STL 排序不对我的向量进行排序<pair<int,int> &gt; 好吧

Question

I am trying to solve a more complex algorithmic problem and a partial requirement involves sorting some pairs of integers.

Here is my code ( I've commented the irellevant part for my question)

#include <fstream>
#include <vector>
#include <queue>
#include <algorithm>
using namespace std;

ifstream fin("ai.in");
ofstream fout("ai.out");

#define MaxN 1001
#define MaxK 150001
//
//int n;
//int t1, t2, s1, s2, s3, s4, r1, r2, r3, r4;
int k;
int x,y;

vector<pair<int,int> >v(MaxK);

int main()
{
//    fin >> n;
//
//    fin >> t1 >> t2 >> s1 >> s2 >> s3 >> s4 >> r1 >> r2 >> r3 >> r4;

    fin >> k;

    for ( int i = 1; i <= k; ++i )
    {
        fin >> x >> y;
        v[i].first = x;
        v[i].second = y;
    }

    sort(v.begin() + 1, v.begin() + n + 1);

    for ( int i = 1; i <= k; ++i, fout << '\n' )
        fout << v[i].first << ' ' << v[i].second;

    fin.close();
    fout.close();
    return 0;
}

For this input

8
1 2
2 3
2 5
4 2
6 2
2 2
2 4
5 2

I get this output

1 2
2 2
2 3
2 5
4 2
6 2
2 4
5 2

Which of course is wrong as you can see the 2 4 pair near the end of output. So how can I solve this?

Answer 1

This part of the program

fin >> k;

for ( int i = 1; i <= k; ++i )
{
    fin >> x >> y;
    v[i].first = x;
    v[i].second = y;
}

sort(v.begin() + 1, v.begin() + n + 1);

where it is not clear what variable n means

Should look the following way

fin >> k;

if ( MaxK < k ) k = MaxK;

for ( int i 0 1; i < k; ++i )
{
    fin >> x >> y;
    v[i].first = x;
    v[i].second = y;
}

sort( v.begin(), v.begin() + k );

The last statement can bewriten also like

sort( v.begin(), std::next( v.begin(), k ) );

provided that header <iterator> will be included

You may set also the range like [1, k + 1 ) but from your post it is not clear why the frist element of the vector should not be filled.

It seems that you get an unexpected result due to setting invalid upper bound of the range.

Answer 2

You probably provide wrong end iterator to sort, as you can see last two entries from your input are the same as in output - sort simply does not include them in sorting.

std::sort will sort pairs, first by first element - and if both are equal it will compare second.