CODEWITHSUNDEEP
c++while-loopistream
user285372
user285372

Reputation:

Indicating end-of-file

In the following simple code snippet, the end-of-file can be easily indicated by pressing ctrl-z.

double x;

while ( cin >> x )
{
    ++count;
    sum += x;
}

But when I try a more complicated program, such as reading in student records to do some processing, I fail to indicate the end-of-file to the program.

For example, I have a struct object called Student_info

struct Student_info
{
    string name;
    double midterm, final;
    vector<double> homework;
};

I have written several functions to read in and store several student records, as follows:

while ( read( cin, record ) )
{
    students.push_back( record );
}

But after inputting a few student records, when I do ctrl-z and hit the Enter to see the results, the command prompt is still waiting for me.

What am I doing wrong?

p.s.1. I separate names and grades with a blank space, and I separate students' details by jumping to the next line, which should be pretty much the same; i.e. some space separator. Right?

p.s.2. This code is adopted from the Accelerated C++ Book, and my knowledge of it is just up until Chapter 5 at the moment...

Thank you in advance,

EDIT

Below are two more functions that I use:

istream& read( istream& is, Student_info& s )
{
    is >> s.name >> s.midterm >> s.final;

    read_hw( is, s.homework );

    return is;
}


istream& read_hw( istream& in, vector<double>& hw )
{
    if ( in )
    {
        hw.clear();

        double x;
        while ( in >> x )
            hw.push_back( x );

        in.clear();
    }

    return in;
}

Upvotes: 3

Views: 927

Answers (2)

Tu Xiaomi
Tu Xiaomi

Reputation: 61

when the cin read the EOF, it only set the instream flag eofbit and failbit. When you use

while(cin>>x)

,it implicit indicate a operator bool (std::cin),it return false when read EOF(the cin >> x return cin). Thus the while loop end.

But when you use

while ( read( cin, record ) )

the cin also return false when it read EOF,but your function read didn't recognize it.And when the while loop continue, the loop occur a new cin. And the important is the failbit flag will save in you stream until you use cin.clear().Thus your code is not good as overload operator >> for your student struct. I am sorry for my English,and hope you can clear.

Upvotes: 1

MSalters
MSalters

Reputation: 179819

The easiest solution would be to write an std::istream& operator>> ( std::istream& in, Student_info&) which behaves similarly to the existing operator>>. You can then use exactly the same loop:

Student_info record;
while ( cin >> record )
{
    ++count;
    students.push_back( record );
}

Inside your operator>>, you should just call operator>> for all members, and then return the istream& in argument.

Upvotes: 2

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