| Re: A simple IF statement question [message #42500] |
Mon, 14 February 2005 13:22 |
Mark Hadfield
Messages: 783
Registered: May 1995
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Senior Member |
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David Fanning wrote:
> David Fanning writes:
>> Here is an article you might want to read:
>>
>> http://www.dfanning.com/code_tips/bitwiselogical.html
>
> It has been pointed out to me that the article is a bit
> deficient in that it doesn't mention the LOGICAL_PREDICATE
> compiler option. If you set:
>
> COMPILE_OPT LOGICAL_PREDICATE
>
> Then 0 is FALSE and everything else is TRUE. That probably
> makes more sense to *everyone*! :-)
The article in question was written not longer before IDL 6.0 went into
beta and was intended to summarise some newsgroup explanations of IDL's
(very confusing) treatment of logical values. I suspect that the article
and/or the newsgroup explanations convinced RSI to clean things up. I've
been meaning to update the article for, oh, 2 years.
Setting COMPILE_OPT LOGICAL_PREDICATE *is* a good idea, I think (if you
have version 6.0 or greater). But there is a catch: the NOT operator can
no longer be used in logical expresssions. For example, 1 is always true
and "NOT 1" evaluates to "-2". this was false under the old rules but is
true when LOGICAL_PREDICATE is in effect. A logical not operator was
introduced in 6.0 and this always works, well, logically.
--
Mark Hadfield "Ka puwaha te tai nei, Hoea tatou"
m.hadfield@niwa.co.nz
National Institute for Water and Atmospheric Research (NIWA)
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| Re: A simple IF statement question [message #42502 is a reply to message #42500] |
Mon, 14 February 2005 13:19  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Michael Wallace writes:
>
> Nice article, but it doesn't show the logical operators &&, || and ~.
> I'm not complaining, but thought that if you're ever bored one day, you
> can explore the facets of ~ and NOT and AND and && or OR and ||.
Uh, you're talking to the wrong guy. Mark Hadfield wrote
that article. :-)
Cheers,
David
P.S. Mark, are you ever bored?
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
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| Re: A simple IF statement question [message #42505 is a reply to message #42502] |
Mon, 14 February 2005 13:09 |
Michael Wallace
Messages: 409
Registered: December 2003
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Senior Member |
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>>> I am using IDL 6.1 on Linux SUSE. I am writing a simple code using the
>>> IF statement and am wondering about the following result:
>>>
>>> IDL> IF 1 THEN PRINT, '1' ELSE PRINT, 'None'
>>> 1
>>> IDL> IF 2 THEN PRINT, '2' ELSE PRINT, 'None'
>>> None
>>> IDL> IF 19 THEN PRINT, '19' ELSE PRINT, 'None'
>>> 19
>>> IDL> IF 24 THEN PRINT, '24' ELSE PRINT, 'None'
>>> None
>>> IDL> IF 0 THEN PRINT, '0' ELSE PRINT, 'None'
>>> None
>>>
>>> Am I wrong when I expect the IF statement to return always TRUE if the
>>> condition is not 0 (I mean something like 1,2,3,4,....)?
>>
>> Yes, you are wrong. :-)
>>
>> Here is an article you might want to read:
>>
>> http://www.dfanning.com/code_tips/bitwiselogical.html
>
>
> It has been pointed out to me that the article is a bit
> deficient in that it doesn't mention the LOGICAL_PREDICATE
> compiler option. If you set:
>
> COMPILE_OPT LOGICAL_PREDICATE
>
> Then 0 is FALSE and everything else is TRUE. That probably
> makes more sense to *everyone*! :-)
Nice article, but it doesn't show the logical operators &&, || and ~.
I'm not complaining, but thought that if you're ever bored one day, you
can explore the facets of ~ and NOT and AND and && or OR and ||.
Actually, I only mention that because I think it's really cool to have
part of an actual sentence that reads aloud as "not and not and and and
and or or and or." And then there's also LOGICAL_OR and LOGICAL_AND
operators. Too many operators!!
Seriously now, using logical not (~) you can achieve the same thing as
the LOGICAL_PREDICATE. Actually ~ is tied to the same definition as
LOGICAL_PREDICATE, so it behaves the same way. If you want a variable
var to evaluate to false when 0 and true otherwise, all you need is
~(~var). If you just did ~var, you'd get the exact opposite of what we
want -- 0 is TRUE (1) and everything else is FALSE (0). The second
logical not flips this result. Just another way to skin a cat.
-Mike
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