| Re: V6.0 indexing change [message #36188 is a reply to message #36186] |
Tue, 12 August 2003 17:45   |
Mark Hadfield
Messages: 783
Registered: May 1995
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Senior Member |
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Andrew Cool wrote:
> Wayne Landsman <landsman@mpb.gsfc.nasa.gov> wrote in message news:<3F39035C.8070409@mpb.gsfc.nasa.gov>...
>
>> I haven't installed V6.0 yet myself but I have been informed by Mike
>> Corcoran of a change that can break some existing code.
>>
>> Formerly, if one had a structure, say a = {x:indgen(10)} which was
>> accessed by a 1 element vector, say i = [3], then the result a.x[i]
>> would be a scalar. In V6.0, however, the result is now a 1 element
>> vector.
>>
>> I have to say that the V6.0 method makes more more sense -- subscripting
>> with a 1 element vector should return a 1 element vector. But it can
>> break existing code because logical tests such as
>>
>> if a.x[i] EQ 3 then stop
>>
>> will no longer work. The fix is simple enough ( set i = i[0] first )
>> but users might want to fix the code before upgrading to V6.0.
>>
>
> You sure about this? Yes, a 1 element vector is now returned, but
> your example
>
> if a.x[i] EQ 3 then stop
>
> works just fine for me under the 6.0 Beta (Win2000), as does
>
> if a.x[i[0]] EQ 3 then stop
And for me under 6.0 final (Win 2000). But then this...
IDL> if [1] eq 1 then print, 'comparison is OK'
...works (ie prints out the message) under IDL 6.0 and 5.6. In other
words an if statement doesn't seem to care if its condition is a scalar
or a 1-element array. News to me!
Still, Wayne's warning is correct: there may be cases where the new,
improved structure indexing in 6.0 will break code.
--
Mark Hadfield "Ka puwaha te tai nei, Hoea tatou"
m.hadfield@niwa.co.nz
National Institute for Water and Atmospheric Research (NIWA)
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