Question

I have a text file, which stores some data. There are several blocks of data formatted in different ways, so I need my program to be able to tell when one block ends and another one starts. Here is a very simple example.

Text file:

a 1
b 2
c 3

qwerty
asdfgh
xcvbnm

My program:

int x;
char a;
string line;

while(ifs >> a >> x)
    cout << a << " " << x << "\n";

cout<<"\n" << "next block of data" << "\n";

while(ifs >> line)
    cout << line;

However it does not read the second half. I thought that formatted reading with >> just discards whitespaces, so I thought that after the first while() fails, the second while() should start reading the second half. Here is my output:

a 1
b 2
c 3

next block of data

Also, if anybody can recommend something to read about this, I would be the most grateful. I searched several books and internet and couldn’t find any clear manual about how the >> operator works.

Answer 1

When operator>> fails, fail bit is set, and all sebsequent extraction will fail, too, unless you clear the error flags. Use ios::clear()

while(ifs >> a >> x)
    cout << a << " " << x << "\n";

ifs.clear();

You can use ios::bad(), ios::fail() and ios::eof() to determine why the extraction failed, so you can act accordingly.