Sort a map by values in C++

This post will discuss how to sort a map by values in C++.

We know that the std::map container sorts its elements by keys by default and not by values. This post provides an overview of some of the available alternatives to accomplish this.

1. Using `std::vector` function

The idea is to convert the std::map into a std::vector of key-value pairs and sort that vector according to the increasing order of its pair’s second value.

#include <iostream>

#include <map>

#include <vector>

#include <algorithm>

typedef std::pair<std::string, int> pair;

int main()

{

// input map

std::map<std::string, int> map = {

{"two", 2}, {"one", 1}, {"four", 4}, {"three", 3}

};

// create an empty vector of pairs

std::vector<pair> vec;

// copy key-value pairs from the map to the vector

std::copy(map.begin(),

map.end(),

std::back_inserter<std::vector<pair>>(vec));

// sort the vector by increasing the order of its pair's second value

// if the second value is equal, order by the pair's first value

std::sort(vec.begin(), vec.end(),

[](const pair &l, const pair &r)

{

if (l.second != r.second) {

return l.second < r.second;

}

return l.first < r.first;

});

// print the vector

for (auto const &pair: vec) {

std::cout << '{' << pair.first << "," << pair.second << '}' << std::endl;

}

return 0;

}

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Output:

{one,1}
{two,2}
{three,3}
{four,4}

2. Using `std::set` function

We can also use std::set instead of std::map. The idea is to insert all the key-value pairs into a std::set constructed using a comparator that orders the pairs by their second value.

#include <iostream>

#include <map>

#include <set>

// The comparison function for sorting the set by increasing order of its pair's

// second value. If the second value is equal, order by the pair's first value

struct comp

{

template<typename T>

bool operator()(const T &l, const T &r) const

{

if (l.second != r.second) {

return l.second < r.second;

}

return l.first < r.first;

}

};

int main()

{

// input map

std::map<std::string, int> map = {

{"two", 2}, {"one", 1}, {"four", 4}, {"three", 3}

};

// create an empty vector of pairs

std::set<std::pair<std::string, int>, comp> set(map.begin(), map.end());

// print the vector

for (auto const &pair: set) {

std::cout << '{' << pair.first << "," << pair.second << '}' << std::endl;

}

return 0;

}

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Output:

{one,1}
{two,2}
{three,3}
{four,4}

3. Using `std::multimap` function

Finally, we can also build another map that uses the original map’s values as keys and the original map’s keys as values. Of course, this will only work if all values in the original map are distinct. If values are not distinct, consider using a std::multimap instead.

#include <iostream>

#include <map>

#include <set>

#include <algorithm>

// Function to convert a `std::map<K, V>` to `std::multimap<V, K>`

template<typename K, typename V>

std::multimap<V, K> invertMap(std::map<K, V> const &map)

{

std::multimap<V, K> multimap;

for (auto const &pair: map) {

multimap.insert(std::make_pair(pair.second, pair.first));

}

return multimap;

}

int main()

{

// input map

std::map<std::string, int> map = {

{"two", 2}, {"one", 1}, {"four", 4}, {"three", 3}

};

// invert the map

std::multimap<int, std::string> multimap = invertMap(map);

// print the multimap

for (auto const &pair: multimap) {

std::cout << '{' << pair.second << "," << pair.first << '}' << std::endl;

}

return 0;

}

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Output:

{one,1}
{two,2}
{three,3}
{four,4}

That’s all about sorting a map by values in C++.

Sort a map by values in C++

1. Using std::vector function

2. Using std::set function

3. Using std::multimap function

Share this:

1. Using `std::vector` function

2. Using `std::set` function

3. Using `std::multimap` function