Question about the getline method...

[Ray]

Hello,

The intent of the following code is to read the first 2 characters from each
line until EOF is reached. It does this correctly on the first line but
stores an empty string for all subsequent lines. Of course I an accomplish
this task in other ways but I was wondering why this approach didn't work.

Thanks


char buf[512];

while (!cin.getline(buf, 3).eof())
{
cout << buf << '\n';
cin.ignore(100, '\n');
}

[Scot Brennecke]

Post by Ray
Hello,
The intent of the following code is to read the first 2 characters from each
line until EOF is reached. It does this correctly on the first line but
stores an empty string for all subsequent lines. Of course I an accomplish
this task in other ways but I was wondering why this approach didn't work.
Thanks
char buf[512];
while (!cin.getline(buf, 3).eof())
{
cout<< buf<< '\n';
cin.ignore(100, '\n');
}

This is why I don't like the iostream classes. After the initial
getline, the state has been set with failbit. It must be reset or
something. I hate these classes and never use them. :)

[Mateusz Loskot]

Post by Scot Brennecke

Post by Ray
Hello,
The intent of the following code is to read the first 2 characters from each
line until EOF is reached. It does this correctly on the first line but
stores an empty string for all subsequent lines. Of course I an accomplish
this task in other ways but I was wondering why this approach didn't work.
Thanks
char buf[512];
while (!cin.getline(buf, 3).eof())
{
cout<< buf<< '\n';
cin.ignore(100, '\n');
}

This is why I don't like the iostream classes. After the initial
getline, the state has been set with failbit. It must be reset or
something. I hate these classes and never use them. :)

This is a well-known and standard behaviour and
Stephan T. Lavavej explains it extensively here:

ID:351636 "STL: ifstream::getline() doesn't set eof flag"

https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=351636

Best regards,

[Mateusz Loskot]

Post by Scot Brennecke

Post by Ray
Hello,
The intent of the following code is to read the first 2 characters from each
line until EOF is reached. It does this correctly on the first line but
stores an empty string for all subsequent lines. Of course I an accomplish
this task in other ways but I was wondering why this approach didn't work.
Thanks
char buf[512];
while (!cin.getline(buf, 3).eof())
{
cout<< buf<< '\n';
cin.ignore(100, '\n');
}

This is why I don't like the iostream classes. After the initial
getline, the state has been set with failbit. It must be reset or
something. I hate these classes and never use them. :)

This is a well-known and standard behaviour and
Stephan T. Lavavej explains it extensively here:

ID:351636 "STL: ifstream::getline() doesn't set eof flag"

https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=351636

Best regards,

[David Wilkinson]

Post by Ray
Hello,
The intent of the following code is to read the first 2 characters from each
line until EOF is reached. It does this correctly on the first line but
stores an empty string for all subsequent lines. Of course I an accomplish
this task in other ways but I was wondering why this approach didn't work.

Try like this

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

int main()
{
string s;
while (getline(cin, s) && !s.empty())
{
cout.write(s.c_str(), 2) << "\n";
}
return 0;
}

[Ray]

Post by David Wilkinson

Post by Ray
Hello,
The intent of the following code is to read the first 2 characters from each
line until EOF is reached. It does this correctly on the first line but
stores an empty string for all subsequent lines. Of course I an accomplish
this task in other ways but I was wondering why this approach didn't work.

Try like this
#include <iostream>
#include <string>
using namespace std;
int main()
{
string s;
while (getline(cin, s) && !s.empty())
{
cout.write(s.c_str(), 2) << "\n";
}
return 0;
}
--
David Wilkinson
Visual C++ MVP
.

David,

Thanks for your suggestion. However, my issue here is not how to accomplish
the task, which I can do, but rather, what precisely is wrong with the
approach I took in the code sample I provided.

Ray

[Tim Roberts]

Post by Ray
Thanks for your suggestion. However, my issue here is not how to accomplish
the task, which I can do, but rather, what precisely is wrong with the
approach I took in the code sample I provided.

Scot told you the answer. Look at the documentation for getline. If the
"get" terminates because it encountered the character limit before it found
the end-of-line character, it sets "failbit" on the stream.

Call cin.clear(); to clear the failbit condition.