Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!gem.mps.ohio-state.edu!ginosko!uunet!virtech!cpcahil
From: cpcahil@virtech.UUCP (Conor P. Cahill)
Newsgroups: comp.lang.c
Subject: Re: swap(x,y)
Message-ID: <1066@virtech.UUCP>
Date: 24 Aug 89 02:45:22 GMT
References: <MYwDEFy00WB842JEUx@andrew.cmu.edu> <1061@virtech.UUCP> <990@m3.mfci.UUCP>
Organization: Virtual Technologies Inc
Lines: 47

In article <990@m3.mfci.UUCP>, karzes@mfci.UUCP (Tom Karzes) writes:
> In article <1061@virtech.UUCP> cpcahil@virtech.UUCP (Conor P. Cahill) writes:
> >In article <MYwDEFy00WB842JEUx@andrew.cmu.edu>, tg1e+@andrew.cmu.edu (Timothy R. Gottschalk) writes:

> This should only be a problem if your machine traps on integer overflow,
> or if your hardware is broken and doesn't give the desired results in
> the presence of integer overflow.  Sane hardware will give you the correct
> low-order bits of the result regardless of overflow.  This will guarantee
> that the above produces correct results, even if the intermediate results
> overflow.


I stand corrected.  You know I was about to say what about unsigned values
where the two operands added up to be greater than what can fit into the
storage area, but I decided to try it on my machine before I opened my big
mouth.  Lucky I did.  Using the following code:

	unsigned short i,j; i = 42000; j = 42001;

	printf("sizeof(i) = %d\n", sizeof(i));

	printf("i = %u, j = %u\n", i, j);
	i += j;
	printf("i = %u, j = %u\n", i, j);
	j = i - j;
	printf("i = %u, j = %u\n", i, j);
	i -= j;
	printf("i = %u, j = %u\n", i, j);

the output was:

	sizeof(i) = 2
	i = 42000, j = 42001
	i = 18465, j = 42001
	i = 18465, j = 42000
	i = 42001, j = 42000

I was about to scream "Compiler Error" when I realized that 18465 - 42001
does underflow back to 42000.

So the original post does work as long as there is no hardware problems
with decimal overflow. 
-- 
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| Conor P. Cahill     uunet!virtech!cpcahil      	703-430-9240	!
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