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| Re: Strange arrary indexing quirk? [message #77179 is a reply to message #77178] |
Thu, 11 August 2011 20:58  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Beaker writes:
> I stumbled across this strange behaviour:
>
> foo = indgen(2,2)
> print,foo[0,10]
> < gives an error, since the second index is out of range>
>
> however,
>
> indx = indgen(5)
> print,foo[0,indx]
> 0
> 2
> 2
> 2
> 2
>
> Does anyone have any insight into why this happens? I'd expect the
> second line to also throw an error. A lot of the inner workings of IDL
> seem pretty arcane and hard to guess at before you try things...
>
> I was being lazy/naughty and using a catch statement to detect when an
> index went out of range, which I knew would happen very occaisonally
> and was trying to avoid the performance hit of checking for it every
> time. This round off instead of an error behaviour made sure that plan
> didn't work!
This is a famous IDL quirk. If you would prefer to have
an error thrown in this situation, use the compile option
STRICTARRSUBS:
Compile_Opt STRICTARRSUBS
Looking up that compiler option will also point you to information
about array subscripts. (At least it will in the IDL 7 on-line help.)
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.idlcoyote.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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| Re: Strange arrary indexing quirk? [message #77180 is a reply to message #77179] |
Thu, 11 August 2011 20:45 |
penteado
Messages: 866
Registered: February 2018
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Senior Member
Administrator |
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On Aug 11, 10:12 pm, Beaker <mattjamesfran...@gmail.com> wrote:
> Does anyone have any insight into why this happens? I'd expect the
> second line to also throw an error. A lot of the inner workings of IDL
> seem pretty arcane and hard to guess at before you try things...
It happens because it was chosen that way when the language was
defined. Quoting from the help:
"STRICTARRSUBS — When IDL subscripts one array using another array as
the source of array indices, the default behavior is to clip any out-
of-range indices into range and then quietly use the resulting data
without error. This behavior is described in Understanding Array
Subscripts. Specifying STRICTARRSUBS will instead cause IDL to treat
such out-of-range array subscripts within the body of the routine
containing the COMPILE_OPT statement as an error. The position of the
STRICTARRSUBS option within the module is not important: All
subscripting operations within the entire body of the specified
routine will be treated this way."
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| Re: Strange arrary indexing quirk? [message #77181 is a reply to message #77180] |
Thu, 11 August 2011 20:42 |
penteado
Messages: 866
Registered: February 2018
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Senior Member
Administrator |
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On Aug 11, 10:12 pm, Beaker <mattjamesfran...@gmail.com> wrote:
> Does anyone have any insight into why this happens? I'd expect the
> second line to also throw an error. A lot of the inner workings of IDL
> seem pretty arcane and hard to guess at before you try things...
>
> I was being lazy/naughty and using a catch statement to detect when an
> index went out of range, which I knew would happen very occaisonally
> and was trying to avoid the performance hit of checking for it every
> time. This round off instead of an error behaviour made sure that plan
> didn't work!
If you do not like that behaviour, use
compile_opt strictarraysubs
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