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From: steve@bat.lle.rochester.edu (Steve Swales)
Newsgroups: comp.lang.idl-pvwave
Subject: Re: Curious array behaviour?
Message-ID: <12793@ur-cc.UUCP>
Date: 15 Mar 91 23:02:33 GMT
References: <275@locke.water.ca.gov>
Sender: news@uhura.cc.rochester.edu
Reply-To: steve@bat.lle.rochester.edu (Steve Swales)
Organization: UofR Laboratory for Laser Energetics
Lines: 38

In article <275@locke.water.ca.gov>, rfinch@caldwr.water.ca.gov (Ralph Finch) writes:
> 
> PV-Wave 3.02 on a Sun 4.
> 
> Array operations are quite powerful in idl-wave and so different from
> other languages that I think I still don't fully understand them.
> Here is something that has me puzzled:
> 
> a=[1,2,3,4,5]
> print,a([-1])
>   -> 1
> print,a(-1)
>   -> ERROR
> 
> Why the difference?

Looks like a 'feature' to me. One explanation for it might be that one
commonly will use this syntax for 'mapping' an array through a lookup
table, and in this case it makes sense to have range checking for
values off the end of the LUT and 'clamp' them to the end values... i.e.:

lut=[0.1,0.5,0.75,0.9,0.95]
values=[0,1,2,3,4,5,-1,11,4,3,2,7]
print,lut(values)
 ->   .100000      .500000      .750000      .900000      .950000      .950000
 ->   .100000      .950000      .950000      .900000      .750000      .950000

Isn't it fun to think up justifications for inconsistent program behavior...
and you get lots of practice working with WAVE/IDL :-).

-steve
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