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From: jss@gold.kpc.com (Jerry Schwarz)
Newsgroups: comp.lang.c++
Subject: Re: Binary Search Using seekg/tellg
Message-ID: <JSS.91Apr9184104@gold.kpc.com>
Date: 10 Apr 91 01:41:04 GMT
References: <71329@brunix.UUCP>
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Organization: Kubota Pacific Computer
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In-Reply-To: sdm@cs.brown.edu's message of 8 Apr 91 21:57:32 GMT
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In article <71329@brunix.UUCP> sdm@cs.brown.edu (Scott Meyers) writes:

   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?


The iostream interface doesn't allow you do to this because it can't
be done portably.  On operating systems with record oriented file
systems there is just no way of doing arbitrary arithmetic on file
positions.  If you aren't worried about portability to such systems
and the system you're on defines |streampos| as an integral type (as
some do) then you can do arithmetic to your hearts content.  

Jerry Schwrz