| Re: Subscripting arrays. [message #84148] |
Thu, 02 May 2013 13:22  |
Michael Galloy
Messages: 1114
Registered: April 2006
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Senior Member |
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On 5/2/13 1:40 PM, rryan@stsci.edu wrote:
> So someone showed me this and I'm trying to wrap my head around it.
> Sorry if it's already posted, I didn't know how to begin to search
> The Group for it...
>
>
> Create an array:
>
> a = intarr(3)
>
> Using IDL 8, subscript with 4 (ie. an index that's too large and
> should be out of bounds)
>
> a[4]=3
>
> Obviously, this crashes.
>
> Conversely subscript with a negative index:
>
> a[-1]=2
>
> does what you expect.
>
> But, now subscript the array, WITH an array:
>
> a[[4]]=2
>
> voila. It works, and simply truncates to the last element of the
> array... Spooky. Now do that with a negative index:
>
> a[[-1]]=4
>
> Like before, it works. But now, it doesn't wrap the index, but
> rather truncates to the first element of the array.
>
> Hmm.. I can't tell if this is the designed behavior, because I have a
> hard time describing it --- it's easier to show it.
Yes, this is the "designed" behavior. You can turn off the odd array
indexing (and just give an error) with compile_opt:
IDL> x = findgen(10)
IDL> print, x[[-1, 0, 9, 10]]
0.00000 0.00000 9.00000 9.00000
IDL> compile_opt strictarrsubs
IDL> print, x[[-1, 0, 9, 10]]
% Array used to subscript array contains out of range subscript: X.
% Execution halted at: $MAIN$
Mike
--
Michael Galloy
www.michaelgalloy.com
Modern IDL: A Guide to IDL Programming (http://modernidl.idldev.com)
Research Mathematician
Tech-X Corporation
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