comp.lang.idl-pvwave archive: archive » Re: Commutativity of multiplication

Home » Public Forums » archive » Re: Commutativity of multiplication

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

Re: Commutativity of multiplication [message #50968 is a reply to message #50961]

Thu, 26 October 2006 01:26

Foldy Lajos
Messages: 268
Registered: October 2001

Senior Member

On Wed, 25 Oct 2006, JD Smith wrote:

> Commutation hasn't been broken, only "type commutation", which doesn't
> really exist. For all purposes, given the limitations of integer
> representation in computers, -500 and 4294966796 *are* the same. I
> could just as easily claim that "adding and subtracting 1 is broken":
>
> IDL> print, 4294967295UL + 1UL
> 0
>
> IDL> print,0b - 1b
> 255
>
> JD
>

If multiplication is commutative, then a*b should be equal to b*a.

IDL> a=-1l
IDL> b= 1ul
IDL> print, a*b eq b*a
1

Fine. If a*b is equal to b*a, then 1.0*(a*b) should be equal to 1.0*(b*a),
too.

IDL> print, 1.0*(a*b) eq 1.0*(b*a)
0

I tend to say that IDL's multiplication is not commutative in
the mathematical sense.

Other languages, like C are "more commutative":

signed int a=-1;
unsigned int b= 1;

printf("%d %d\n", a*b==b*a, 1.0*(a*b)==1.0*(b*a));

prints 1 1 (the result is signed int both for a*b and b*a).

regards,
lajos

Report message to a moderator

[Message index]

		Re: Commutativity of multiplication By: JD Smith on Thu, 26 October 2006 10:47
		Re: Commutativity of multiplication By: JD Smith on Thu, 26 October 2006 10:37
		Re: Commutativity of multiplication By: Braedley on Thu, 26 October 2006 10:24
		Re: Commutativity of multiplication By: Foldy Lajos on Thu, 26 October 2006 01:26
		Re: Commutativity of multiplication By: JD Smith on Wed, 25 October 2006 16:09
		Re: Commutativity of multiplication By: David Fanning on Wed, 25 October 2006 15:33
		Re: Commutativity of multiplication By: Sven Geier on Wed, 25 October 2006 14:54
		Re: Commutativity of multiplication By: K. Bowman on Wed, 25 October 2006 13:42
		Re: Commutativity of multiplication By: David Fanning on Wed, 25 October 2006 13:54

Previous Topic:	Syntax errors
Next Topic:	Re: !P.SYMSIZE still broken? after 5 years?

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Fri Oct 10 22:54:12 PDT 2025

Total time taken to generate the page: 0.88541 seconds