Writing your custom sort functions in C++

1 minute read


In this blog post I will quickly go over how to define your custom comparator for sort() for your data structure in C++ based on two example cases.

I decided to start programming in C++ again after a long time (end of 2017!!) because I was practising algorithm-style problems for job interviews and C++ seemed like a natural choice. It’s Standard Template Library makes working with data structures very easy by providing a uniform api and inbuild data structures. I noticed that a lot of the questions that I came across required me to overload the comparator operator in C++ sort() function since I was working with custom data structures. Next, I will mention two example cases of such problems and how I went around writing the sort comparator for my requirements.

Case 1. Sort a vector<vector<int>> based on two different conditions

struct contestComparator
    bool operator ()(const vector<int> & contest1, const vector<int> & contest2)
        if(contest1[1] != contest2[1])
            return contest1[1] >  contest2[1]; // consider important contests first
        return contest1[0] < contest2[0];

// Complete the luckBalance function below.
int luckBalance(int k, vector<vector<int>> contests) {
    int luck_balance = 0;
    int len = contests.size();
    sort(contests.begin(), contests.end(), contestComparator());
    int num_important = 0;
    for(int i = 0; i<len && contests[i][1] == 1; i++)
    k = num_important - k;
    for(int i = 0; i < len; i++)
        cout<<contests[i][0]<<" ";
            luck_balance -= contests[i][0];
            luck_balance += contests[i][0];
    return luck_balance;

Case 1. Sort a vector<pair<int, int>> based on the first number in the pair

struct pairComp
    bool operator ()(const pair<int, int> & pair1, const pair<int, int> & pair2)
        return pair1.first < pair2.first;

// Complete the whatFlavors function below.
void whatFlavors(vector<int> cost, int money) {
    vector<pair<int,int>> costidx;
    for(int i=0; i<cost.size(); i++)
        costidx.push_back(make_pair(cost[i], i+1));
    sort(costidx.begin(), costidx.end());
    int i=0;
    int j=cost.size()-1;
        if(costidx[i].first + costidx[j].first > money)
        else if(costidx[i].first + costidx[j].first < money)
    int min = (costidx[i].second < costidx[j].second)? costidx[i].second: 
    int max = (costidx[i].second > costidx[j].second)? costidx[i].second: 
    cout<<min<<" "<<max<<endl;