Re: complexity for tellg()

From:

"Carlo Capelli" <carlo.capelli@rdbos.it>

Newsgroups:

comp.lang.c++

Date:

Tue, 20 Feb 2007 08:57:01 +0100

Message-ID:

<GSxCh.15006$aF5.5339@tornado.fastwebnet.it>

Of course you can approach the problem computing the position yourself, if
you know the size of the input read.
Not elegant, but it works for simple cases...

     std::ifstream in("Y:/Data/workspaces/tob4f/tob4f.dat");
     size_t pos = 0;
     std::string line;
     while(in){
        // int pos = in.tellg();
         std::getline(in,line);
         pos += line.length() + 2; // account for line terminator...
     }

Bye Carlo

"toton" <abirbasak@gmail.com> ha scritto nel messaggio
news:1171952853.299366.190420@q2g2000cwa.googlegroups.com...

Hi,
I am reading a big file , and need to have a flag for current file
position so that I can store the positions for later direct access.
However it looks tellg is a very costly function ! But it's code says
it should just return the current buffer position , thus should be a
very low cost function.
To explain,
{
boost::progress_timer t;
std::ifstream in("Y:/Data/workspaces/tob4f/tob4f.dat");
std::string line;
while(in){
int pos = in.tellg();
std::getline(in,line);
}
}
This code takes 0.58 sec in my computer, while if I uncomment the line
in.tellg() , it takes 120.8 sec (varies a little )

can anyone say the reason & the possible workout ?
I amusing MS Visual Studio 7.1 and the std library provided by visual
studio 7.1