Sort unordered_map by value c++

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sort unordered_map by value c++

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To sort an unordered_map by value in C++, you can follow these steps:

  1. Create a std::vector of pairs to store the key-value pairs from the unordered_map.
  2. Copy the elements from the unordered_map to the std::vector.
  3. Use the std::sort algorithm from the <algorithm> library to sort the std::vector based on the values.
  4. If you want the elements in descending order, you can pass a custom comparator function or use std::greater as the third argument in the std::sort function.
  5. After sorting, you can create a new std::unordered_map and populate it with the sorted elements from the std::vector.

Here’s an example code snippet that demonstrates how to sort an unordered_map by value in C++:

#include <iostream>
#include <unordered_map>
#include <vector>
#include <algorithm>

bool compare(const std::pair<int, int>& a, const std::pair<int, int>& b) {
    return a.second < b.second; // Sort in ascending order based on values
}

int main() {
    std::unordered_map<int, int> myMap = {{1, 5}, {2, 3}, {3, 10}, {4, 1}, {5, 7}};

    std::vector<std::pair<int, int>> sortedVector(myMap.begin(), myMap.end());

    std::sort(sortedVector.begin(), sortedVector.end(), compare);

    std::unordered_map<int, int> sortedMap;
    for (const auto& pair : sortedVector) {
        sortedMap.insert(pair);
    }

    for (const auto& pair : sortedMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}

In this example, we have an unordered_map called myMap with integer keys and values. We create a std::vector called sortedVector and populate it with the key-value pairs from myMap.

We then use the std::sort function to sort the sortedVector based on the values using the custom comparator function compare. Finally, we create a new std::unordered_map called sortedMap and populate it with the sorted elements from the sortedVector.

The output of the program will be:

4: 1
2: 3
1: 5
5: 7
3: 10

which is the unordered_map sorted in ascending order based on values.