How to Custom Sort a Vector of Pairs Based on the Second Element in C ?

Custom Sorting of Vectors of Pairs

Consider the scenario where you want to sort a vector of pairs (

std::vector<std::pair<int, int>>

) based on the second element of each pair in increasing order.

To achieve this without implementing a separate function object, you can utilize the third parameter of

std::sort

, which accepts a custom comparator. This comparator compares two pairs based on their second elements using the provided comparator.

For example, using a C 11 compiler, you can define a comparator with a lambda expression:

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

Alternatively, define a custom struct that overloads the

operator()

function to perform the comparison:

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

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

If you desire a generic solution that can be reused with different types and comparators, create a template like this:

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);
    }
};

With this template, you can sort pairs using any custom comparator:

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

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