| Re: Subscripting arrays. [message #84230 is a reply to message #84148] |
Fri, 03 May 2013 13:09  |
Russell Ryan
Messages: 122
Registered: May 2012
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Senior Member |
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A ha. Sounds reasonable, but I'm not sure why it's "designed" this way, but c'est la vie. I passed your response on to the friend, who groaned. We nearly said (in unison), I never pay much attention to those compiler options.
Thanks!
Russell
On Thursday, May 2, 2013 4:22:52 PM UTC-4, Mike Galloy wrote:
> On 5/2/13 1:40 PM, u wrote:
>
>> So someone showed me this and I'm trying to wrap my head around it.
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>> Sorry if it's already posted, I didn't know how to begin to search
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>> The Group for it...
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>>
>
>>
>
>> Create an array:
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>>
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>> a = intarr(3)
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>>
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>> Using IDL 8, subscript with 4 (ie. an index that's too large and
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>> should be out of bounds)
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>>
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>> a[4]=3
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>>
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>> Obviously, this crashes.
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>>
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>> Conversely subscript with a negative index:
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>>
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>> a[-1]=2
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>>
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>> does what you expect.
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>>
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>> But, now subscript the array, WITH an array:
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>>
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>> a[[4]]=2
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>>
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>> voila. It works, and simply truncates to the last element of the
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>> array... Spooky. Now do that with a negative index:
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>>
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>> a[[-1]]=4
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>>
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>> Like before, it works. But now, it doesn't wrap the index, but
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>> rather truncates to the first element of the array.
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>>
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>> Hmm.. I can't tell if this is the designed behavior, because I have a
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>> hard time describing it --- it's easier to show it.
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>
>
> Yes, this is the "designed" behavior. You can turn off the odd array
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> indexing (and just give an error) with compile_opt:
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>
>
> IDL> x = findgen(10)
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> IDL> print, x[[-1, 0, 9, 10]]
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> 0.00000 0.00000 9.00000 9.00000
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> IDL> compile_opt strictarrsubs
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> IDL> print, x[[-1, 0, 9, 10]]
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> % Array used to subscript array contains out of range subscript: X.
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> % Execution halted at: $MAIN$
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>
>
> 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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