reading pairs of data from a file - c++

Question

I am trying to read pairs of data from a file using the following code.
#include <iostream>
#include <fstream>
#include <vector>
using namespace std;
int main()
{
//**** Opening data file ****
ifstream infile;
infile.open("reg_data_log.inp");
if (infile.is_open())
{
cout << "File successfully open" << endl;
}
else
{
cout << "Error opening file";
}
//**** Reading data ***
vector<pair<double, double> > proteins;
pair<double, double> input;
while (infile >> input.first >> input.second)
{
proteins.push_back(input);
}
//**** Show data in screen ****
cout << "Proteins analized: " << proteins.size() << endl;
for(unsigned int i=0; i<proteins.size(); i++)
{
cout << i.first << ", " << i.second << endl;
}
}
when compiling I have the following message
"65:13: error: request for member ‘first’ in ‘i’, which is of non-class type ‘unsigned int’"
"65:13: error: request for member ‘first’ in ‘i’, which is of non-class type ‘unsigned int’"
I cannot get my head around the problem. Can anybody help?

Answer 1

Look further at your loop
for(unsigned int i=0; i<proteins.size(); i++)
{
cout << i.first << ", " << i.second << endl;
}
You are trying to access the property first of the integer value i. integers do not have that property it is a property of a pair object.
I think you are getting confused between iterating using iterator and indices. The simple fix is to use a range based for loop, as already suggested in comments.
for(auto d: proteins)
{
cout << d.first << ", " << d.second << endl;
}
Now what you have as is the element from the vector and not an integer. You can now access first and second.
If you cannot use range based for loop and auto, then you can use the old iterator for loop way.
for(vector<pair<double, double> >::iterator it = proteins.begin();
it != proteins.end();
++it)
{
cout << it->first << ", " << it->second << endl;
}
and someone else has already posted the how it can be done using indices, so I won't repeat that here.

Answer 2

As John Mopp mentions in his comment, indeed you arev referencing an int and not a pair-type. The following loop will most likely fix your issues:
cout << "Proteins analized: " << proteins.size() << endl;
for(unsigned int i=0; i<proteins.size(); i++)
{
cout << proteins[i].first << ", " << proteins[i].second << endl;
}
I did not test this yet, but I am pretty convinced this solves your problem.