Skip lines in std::istream

Question

I\'m using std::getline() to read lines from an std::istream-derived class, how can I move forward a few lines?

Do I have to just read and discard them?

Answer 1

Yes use std::getline unless you know the location of the newlines.

If for some strange reason you happen to know the location of where the newlines appear then you can use ifstream::seekg first.

You can read in other ways such as ifstream::read but std::getline is probably the easiest and most clear solution.

Answer 2

No, you don't have to use getline

The more efficient way is ignoring strings with std::istream::ignore

for (int currLineNumber = 0; currLineNumber < startLineNumber; ++currLineNumber){
    if (addressesFile.ignore(numeric_limits<streamsize>::max(), addressesFile.widen('\n'))){ 
        //just skipping the line
    } else 
        return HandleReadingLineError(addressesFile, currLineNumber);
}

HandleReadingLineError is not standart but hand-made, of course. The first parameter is maximum number of characters to extract. If this is exactly numeric_limits::max(), there is no limit: Link at cplusplus.com: std::istream::ignore

If you are going to skip a lot of lines you definitely should use it instead of getline: when i needed to skip 100000 lines in my file it took about a second in opposite to 22 seconds with getline.

Answer 3

Edit: You can also use std::istream::ignore, see https://stackoverflow.com/a/25012566/492336

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Do I have to use getline the number of lines I want to skip?

No, but it's probably going to be the clearest solution to those reading your code. If the number of lines you're skipping is large, you can improve performance by reading large blocks and counting newlines in each block, stopping and repositioning the file to the last newline's location. But unless you are having performance problems, I'd just put getline in a loop for the number of lines you want to skip.

Answer 4

For what it's worth:

void skip_lines(std::istream& pStream, size_t pLines)
{
    std::string s;
    for (; pLines; --pLines)
        std::getline(pStream, s);
}