How Can I Sort a Vector of Pairs Based on the Second Element in C ?

Sorting Vectors of Pairs Using the Second Element

Sorting vectors of pairs based on the second element is a common programming task. This article explores various methods to accomplish this using existing STL components and std::less.

Method 1: Custom Comparator

A custom comparator is a common approach. Create a struct with an overridden operator() function that compares pairs based on their second elements. For example:

struct sort_pred {
    bool operator()(const std::pair<int, int>& left, const std::pair<int, int>& right) {
        return left.second < right.second;
    }
};

Then, call sort() with this comparator as the third argument:

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

Method 2: Lambda Expression (C 11)

In C 11, you can use a lambda expression to define the comparison function:

std::sort(vec.begin(), vec.end(), [](const std::pair<int, int>& left, const std::pair<int, int>& right) {
    return left.second < right.second;
});

Method 3: Generic Template-Based Comparator (C 11)

For reusable code, create a generic template-based comparator:

template <class T1, class T2, class Pred = std::less<T2>>
struct sort_pair_second {
    bool operator()(const std::pair<T1, T2>& left, const std::pair<T1, T2>& right) {
        Pred p;
        return p(left.second, right.second);
    }
};

Then, call sort() with this template-based comparator:

std::sort(vec.begin(), vec.end(), sort_pair_second<int, int>());

Recommendation

For reusability, the generic template-based comparator is a powerful option. However, for simplicity, writing a custom comparator or lambda expression is often sufficient.

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