Question

I’m doing some programming practice right now by trying to sort a 2D array on it’s first “column”.

I am reading input from a file:

100 5
8 80
5 20
9 40
3 10
6 30

This is my code:

#include <cstdio>
#include <iostream>
#include <fstream>
#include <vector>
#include <map>
#include <algorithm>

using namespace std;

bool helper(vector<long> k, vector<long> l)
{
   return (k[0] < l[0]);
}

int main()
{
   ifstream fi("milk.in");
   ofstream fo("milk.out");
   long price = 0, n, m, i, p, a;
   vector< vector<long> > farmers;
   vector<long> farmer(2,0);
   fi >> n >> m;

   for (i=0; i<n; ++i)
   {
      fi >> p >> a;
      farmer[0] = p;
      farmer[1] = a;
      farmers.push_back(farmer);
   }


   sort(farmers.begin(),farmers.end(),helper);


   for (i=0; i<m; ++i)
   {
      cout << farmers[i][0] << " " << farmers[i][1] << endl;
   }

   return 0;
}

As you can see, I try to sort the input by it’s first column (I don’t care about the first line at the moment).

However, this is the result:

3 10
5 20
6 30
6 30
6 30

This is the expected result:

3 10
5 20
6 30
8 80
9 40

I can’t figure it out.

Answer 1

Your first line of your milk.in:

100 5

You’re going to end up looping through trying to read in 100 inputs from this file because n = 100.

fi >> n >> m;
for (i=0; i<n; ++i)

If you change milk.in to:

5 5
8 80
5 20
9 40
3 10
6 30

That seems to work.

Perhaps a better idea is to just check if you’re done reading input from the filestream:

for(i = 0; i < n; ++i)
{
    if(!(fi >> farmer[0] >> farmer[1])) break;

    farmers.push_back(farmer);
}