Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!brunix!sdm
From: sdm@cs.brown.edu (Scott Meyers)
Newsgroups: comp.lang.c++
Subject: Binary Search Using seekg/tellg
Message-ID: <71329@brunix.UUCP>
Date: 8 Apr 91 21:57:32 GMT
Sender: news@brunix.UUCP
Reply-To: sdm@cs.brown.edu (Scott Meyers)
Organization: Brown University Department of Computer Science
Lines: 43

Regarding direct access in input streams, the istream man page says:

          insp=&ins.seekg(off,dir)
               Repositions  ins.rdbuf()'s   get   pointer.    See
               sbuf.pub(3C++) for a discussion of positioning.

          insp=&ins.seekg(pos)
               Repositions  ins.rdbuf()'s   get   pointer.    See
               sbuf.pub(3C++) for a discussion of positioning.

          pos=ins.tellg()
               The current position of ios.rdbuf()'s get pointer.
               See  sbuf.pub(3C++)  for a discussion of position-
               ing.

Okay, this is from the sbuf.pub man page:

     pos=sb->seekpos(pos, mode)
          Repositions the streambuf get  and/or  put  pointer  to
          pos.  mode specifies which pointers are affected as for
          seekoff().  Returns pos (the argument) or  EOF  if  the
          class  does  not  support  repositioning  or  an  error
          occurs.  In general a streampos should be treated as  a
          "magic cookie" and no arithmetic should be performed on
          it.

So how do I implement a binary search using iostream I/O?  I need to be
able to get two stream positions (corresponding to the top and bottom of
the region I'm about to bisect) that I can perform arithmetic on.  For
example, during the first iteration of the search, I want to go to the top
of the file and get a streampos, call it TOP, then to the BOTTOM of the
file to get another streampos, call it BOTTOM, then I want to jump to the
position in the file corresponding to (BOTTOM-TOP)/2.  However, the man
page says that streampos objects don't have to obey the laws of arithmetic.
What do I do?

Thanks,

Scott


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