sorting of pairs by first element

Question

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

 struct myclass
{
    bool operator()(pair<int,int> &left, pair<int,int> &right)
    {
        return (left.first < right.first);
    }
 } x;

int main()
{
     int i;
     vector<pair<int,int> > a;
    a.push_back(make_pair(1,2));
    a.push_back(make_pair(1,0));
    a.push_back(make_pair(2,2));
    a.push_back(make_pair(2,1));
     a.push_back(make_pair(3,2));
    a.push_back(make_pair(1,5));

sort(a.begin(),a.end(),x);
for(i=0;i<6;i++)
{
    printf("%d %d\n",a[i].first,a[i].second);
}
return 0;
}

what is the problem with my code sorting vector of pairs by first element ? It is giving [Error] expected primary-expression before 'x' .

You can get some hints here [How do I sort a vector of pairs based on the second element of the pair](http://stackoverflow.com/questions/279854/how-do-i-sort-a-vector-of-pairs-based-on-the-second-element-of-the-pair) — Bo Persson, Mar 09 '16 at 07:30
Your code, as it currently standards does not produce that error; http://coliru.stacked-crooked.com/a/755a9f0655f68cf9 — Niall, Mar 09 '16 at 07:36
In my compiler It is giving [Error] expected primary-expression before 'x' . — Shivam Chauhan, Mar 09 '16 at 07:43
Which means you are using Orwell Dev-C++ which in uses recent versions of `g++`, the same is used by coliru. — legends2k, Mar 09 '16 at 07:51
Btw, `std::pair` already has a lexicographical order, so in presented case, you may use default comparer. — Jarod42, Mar 09 '16 at 09:25

legends2k · Accepted Answer · 2016-03-09T08:10:33.940

1

Live example.

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

using namespace std;
using IntPair = pair<int, int>;

int main()
{
    vector<IntPair> a = {
        make_pair(1,2),
        make_pair(1,0),
        make_pair(2,2),
        make_pair(2,1),
        make_pair(3,2),
        make_pair(1,5)
    };

    sort(a.begin(), a.end(), [](const IntPair &left, const IntPair &right)
         {
             return (left.first < right.first);
         });

    for(auto const & i : a)
    {
        cout << i.first << ' ' << i.second << '\n';
    }
}

You don't need the class you have defined nor the global variable x. With modern C++, you don't have to define a functionoid or a functor explicitly, just use a lambda.

edited Mar 09 '16 at 08:10

answered Mar 09 '16 at 07:33

legends2k

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sir but why it gives error in my solution " expected primary-expression before 'x'" – Shivam Chauhan Mar 09 '16 at 07:41
Sorry, I got confused by the "You don't need the class" and the long line of code. – juanchopanza Mar 09 '16 at 08:09
No problem, I didn't add much explanation to the code, so partly my bad too :) – legends2k Mar 09 '16 at 08:09

sorting of pairs by first element

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