| Re: IDL 8.0 compile_opt changes [message #69452 is a reply to message #69187] |
Thu, 07 January 2010 09:56   |
Michael Galloy
Messages: 1114
Registered: April 2006
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Senior Member |
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On 1/7/10 8:27 AM, Maarten wrote:
> On Jan 6, 10:25 pm, mgalloy<mgal...@gmail.com> wrote:
>> On 1/6/10 7:01 AM, Maarten wrote:
>>
>>> I do love the idea of negative indices, although I'd like them to mean
>>> the same as in Python. The samples Ive seen so far are off by one.
>>
>> My understanding of the negative indices proposal in IDL was that they
>> would be the same as in Python:
>>
>>>> > a = [1, 2, 3, 4]
>>>> > a[-1]
>> 4
>>>> > a[-2]
>> 3
>
> Yes, in this case it is the same. The (subtle) difference comes in for
> ranges.
>
> Python:
>>>> a = [1,2,3,4]
>>>> a[-1]
> 4
>>>> a[-2]
> 3
>>>> a[1:-1]
> [2, 3]
>
> The last one is the one I'm concerned about, as python does not
> include the last index in the range.
>
> IDL> a = [1,2,3,4]
> IDL> print, a[3]
> 4
> IDL> print, a[1:3]
> 2 3 4
>
> (index 3 is equivalent to index -1).
>
> Now you could say that Python and IDL already disagree here, but the
> off-by-one is worth mentioning anyway.
I think we are agreeing here, but just to be sure: Python and IDL would
be specifying the endpoints of the range in the same way, it's just that
Python always includes the start index and excludes the end index (even
if not using negative indices):
>>> a = [1, 2, 3, 4]
>>> a[1:3]
[2, 3]
Mike
--
www.michaelgalloy.com
Research Mathematician
Tech-X Corporation
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