Question about how ifstream reads

[mixelplik]

I'm trying to get a better understanding of ifstream, and was playing with it using the program below. It's purpose was to grab the first and last entries in the file, instead I get some perplexing output leading me to realize I've very little clue as to how streams really work. I know the simple book nuts and bolts, but putting it into practice is different. (and yes I am avoiding .eof() on purpose)

#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    ifstream names;
    names.open("P25.txt");

    string firstInFile;
    string lastInFile;
    string next;
    string students;


    while(names >> students)
    {
        names >> firstInFile;
        names >> lastInFile;
    }

    names.close();

    cout << firstInFile << endl;
    cout << lastInFile << endl;


    return 0;
}


/*
P25.txt
-------
Amy
Angela
Vicky
KC
Stephanie
Mark
Andrea
Tom
*/

What I'm getting in output is:
Tom
Mark

If I change the code a little like this:

    names >> firstInFile;

    while(names >> students)
    {

        names >> lastInFile;
    }

The output is :
Amy
Andrea

I get Amy, as it's the first entry in the file, but why does it stop at Andrea, when there is one more entry it can read in.

Also,
If I just have this:

 while(names >> students)
        {

            names >> lastInFile;
        }

With no other reading, it does indeed return the last entry in the file.

I know this is a pretty simple question, but I'm really trying to get a decent grasp on what's going on.

[mixelplik]

I added a counter to trying and gauge where the read head is:

/*
25. Using Files Student Line Up
Modify the Student Line Up program described in Programming Challenge 14 so that
it gets the names from a file. Names should be read in until there is no more data to
read. If you have downloaded this books source code from the companion Web site,
you will nd a le named LineUp.txt in the Chapter 05 folder. You can use this le to
test the program. (The companion Web site is at www.pearsonhighered.com/gaddis.)
*/

#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    ifstream names;
    names.open("P25.txt");

    string firstInFile;
    string lastInFile;
    int i = 0;
    string students;
    i++;
    names >> firstInFile;
 names.close();
 names.open("P25.txt");
    while(names >> students)
    {
        names >> lastInFile;
        i++;
    }

    names.close();

    cout << firstInFile << endl;
    cout << lastInFile << endl;
    cout << i << endl;


    return 0;
}


/*
P25.txt
-------
Amy
Angela
Vicky
KC
Stephanie
Mark
Andrea
Tom
*/

and this is my output:
Amy
Tom
5

Process returned 0 (0x0) execution time : 0.047 s
Press any key to continue.

Why is it at 5? Arg, this is so confusing.

[gerard4143]

Try running this program with your data.

#include <iostream>
#include <fstream>

using namespace std;

int main(int argc, char** argv) 
{
    ifstream names;

    names.open("P25.txt");

    string data;

    while(names >> data)
    {
        cout << "You read->" << data << "<-into data!" << std::endl;
    }

    names.close();

    return 0;
}

[mixelplik]

I actually tried that, and for somereason nothing is displayed. I'm using codeblocks, though I'm not sure if that has anything to do with it.

[gerard4143]

Did you pass std::endl or "\n" to std::cout?

[mixelplik]

I cut and pasted your code. Which from what I can see should work, or at least spit somthing out. This is literally what's in my text file:

Amy
Angela
Vicky
KC
Stephanie
Mark
Andrea
Tom

[gerard4143]

Are you sure the datafile is in the same folder as the executable.

[mixelplik]

Yeah, I am able to run the code I posted above just fine.

[mike_2000_17]

Why is it at 5? Arg, this is so confusing.

Because you are reading two names per iteration when you do this:

while(names >> students)
{
    names >> lastInFile;
    i++;
}

You read one name into students and then another into lastInFile. Because you have 8 names, and you first read one before the loop, you then get 7 names to read in the loop, which takes 4 iterations. This is why you get 5 printed out (1 + 4). You need to read only one name per iteration:

while(names >> lastInFile)
{
    i++;
}

and that should give you the correct result (8) for the counter, and lastInFile should contain the last valid name from the file.

[mixelplik]

AHA! That makes total sense, in the while it's acutually performing the function, FACE PALM (to self stoopid rookie!) Thanks Mike_2000_17.

Though I'm not sure why I can't ghet gerad4143's code to work. I'll keep playing with, my big question was answered. Thanks guys!