot sort the elements by their value.

You need to copy the contents to another data structure (for example std::vector<std::pair<string, int> >) which can be sorted.

Here is a predicate that can be used to sort such a vector. Note that sorting algorithms in C++ standard library need a “less than” predicate which basically says “is a smaller than b”.

bool cmp(std::pair<string, int> const &a, std::pair<string, int> const &b) {
  return a.second < b.second;
}