CODEWITHSUNDEEP
c++sortingvectorstl
Ildar Zaripov
Ildar Zaripov

Reputation: 551

How to sort vector < pair <int, int> >?

I would like to sort vector by its second element. The code is following:

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;

bool compare(const pair<int,int>&i, const pair<int,int>&j){
    return i.second < j.second;
}

int main()
{
    vector<pair<int,int> >a(100);

    // values for test
    a[0].first=0; a[0].second=21;
    a[1].first=1; a[1].second=100;
    a[2].first=2; a[2].second=100;
    a[3].first=3; a[3].second=30;
    a[4].first=4; a[4].second=17;

    sort(a.begin(),a.end(),compare);

    for(int i=0;i<5;i++)
        cout << a[i].second << " " << a[i].first << endl;
}

The program returns

0 0
0 0
0 0
0 0
0 0

Where I made a mistake?

Upvotes: 0

Views: 1458

Answers (3)

HelloWorld123456789
HelloWorld123456789

Reputation: 5359

Since the rest 95 elements are not assigned, they are by default (0,0). So you need to sort the first 5 elements only.

sort(a.begin(),a.begin()+5,compare);

Upvotes: 3

Ivaylo Strandjev
Ivaylo Strandjev

Reputation: 70931

The sorting of a vector<pair<int, int> > works just the way you use it. However you have another problem. This code - vector<pair<int,int> >a(100); creates a vector of size 100 and fills it with pairs of (0,0). Thus when you sort it the first 5 elements are (0,0).

Upvotes: 3

dlf
dlf

Reputation: 9373

Your vector is of size 100, but you only populated 5 elements. The rest will all be (0, 0), which your compare() correctly puts at the front.

Upvotes: 2

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