C++: How do I open file and getLine() from the middle instead of the beginning?

[SNYP40A1]

I have some very large text files, ~ 46 - 90 MB that I am parsing, but the data that I really care about is always at the end, around the last 200 kb. I get this data by opening the file and then doing

ifstream textfile;
textfile = open("log.txt");

while(!textfile.fail())
{
string nextline;
getLine(nextline, textfile);
//more code...
}

The above might have some errors, but you get the idea. Instead of starting the getLine() routine at the beginning of the file, how can I start closer to the end? Say 99% through the file.

[LaRoza]

I found this in the standard library: http://www.cppreference.com/cppio/seekg.html

[SNYP40A1]

I found this in the standard library: http://www.cppreference.com/cppio/seekg.html

Hi LaRoza, I saw that function too, but it does not do me much good w/o knowing the position that I want to seek to. How do I say, "seek to line number (#total lines * 0.99)"?

[LaRoza]

Hi LaRoza, I saw that function too, but it does not do me much good w/o knowing the position that I want to seek to. How do I say, "seek to line number (#total lines * 0.99)"?

http://www.cplusplus.com/doc/tutorial/files.html

See about 3/4 the way down. You can read files from the end.

[SNYP40A1]

http://www.cplusplus.com/doc/tutorial/files.html

See about 3/4 the way down. You can read files from the end.

Thanks, that did the trick! Here's the code I made based on your reference:

ifstream datafile;
datafile.open(filepath.c_str());
int begin = datafile.tellg();
datafile.seekg (0, ios::end);
int end = datafile.tellg();
int newpos = end - begin;
newpos = 0.95*newpos;
datafile.seekg(newpos, ios::beg);