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 #50961 is a reply to message #50959]

Thu, 26 October 2006 10:24

Braedley
Messages: 57
Registered: September 2006

Member

JD Smith wrote:
> On Wed, 25 Oct 2006 14:54:59 -0700, Sven Geier wrote:
>
>> David Fanning wrote:
>> [...]
>>> It also explains this:
>>>
>>> IDL> f = 500L
>>> IDL> g = 1UL
>>> IDL> help, g*(-f)
>>> <Expression> ULONG = 4294966796
>>> IDL> help, (-f)*g
>>
>> Wow. This is ... uhm ... "more interesting than I thought". There's whole
>> realms of oddity here that I never knew existed:
>>
>> IDL> help,f,g
>> F LONG = -500
>> G ULONG = 1
>> IDL> print,f*g
>> -500
>> IDL> print,g*f
>> 4294966796
>> IDL> if f*g eq g*f then print,f*g," equals ",g*f
>> -500 equals 4294966796
>
> The bit pattern for these two numbers is *exactly* the same. The only
> difference is the "type" that IDL assigns them, which affects how the
> numbers are printed (and only how they are printed). That type (for
> IDL) is determined by the "left-most" variable or constant in the
> expression, with the exception that if anything to the right has a
> larger range, it will be used as the type (e.g. promotion to float,
> double, ULONG64, etc.), hence:
>
> IDL> f=1.0
> IDL> l=100UL
> IDL> help,l+f
> <Expression> FLOAT = 101.000
>
> When the range is the same, e.g. UL and L, leftmost always wins.
>
> 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

IDL must make a choice as to which type to use, since the length of
ranges of LONG and ULONG are exactly the same, but there is only 50%
overlap. The result from the multiplication may be within the range of
both, in which case everything is fine. However, if the result is
negative, the result will be outside the range of ULONG. Likewise, if
the two numbers are sufficiently large, the result will be outside the
range of LONG. IDL doesn't know beforehand what the result will be,
and therefore assigns the type of the leftmost variable.

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: Sat Oct 11 10:44:11 PDT 2025

Total time taken to generate the page: 1.04036 seconds