abcdefabcdefabcdefab
1234567 abcdefabcdefabcdefab
9876543 abddefbbbdefcbcdefac
5554446 abcdefabcdefabcdef
4445556 abcdefabcdefabcdefabcd
3332221 abcdefghijklmnopqrst

1234567 20
9876543 15
5554446 Too few answers.
4445556 Too many answers.

#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main ()
{
    ifstream key;
    ofstream results;
   
    string keyAnswer;
    string stuAnswer;   
   
    int studentID = 9999999;   
    int n = 0;   
    int numCorrect = 0;   
       
    key.open("exams.txt");
    results.open("results.txt");

    key >> keyAnswer;
    key >> studentID;
    key >> stuAnswer;
           
    while ( !key.eof() )
    {
        // checking for invalid input values (letters other than a-f)
        while ( n < keyAnswer.length() )
        {
            string typecheck = stuAnswer.substr(n, 1);
            if ( typecheck > "f" || typecheck < "a" )
            {
                 results << studentID << " ";
                 results << "Invalid answers." << endl;                                 
            }
            n++;   
        }
         
        n = 0;   
       
        // checking for answers with a lack of input values
        if ( stuAnswer.length() < keyAnswer.length() )
        {
             results << studentID << " ";
             results << "Too few answers." << endl;
        }
       
        // checking for answers with too many input values
        else if ( stuAnswer.length() > keyAnswer.length() )
        {
             results << studentID << " ";         
             results << "Too many answers." << endl;             
        }
       
        // the correct number of input values, but how many of them are correct?
        else if ( stuAnswer.length() == keyAnswer.length() )
        {
             while ( n < keyAnswer.length() )
             {
                   string stuCheck = stuAnswer.substr(n, 1);
                   string keyCheck = keyAnswer.substr(n, 1);               
                   
                   if ( stuCheck == keyCheck )
                         numCorrect++;
                                       
                   n++;
             }
   
             results << studentID << " ";         
             results << numCorrect << endl;
             numCorrect = 0;         
        }
       
        key >> studentID;
        key >> stuAnswer;
    }   

    key.close();
    results.close();
       
    cout << "Answers checked.  Please check output file for results.\n";
    system("pause");
    return 0;
}

1234567 20
9876543 15
5554446 Too few answers.
4445556 Too many answers.
3332221 Invalid answers.
3332221 Invalid answers.
3332221 Invalid answers.
3332221 Invalid answers.
3332221 Invalid answers.
3332221 Invalid answers.
3332221 Invalid answers.
3332221 Invalid answers.
3332221 Invalid answers.
3332221 Invalid answers.
3332221 Invalid answers.
3332221 Invalid answers.
3332221 Invalid answers.
3332221 Invalid answers.
3332221 6

key >> in;
while(!key.oef()){

   ........
   .................
   .........


   key >> in;//on the last data entry, the instream stops at the end
}

while(true){
   key >> in;

        ............
        ...............
        ..........

        if(key.eof)
             break;
}