Path: utzoo!utgpu!cunews!bnrgate!bwdls58!!stetner
From: stetner@.bnr.ca (Douglas Stetner)
Newsgroups: comp.lang.c
Subject: Re: Problem with post-incrementation
Message-ID: <6509@bwdls58.bnr.ca>
Date: 18 Apr 91 01:06:08 GMT
References: <6530@s3.ireq.hydro.qc.ca>
Sender: news@bwdls58.bnr.ca
Reply-To: stetner@bwdlh507.bnr.ca (Douglas Stetner)
Organization: Bell-Northern Research, Ltd.
Lines: 49

In article <6530@s3.ireq.hydro.qc.ca> godin@ireq.hydro.qc.ca (Andre Godin 8926) writes:
>
>	Every times I call this function, the value of 'str' 
>	is always tmp_0 and 'i' never gets incremented. If
>	we use the pre-increment operator, the function works 
>	properly. Why post-incrementing doesn't work?
>
>	(I'm using cc under SunOs 4.0.3.)
>
>char *
>fnc() /*
>----- */
>{
>static	int	i = 0;
>static	char	str[256];
>
>	sprintf(str, "tmp_%d", i = (i++) % 3600);
>
>	return (str);
>}

Think about the way this will get compiled
i = (i++) % 3600
      |
      V
i = ( 0 ) % 3600   /* i has been incremented, i = 1 */
          |
	  V
i =       0        /* the assignment takes place and i = 0 */

Pre increment works because it evaluates this way...
i = (++i) % 3600
      |
      V
i = ( 1 ) % 3600   /* i has been incremented, i = 1 */
          |
	  V
i =       1        /* the assignment takes place and i = 1 */

It would be much better to either do the sprintf then twiddle i
or to twiddle i then do the sprintf, as this would be more readable
and will most likley make no difference to the size/speed of the
resulting executable.
not produce 'smaller|quick
--
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Douglas G. Stetner, 4S75  |  Bell-Northern Research  | My opinions are my own.
      stetner@bnr.ca      |  P.O. Box 3511, Stn. C   | (613) 828-6321  (home)
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