Why the program prints twice the default condition instead of quitting the while loop when i enter the exit condition?

Question

I'm a beginner and I copied this code from a book but it doesn't work because when I enter EOF, the program instead of quitting the while loop prints the default message twice. It is like a hour that i search for a solution but nothing that i've tried works, thanks for help. the program is supposed to count the number of each kind of grades.

#include "stdafx.h"
#include <iomanip>
#include <iostream>
#include <math.h>
#include <ctype.h>

using namespace std;


int main()
{ 

    int grade,
        aCount = 0,
        bCount = 0,
        cCount = 0,
        dCount = 0,
        fCount = 0;



    cout << "Enter the letter grades (EOF to quit) ->>";

    while ((grade = cin.get()) != 'EOF')  {

        switch (grade) {

        case 'A':
            ++aCount;
            break;

        case 'B':
            ++bCount;
            break;

        case 'C':
            ++cCount;
            break;

        case 'D':
            ++dCount;
            break;

        case 'F':
            ++fCount;
            break;

        case '\n':
        case '\t':
        case ' ':
            break;

        default :
            cout << "Invalid letter grade entered. Enter a new grade." << endl;
            break;

        }
    }

Not sure if duplicate, but at least worth a read: [Why is iostream::eof inside a loop condition considered wrong?](https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-considered-wrong). — Algirdas Preidžius, Jul 10 '17 at 11:39
@Borgleader As mentioned it is valid, but it's implementation-specific what happens with it. — Some programmer dude, Jul 10 '17 at 11:42
https://stackoverflow.com/a/3684046/4341534 Relevant answer to the single quote semantics. Note that it not defined in the C standard. — patatahooligan, Jul 10 '17 at 11:43
the book title is "DEITEL C++" , it is supposed to be a beginner book — Michele Deodato, Jul 10 '17 at 11:48
@AlgirdasPreidžius: Not really. The loop semantics (aside from the `EOF` spelling) are correct here. — Lightness Races in Orbit, Jul 10 '17 at 11:49
Your compiler would have made this clear had you compiled with warnings on. — underscore_d, Jul 10 '17 at 11:50
@LightnessRacesinOrbit Probably, at this point, whenever I see `eof` used in C++ code, I suspect that there might be error involved, without even looking at the code :/ — Algirdas Preidžius, Jul 10 '17 at 11:52
I've changed the condition to while ((grade = cin.get()) != -1 ) but it doesn't work — Michele Deodato, Jul 10 '17 at 12:03
Please do not butcher the question after answers have been posted. Your edited approach actually has more problems than the original one. — Lightness Races in Orbit, Jul 10 '17 at 12:45

score 6 · Answer 1 · answered Jul 10 '17 at 11:48

6

The documentation for cin.get() tells us that the return condition you are looking for is EOF.

while ((grade = cin.get()) != EOF)  {

That's it — no quotation marks! It's a macro that expands to an integer. What you did was to write a "multi-byte character literal" 'EOF', which has implementation-defined meaning and no real relevance here.

Either you copied from the book wrong, or the book is wrong.

answered Jul 10 '17 at 11:48

Lightness Races in Orbit

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1,055

i'm sure i copied right, i've already checked a thousand time, furthermore i tried change the condition to – Michele Deodato Jul 10 '17 at 11:55
while ((grade = cin.get()) != -1 ) but it doesn't work – Michele Deodato Jul 10 '17 at 11:56
1

@MicheleDeodato: Nobody told you to do that. It is `EOF`. Not `'EOF'`, not `-1`, but `EOF`. Just write that. – Lightness Races in Orbit Jul 10 '17 at 12:44
you're right it was without quotation marks, but it still doesn't work, i really don't know what to do. – Michele Deodato Jul 11 '17 at 09:55
@MicheleDeodato: Do exactly what I told you to do. – Lightness Races in Orbit Jul 11 '17 at 10:27

taha · Answer 2 · 2017-07-11T16:34:18.263

the docs say: cin.get() extracts a single character from the stream as unformatted input.

when you enter -1 it's considered as 2 seperate inputs - and 1 both of them dont match any case that's why the default message get printed twice and the condition to exit the while loop is never met

Unlike files console doesnt have an EOF flag. Nonetheless you can simulate it by typing Ctrl + Z

A simple workaround is to change the condition, something like entering 'X' exits the loop while ((grade = cin.get()) != 'X')

Why the program prints twice the default condition instead of quitting the while loop when i enter the exit condition?

2 Answers2