| Re: array subscript conversion [message #55033 is a reply to message #55032] |
Wed, 25 July 2007 10:46   |
Dick Jackson
Messages: 347
Registered: August 1998
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Senior Member |
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Hi,
"F�LDY 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!):
IDL> help,-1ull
<Expression> ULONG64 = 18446744073709551615
That's why a[[-1ull]] gives 9, as the subscript is (somewhat) larger than the
maximum index in the array a. It is similar to a[[11]] below...
IDL> help,long(-1ull)
<Expression> LONG = -1
This is similar to a[[-1]] below...
IDL> print,a[[-1]]
0
IDL> print,a[[11]]
9
And we know this is different from simple subscripting which doesn't allow
out-of-range values:
IDL> print,a[-1]
Attempt to subscript A with <INT ( -1)> is out of range.
Execution halted at: $MAIN$
IDL> print,a[11]
Attempt to subscript A with <INT ( 11)> is out of range.
Execution halted at: $MAIN$
--
Cheers,
-Dick
--
Dick Jackson Software Consulting http://www.d-jackson.com
Victoria, BC, Canada +1-250-220-6117 dick@d-jackson.com
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