C++ formatted input: how to 'skip' tokens?

Question

Hi all!

Suppose I have an input file in this format:

VAL1 VAL2 VAL3
VAL1 VAL2 VAL3

I'm writing a program that would be interested only in VAL1 and VAL3. In C, if i wanted to 'skip' the second value, I'd do as follows:

char VAL1[LENGTH]; char VAL3[LENGTH];
FILE * input_file;
fscanf(input_file, "%s %*s %s", VAL1, VAL3);

Meaning, I'd use the "%*s" formatter to make fscanf() read this token and skip it. How do I do this with C++'s cin? Is there a similar command? Or do I have to read to a dummy variable?

Thanks in advance.

Answer 1

I would just read it into a dummy variable. If you do need it eventually, it will be available.

Answer 2

You can do it in a much easier way with getline(). Just use it to grab the entire line, and then parse out the values in between the tokens (use strtok?)

There are a whole bunch of other issues with getline(), but it should work for your problem.

Answer 3

You can use

cin.ignore(256, ' ');

This will ignore everything up to either 256 characters or a white space.

Edit (formatting and...): alternatives you can do:

int a, b;
cin >> a >> b >> b;

Answer 4

There's an ignore function available:

std::cin << val1;
std::cin.ignore (9999, ' ');
std::cin << val3;

The first argument defines the number of characters to skip, the second parameter is the delimiter to stop skipping at.

You could wrap that up in a user-defined manipulator, since it's ugly.

---

Here's the custom manipulator:

template <class charT, class traits>
inline std::basic_istream<charT, traits> &
ignoreToken (std::basic_istream<charT, traits> &strm)
{
    strm.ignore (9999, ' ');
    return strm;
}

And you could use that like this:

cin >> val1 >> ignoreToken >> val3 >> ignoreToken >> val5;

(ps -- I didn't compile this, so there maybe a typo).