Reading files

[Ramzi89]

Hi

Can you use the cin function on a string variable?

I am trying to read input from a file line by line. I want to be able to output the correct lines to a file and the incorrect to a file.

Each line holds three pieces of information separated by a space and then finally a newline.

So i want to read the input line by line using getline. Then want to take each piece of information for validation?

Can i do this by using cin on the value returned by the getline or must i read it char by char?

[Chervil]

You could use getline to read the whole line into a string. Then use a stringstream to easily read the three parts of that string.

    string line = "one 123 two";
    int b;
    string a, c;
    istringstream ss(line);
    ss >> a >> b >> c;
	
    cout << "a: " << a << " b: " << b << " c: " << c << endl;

Output:

a: one b: 123 c: two

[Ramzi89]

OK i' trying to get my head around stringstream and how you would test for errors.

what on earth is going on here:

#include <string>       // std::string
#include <iostream>     // std::cout
#include <sstream>      // std::stringstream

using namespace std;

int main () {

  stringstream ss;

  ss << 100 << 'f'<< "hello" << 200;

  int foo,bar;

  string s;
  ss >> foo >> s >> bar;

  cout << "foo: " << foo << '\n';
  cout << "s:" << s << endl;
  cout << "bar: " << bar << '\n';

  return 0;
}

the output is:

foo: 100
s:fhello200
bar: 134515753

cos theres no white space between f and hello.

Fair enough.

Why then does it stop when it encounters the char f but does not stop when it encounters the int 200?

[Chervil]

When you do this, the process first skips any whitespace, then continues to extract characters so long as they form a valid part of an integer.

int foo;
ss >> foo;

On the other hand, this will first skip any whitespace, them consider any subsequent characters as a valid part of a string until the next whitespace (or end of stream) is encountered.

string s;
ss >> s;

Still, the original question said each part was separated by a space. So you could have done something like this:
ss << 100 << ' ' << 'f' << ' ' << "hello" << ' ' << 200;

[Ramzi89]

OK thanks. Your right about the question anyway. But out of interest in my example why does it stop when it encounters the char f but does not stop when it encounters the int 200?

[Chervil]

I tried to explain previously.
'f' is not part of a valid integer so extraction stops there.
'2' is a valid character which can be part of a string, so extraction does not stop.

[Ramzi89]

Thanks Chevril. So everything in the stringstream is stored as a char and it all depends on the type of extraction?

[Chervil]

That's right. A stringsteam behaves in many ways just like a disk file, or like an ordinary cin / cout. That's because they are all types of stream with similar characteristics.