| Re: Logical vs. Bitwise boolean operators [message #29156 is a reply to message #29155] |
Fri, 01 February 2002 12:47  |
Jeff Guerber
Messages: 41
Registered: July 2000
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Member |
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On Fri, 1 Feb 2002, James Kuyper wrote:
> Fortran has the logical boolean operators .AND., .OR., and .NOT., and I
> naively expected the correspondingly named IDL operators to be the same.
> However, in IDL, those are bitwise operators, rather than logical ones
> (my copy of the manual just calls them 'boolean', without specifying
> whether they are logical or bitwise). As a result, the following code
> did not do what I expected:
>
> IF (!D.FLAGS AND 8) AND (ratio LE minrat) THEN BEGIN
James, as an aside, this statement tries use AND both bitwise _and_
logically. But I understand your point...
> I can always code around this limitation. The solution I came up with
> was to change it to:
>
> IF ((!D.FLAGS AND 8) EQ 8) AND (ratio LE minrat) THEN BEGIN
When confronted with a similar situation I like to use "IF ((!D.FLAGS
AND 8) NE 0) ...", which is a little more general.
> However, it seems to me that there should be a better way to do this.
I agree! I've argued for logical operators before, and also for a true
logical variable type like Fortran's. (I find the practice of using
integers for this to be the source of considerable confusion, especially
when coupled with the arcane rules for what's true and what's false, and
the bitwise operators.) But, so far Boulder has not heard.
Jeff Guerber
Raytheon ITSS
NASA/Goddard, code 971
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