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Re: Subscripting multidimensional arrays [message #37278 is a reply to message #37275]

Fri, 12 December 2003 08:53

JD Smith
Messages: 850
Registered: December 1999

Senior Member

On Fri, 12 Dec 2003 07:55:16 -0700, Christopher Lee wrote:

> In article <nwfCb.37807$SU2.20541@newssvr29.news.prodigy.com>, "Jonathan
> Greenberg" <greenberg@ucdavis.edu> wrote:
>
>
>> Hi all -- I was hoping to get some help with converting a vector which
>> contains the x,y,z position for a value I want to exract from a
>> multidimensional array -- I understand that using an array to subscript
>> another array requires knowing the linear subscript position. For
>> example: a = 0 10 20
>> 30 40 50
>> 60 70 80
>>
>> 90 100 110
>> 120 130 140
>> 150 160 170
>> I have a vector which is defined as:
>> locationvector=[2,2,2]
>> I want to extract the value at that position (e.g. a[2,2,2] = 170), but
>> I can't do a:
>> a[locationvector] --> I apparently have to convert the locationvector
>> to that linear position. How do I do this? Does IDL have a built in
>> function that will do this conversion, or is there an easy formula for
>> doing this conversion in ANY dimension? Thanks! --j
>>
>>
> function element, array, loc_vector
>
> s=size(array)
> d=s[1:s[0]] ;dimensions
> e=lonarr(s[0]) ;product of dimensions e[0]=1L for i=1L, s[0]-1 do
> e[i]=e[i-1]*d[i-1] ;e is the number of elements each dimension contains
> return, total(loc_vector*e)
>
> end
>
> seems to work, there must be a better way though...

PRODUCT works nicely for this:

function linear_indices,array,vec_indices
s=size(array,/DIMENSIONS)
nd=n_elements(s)
if nd eq 1 then return,s[0]
return,long(total([1.,product(s[0:nd-1],/CUMULATIVE)]*vec_in dices))
end

to go the other direction, IDL6 offers ARRAY_INDICES. Or you can always
just resort to:

a[vec[0],vec[1],vec[2]]

A take home problem would be to modify this such that NxM input vectors,
where N is the number of dimensions of "array", will return a vector of
length M containing all the 1-D indices. Hints: REBIN/REFORM and the
"dimension" argument to TOTAL.

JD

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[Message index]

		Re: Subscripting multidimensional arrays By: JD Smith on Fri, 12 December 2003 13:35
		Re: Subscripting multidimensional arrays By: Chris Lee on Fri, 12 December 2003 13:15
		Re: Subscripting multidimensional arrays By: R.G. Stockwell on Fri, 12 December 2003 11:46
		Re: Subscripting multidimensional arrays By: R.G. Stockwell on Fri, 12 December 2003 11:44
		Re: Subscripting multidimensional arrays By: JD Smith on Fri, 12 December 2003 08:53
		Re: Subscripting multidimensional arrays By: R.G. Stockwell on Fri, 12 December 2003 08:57
		Re: Subscripting multidimensional arrays By: Chris Lee on Fri, 12 December 2003 06:55
		Re: Subscripting multidimensional arrays By: Jonathan Greenberg on Sat, 13 December 2003 12:19

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