How to Sort Map by Value in C++: Simple Guide

In C++, you cannot directly sort a std::map by its values because it sorts by keys by default. Instead, copy the map's key-value pairs into a std::vector of pairs and then sort this vector using a custom comparator that compares the values.

Syntax

To sort a map by value, follow these steps:

• Copy the map elements into a std::vector<std::pair<Key, Value>>.
• Use std::sort with a custom comparator that compares the second element (value) of each pair.
• Iterate over the sorted vector to access elements in value order.

cpp

std::map<Key, Value> myMap;
std::vector<std::pair<Key, Value>> vec(myMap.begin(), myMap.end());

std::sort(vec.begin(), vec.end(), [](const auto &a, const auto &b) {
    return a.second < b.second; // sort by value ascending
});

Example

This example shows how to sort a std::map<std::string, int> by its values in ascending order and print the sorted pairs.

cpp

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

int main() {
    std::map<std::string, int> myMap = {
        {"apple", 5},
        {"banana", 2},
        {"cherry", 7},
        {"date", 3}
    };

    // Copy map to vector
    std::vector<std::pair<std::string, int>> vec(myMap.begin(), myMap.end());

    // Sort vector by value
    std::sort(vec.begin(), vec.end(), [](const auto &a, const auto &b) {
        return a.second < b.second;
    });

    // Print sorted vector
    for (const auto &pair : vec) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}

Output

banana: 2
date: 3
apple: 5
cherry: 7

Common Pitfalls

Common mistakes when sorting a map by value include:

• Trying to sort the std::map directly, which is impossible because it is always sorted by keys.
• Not copying elements to a vector first.
• Forgetting to use a custom comparator that compares the values (second element of pairs).
• Modifying the map while iterating over the sorted vector, which can cause inconsistencies.

cpp

/* Wrong approach: Trying to sort map directly (won't compile) */
// std::sort(myMap.begin(), myMap.end(), [](const auto &a, const auto &b) {
//     return a.second < b.second;
// });

/* Correct approach: Copy to vector and sort */
std::vector<std::pair<Key, Value>> vec(myMap.begin(), myMap.end());
std::sort(vec.begin(), vec.end(), [](const auto &a, const auto &b) {
    return a.second < b.second;
});

Quick Reference

Summary tips for sorting a map by value in C++:

• Use std::vector to hold map entries.
• Sort with std::sort and a lambda comparing pair.second.
• Remember std::map itself cannot be reordered by value.
• Use this method when you need ordered access by values.

Key Takeaways

You cannot sort a std::map by value directly because it sorts by keys.

Copy map entries into a std::vector of pairs to sort by value.

Use std::sort with a custom comparator comparing the second element of pairs.

Iterate over the sorted vector to access elements in value order.

Avoid modifying the map while using the sorted vector to prevent inconsistencies.