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From: jlg@cochiti.lanl.gov (Jim Giles)
Newsgroups: comp.lang.c
Subject: Re: Problem with post-incrementation
Message-ID: <21400@lanl.gov>
Date: 15 Apr 91 17:26:28 GMT
References: <6530@s3.ireq.hydro.qc.ca>
Sender: news@lanl.gov
Organization: Los Alamos National Laboratory
Lines: 19


	sprintf(str, "tmp_%d", i = (i++) % 3600);

I have seen this bug in a number of C programs (including the _second_
edition of a published book on graphics in C).  It is one of the
drawbacks of side-effect operators that such errors are made and
remain undetected.

The problem is that i = (i++) etc.  assigns the value of the expression
to the variable i.  Depending on the order that the side-effects are
carried out, this may happen before or after the side-effect for the
incrementation operator.  If the assignment happens _after_ the increment,
then i will get zero every time.  What you want to do is this:

      sprintf(str, "tmp_%d", i = (i+1) % 3600);

You only need i to be assigned once anyway.

J. Giles