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Re: array subscript conversion [message #55031]

Wed, 25 July 2007 11:26

Foldy Lajos
Messages: 268
Registered: October 2001

Senior Member

On Wed, 25 Jul 2007, Dick Jackson wrote:

> Hi,
>
> "FÖLDY Lajos" <foldy@rmki.kfki.hu> wrote in message
> news:Pine.LNX.4.64.0707251721430.28405@bifur.rmki.kfki.hu...
>> Hi guys,
>>
>> according to the manual, array subscripts are converted to long (or long64 on
>> 64 bit systems) before use if necessary, so an explicit conversion should not
>> affect the result.
>>
>> IDL> print, !version
>> { x86 linux unix linux 6.3 Mar 23 2006 32 64}
>> IDL>
>> IDL> a=lindgen(10)
>> IDL> print, a[[long(-1ull)]]
>> 0
>> IDL> print, a[[-1ull]]
>> 9
>>
>> Is it a bug or I am missing something?
>
> I think you're expecting -1ull to be negative, but the 'u' in 'ull' means
> 'unsigned'. What you end up with instead of -1 is the largest 64-bit integer
> (this is a nice shortcut when it's actually what you want to do!):
>

No, I have expected LONG(-1ull) to be used for subscripting, which is
really negative. As Mike wrote, integers are not converted to LONG, that
was my wrong assumption.

I am trying to find differences between IDL and FL behavior, and this is
one example. In FL, I always convert non-LONG subscripts to LONG. (It's a
pity we have no formal definition of IDL syntax and semantics.)

regards,
lajos

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[Message index]

		Re: array subscript conversion By: Foldy Lajos on Wed, 25 July 2007 11:26
		Re: array subscript conversion By: Foldy Lajos on Wed, 25 July 2007 11:04
		Re: array subscript conversion By: Dick Jackson on Wed, 25 July 2007 10:46
		Re: array subscript conversion By: Michael Galloy on Wed, 25 July 2007 09:09

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