| Re: Loop Arrays [message #27240 is a reply to message #27110] |
Mon, 15 October 2001 14:43   |
Martin Downing
Messages: 136
Registered: September 1998
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Senior Member |
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"Ken Mankoff" <mankoff@I.HATE.SPAM.cs.colorado.edu> wrote in message
news:Pine.LNX.4.33.0110091423020.29204-100000@snoe.colorado. edu...
> On Tue, 9 Oct 2001, Mark Hadfield wrote:
>
>> From: "Ken Mankoff" <mankoff@lasp.colorado.edu>
>>> I am interested in creating circular arrays, where subscripts that
would
>>> be out-of-bounds on a regular array just start indexing on the other
side
>>> of the array.
>>
>> You can do quite a lot with ordinary arrays using arrays of indices, eg
>>
>> a = indgen(10)
>> print, a[ [0,10,20,100] mod n_elements(a)]
>>
>
> This is the technique I have been using. However there are 2 cases it does
> not cover:
>
> 1) negative indexes require a few more lines of code to get your example
> to work. I would recode it as:
>
> a = indgen( 10 )
> indexes = [ 0,10,20,100,-10,-22 ] ;;; or some other values...
> ind = indexes mod n_elements( a )
> neg = where( ind lt 0, num )
> if ( num ne 0 ) then ind[ neg ] = ind[ neg ] + n_elements( a )
> print, a[ ind ]
>
> 2) subscript ranges. You cannot do:
> print, a[ 8:12 mod n_elements(a) ]
>
> It is these two specific abilities that I would like to have.
>
> -k.
Hi Ken,
This discussion makes for interesting reading. However, except for arrays
representing objects with circular indexing logic, such as closed
polygons for instance, I'm not sure it is productive to prevent IDL from
pointing out that you have run off the end of an array!
Anyway, there is a way you can code range indexing above for circular
arrays:
eg for indexing a[b:c] do the following:
IDL> a = indgen(10) ; to be interpreted as a circular array
IDL> b = 9 & c = 13
IDL> print, a[ (indgen(c-b)+b) MOD n_elements(a) ]
; read as a[b:c]
9 0 1 2
IDL> b = 9 & c = 23
IDL> print, a[ (indgen(c-b)+b) MOD n_elements(a) ] ; read as a_circ[b:c]
9 0 1 2 3 4 5 6 7 8 9 0 1 2
-Is that of any use to you?
regards
Martin
>
> --
> Ken Mankoff
> LASP://303.492.3264
> http://lasp.colorado.edu/~mankoff/
>
>
>
>
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