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Re: Warning: IDL 8.0 alters the behaviour of existing valid programs without any notice! [message #72180 is a reply to message #72179] Wed, 18 August 2010 05:14 Go to previous messageGo to previous message
Maarten[1] is currently offline  Maarten[1]
Messages: 176
Registered: November 2005
Senior Member
On Aug 18, 1:59 pm, svhhaugan <s.v.h.hau...@gmail.com> wrote:
> pro test,data
> catch,error
> if error ne 0 then begin
> catch,/cancel
> print,"Guess that didn't work"
> return
> end
> data[where(data eq min(data)-1)] = 50
> end
>
> Perfectly valid code, makes sense in IDL 7.1 although the catch,error
> part would most likely be done "with human intervention".

I just looked up the help for where in IDL 7. This is what is
prominently displayed on that page:

Note: When WHERE Returns –1

If all the elements of Array_Expression are zero, WHERE returns a
scalar integer with a value of –1. Attempting to use this result as an
index into another array results in a "subscripts out of bounds"
error. In situations where this is possible, code similar to the
following can be used to avoid errors:

; Use Count to get the number of nonzero elements:
index = WHERE(array, count)

; Only subscript the array if it's safe:
IF count NE 0 THEN result = array[index]

And yes, you can use the try/catch exception method of handling this,
but I would pose that testing for an item not found message and then
using an IF statement to handle that common situation makes more sense
than relying on an error. So in that sense the code was never valid or
sensible in the first place.

Does that mean no one will be caught off-guard by this? No. Is that an
issue? Some people just get what they deserve. The added benefit of
being able to count from the end of an array is certainly worth some
trouble.

Maarten

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