Sorting multidimensional array C++ strange behaviour - c++

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);
}

Answer 2

You need this loop to load the data from the file in:
And remember to close the file.
while(!fi.eof())
{
fi >> n >> m;
farmer[0] = n;
farmer[1] = m;
farmers.push_back(farmer);
}
fi.close( );
Also, I made some minor changes to your 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()
{
long price = 0, n, m, i, p=0, a=0,numlines=0;
vector< vector<long> > farmers;
vector<long> farmer(2,0);
cout<<"\nLoading data from file: milk.in\n\n";
ifstream fi("milk.in");
while(!fi.eof())
{
fi >> n >> m;
numlines++;
cout<<n<<" "<<m<<"\n";
farmer[0] = n;
farmer[1] = m;
farmers.push_back(farmer);
}
fi.close( );
cout<<"--- "<<numlines<<" lines loaded\n";
cout<<"\n---------------------\n";
cout<<"\nSorted data:\n\n";
sort(farmers.begin(),farmers.end(),helper);
ofstream fo("milk.out");
for (i=0; i<numlines; ++i)
{
cout << farmers[i][0] << " " << farmers[i][1] << endl;
fo<< farmers[i][0] << " " << farmers[i][1] << endl;
}
fo.close();
cout<<"\n---------------------\n";
return 0;
}
Output:
Sorted data:
3 10
5 20
6 30
8 80
9 40
100 5