Path: utzoo!attcan!uunet!jarthur!nntp-server.caltech.edu!seismo.gps.caltech.edu!bruce
From: bruce@seismo.gps.caltech.edu (Bruce Worden)
Newsgroups: comp.lang.c
Subject: Re: Order of evalution of expressions.
Message-ID: <1990Sep25.000621.6980@nntp-server.caltech.edu>
Date: 25 Sep 90 00:06:21 GMT
References: <6398@castle.ed.ac.uk>
Sender: bruce@seismo.gps.caltech.edu (Bruce Worden)
Organization: California Institute of Technology, CA
Lines: 33
Nntp-Posting-Host: sis.gps.caltech.edu

In article <6398@castle.ed.ac.uk> elee24@castle.ed.ac.uk (H Bruce) writes:
>Is the line
>
>value = *ptr - *ptr++;
>
>sensible C ?

As others have pointed out, it is not; the behavior is undefined and
therefore may be implementation dependent.  

>If not what is the fastest way of computing this type of expression ?
>Would the following lines be optimized by a compiler (so that value is
>not loaded twice) ?
>value = *ptr;
>value -= *ptr++;

Remember, the increment takes place after the value is used, so you are
effectively subtracting *ptr from itself.  I suspect that what you want 
to do is:

	value = ptr[0] - ptr[1];
	ptr++;

As was discussed here a couple of weeks ago, this form should be as
efficient as the form:

	value = *ptr - *(ptr+1);
	ptr++;

The first form is, I think, clearer.
--------------------------------------------------------------------------
C. Bruce Worden                            bruce@seismo.gps.caltech.edu
252-21 Seismological Laboratory, Caltech, Pasadena, CA 91125