File I/O question

[jjmounes]

I am studying file I/O and writing simple programs. This one has an issue I don't understand and any enlightenment would be greatly appreciated.

Here is the code:

#include <fstream>
#include <iostream>

using namespace std;

int main()
{
	// declare variables
	int score, totalScore = 0, count = 0; // count is count of students(or scores)
	int numA = 0, numB = 0, numC = 0, numD = 0, numU = 0; // number of As, ..., etc
	ifstream input;
	ofstream output;

	// open file scores.dat for read. 
	// if open file for read fails, print out error message and exit the program (see demo for details)
	input.open("scores.dat");

	if(input.fail())
	{
		cout << "Unable to open file scores.dat to read";
		system("pause");
		exit(1);
	}

	// as long as there is a score in file to read
	while(input >> score)
	{
		// read a score from 	
		input >> score;
		
		// update total score
		totalScore = totalScore + score;

		// update the number of As, Bs, Cs, Ds, and Us
		switch(score/10)
		{
		case 10: case 9:
			numA++;
			break;
		case 8:
			numB++;
			break;
		case 7:
			numC++;
		case 6:
			numD++;
		default:
			numU++;
			break;
		}

	// update the count of students
	count++;
	}
	


	// if count of students is 0, print out error message and exit program
	if(count == 0)
	{
		cout <<  "Student count is 0. Program exit.";

	}

	else
	{
		// open file YourName.txt for write
		output.open("MounesJeremy.txt");

		// set correct ios flags 
		output.setf(ios::fixed);
		output.setf(ios::showpoint);
		output.precision(2);

		// output result to the file
		output << "\n\n\t\tTest Statistics";
		output << "\n\n\tTotal number of students: " << count;
		output << "\n\tNumber of A's: " << numA;
		output << "\n\tNumber of B's: " << numB;
		output << "\n\tNumber of C's: " << numC;
		output << "\n\tNumber of D's: " << numD;
		output << "\n\tNumber of U's: " << numU;
	}

	cout << "\n\tThe test stats were output to MounesJeremy.txt";
	cout << "\n\tThank you for using MyTeachingAssistant software\n\t";
	
	// close files
	input.close();
	system("pause");
	return 0;
}

The problem is, while this method seems to work, when a list of 26 integers is given:

		Test Statistics

	Total number of students: 13
	Number of A's: 4
	Number of B's: 3
	Number of C's: 2
	Number of D's: 5
	Number of U's: 6 

It reads exactly half before stopping.

Why?

[shacktar]

Here you're doing two reads (one within the while statement and you read the next one right after):

while(input >> score)
{
    // read a score from 	
    input >> score;

[jjmounes]

Thanks.

That should have been obvious, but the use of an input/output as a while loop exit condition was confusing to me.

[shacktar]

What's happening there is it does the read and then input itself is used as the condition. Using an instance of ifstream or ofstream as a condition is basically the same thing as saying myStreamInstance.good().

[jjmounes]

I wasn't aware the declaration of a do/while loop allowed for operations of any kind. I thought while loops were conditional only, with for loops reserved for that type of thing.

Would this work, then?

while(i++)
{
do something

if(i > 5) { break; }
}

Mark as solved, yes I am awar of the dumb stuff in that program, such as the missing break;'s