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Re: CASE statement [message #26539 is a reply to message #26530]

Fri, 07 September 2001 20:18

Jeff Guerber
Messages: 41
Registered: July 2000

Member

On Fri, 7 Sep 2001, Brian Jackel wrote:

> CASE (1) OF
> (this OR that): x=1
> (something AND somethingelse): x=2
> (NOT theotherthing): x=3

Careful!!! Notice:

IDL> print, not 0, not 0b, not 1, not 1b
-1 255 -2 254
IDL> print, (not (1 eq 2)) eq (1 ne 2)
0
IDL>

Unfortunately, AND, OR, XOR, and NOT all operate _bitwise_, at least on
integral types. The first two selectors in your CASE should be OK if the
operands are all expressions that evaluate to 0 or 1 (which the
relationals EQ, NE, GT, GE, LT, and LE all do; but watch out for the
general case!). The third one very likely won't do what you intend.

This is why I've argued a couple times for a true logical type, like
Fortran's, with "true" and "false" system constants and Boolean operators
that always return them. More recently, though, I discovered some
examples in the manual that use the 0 or 1 returned from the relationals
in arithmetic expressions, so I guess we're stuck with them; I'd be mostly
satisfied with operators that return only 0 or 1, and constants !true=1
and !false=0. (Hmmmm. Perhaps a type that can _only_ have the values 0
or 1, with operand conversion based on truth value??)

A while back, I tried to think up a function to evaluate the truth of
its argument, for a general case, and return 0 or 1, for use in situations
like this. The only thing I could come up with involved a loop over all
its elements, enclosing an IF (or equivalent a?b:c) statement. OK for
scalars, not so good for arrays.

> ELSE:MESSAGE,'Error- no match found for case statement'
> ENDCASE
>
> Kinda ugly, but gets the job done.

Agreed, and I use the idiom myself; but, you've got to be more careful
with it than I for one think you should have to be!

Jeff Guerber
Raytheon ITSS
NASA Goddard Space Flight Ctr
Oceans & Ice Branch

P.S. I've long thought that a language where comparisons can be
distributed, as in English, would be very handy:

if expression eq A or B or C or D then...

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[Message index]

		Re: CASE statement By: R.Bauer on Mon, 10 September 2001 04:54
		Re: CASE statement By: Martin Downing on Mon, 10 September 2001 04:45
		Re: CASE statement By: Jeff Guerber on Fri, 07 September 2001 20:18
		Re: CASE statement By: gmccabe14 on Fri, 07 September 2001 18:44
		Re: CASE statement By: Pavel A. Romashkin on Fri, 07 September 2001 16:16
		Re: CASE statement By: Kristian Kjaer on Fri, 07 September 2001 15:17
		Re: CASE statement By: Pavel A. Romashkin on Fri, 07 September 2001 14:30
		Re: CASE statement By: Brian Jackel on Fri, 07 September 2001 14:41
		Re: CASE statement By: David Fanning on Fri, 07 September 2001 14:00
		Re: CASE statement By: Stein Vidar Hagfors H[1] on Fri, 07 September 2001 13:39
		Re: CASE statement By: David Fanning on Fri, 07 September 2001 13:23
		Re: CASE statement By: marc schellens[1] on Wed, 12 September 2001 23:01
		Re: CASE statement By: george.mccabe on Wed, 03 October 2001 11:49

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