While I almost hate to say it (as I have just learned how to use this
recently) this could be a job for histogram and reverse_indices.
http://www.dfanning.com/tips/histogram_tutorial.html
It seems to me that one way to do this is to:
- generate the mask [0,0,2,1,3,4,5,2,1,5,7,1,1,1] (same mask values
stuff)
- use histogram on the mask with binsize 1 which will then use put
each mask value in its own bin, from there the reverse_indices thing
is the trick.
I have experimented with histogram() and found that if you are pulling
one or two sets out of an array, then where() is faster, pulling three
or so different sets out then where() and histogram() are comparable,
and beyond that histogram() is significantly faster.
In a previous post is a touch of code to make the reverse_indices part
easier:
http://groups.google.com/group/comp.lang.idl-pvwave/browse_t hread/thread/9ccdf9fabc564f78/93e52a4ba6173817#93e52a4ba6173 817
but it uses idl pointer stuff which always confises the hell out of me
(this isn't C after all) so I wrote this one, and the example in the
doc header could be close to your problem, if I understand correctly.
;+
; NAME:
; reverse_indices
;
;
; PURPOSE:
; use histogram to pull out regions using the reverse_indices keyword
;
;
; INPUTS:
; histo - a histogram from the histogram() function
; ri - the histogram() reverse_indices keyword output
;
;
; OPTIONAL INPUTS:
; bin - the bin you want the indices for
;
;
; KEYWORD PARAMETERS:
; (all must be specified to use value)
; OMIN - omin keyword output from histogram()
; OMAX - omax keyword output from histogram()
; VALUE - specify a bin by value instead of number
;
;
; OUTPUTS:
; out - indices in that bin of a histogram
;
;
; OPTIONAL OUTPUTS:
; none
;
;
; COMMON BLOCKS:
; none
;
;
; SIDE EFFECTS:
; none
;
;
; RESTRICTIONS:
; none
;
;
; EXAMPLE:
; IDL> data=fix(randomu(101,25)*12)
; IDL> h=histogram(data, OMIN=omin, OMAX=omax, REVERSE_INDICES=ri,
binsize=2, /nan)
; IDL> print, data[sort(data)]
; 0 0 0 1 1 1 1
2 3 3 4 4 4 4 4
; 5 6 7 9 10 10 11 11
11 11
; IDL> print, data[reverse_indices(h, ri, 2)]
; 5 4 4 4 4 4
; IDL> print, data[reverse_indices(h, ri, value=3.5 , OMIN=omin,
OMAX=omax)]
; 2 3 3
;
;
;
; MODIFICATION HISTORY:
;
; Mon Feb 12 15:14:46 2007, Brian Larsen
; <larsen@ssel.montana.edu>
;
; written and tested
;
;-
FUNCTION reverse_indices, histo, ri, bin, OMIN=omin, OMAX=omax,
VALUE=value
;;;;;;;;;;; error checking ;;;;;;;;;;;;;;;;;;;
IF N_ELEMENTS(histo) EQ 0 THEN $
message, /ioerror, 'NULL histogram input'
IF N_ELEMENTS(ri) EQ 0 THEN $
message, /ioerror, 'NULL REVERSE INDICES input'
IF N_ELEMENTS(bin) EQ 0 AND N_ELEMENTS(value) EQ 0 THEN $
message, /ioerror, 'Must either specify bin or value'
IF N_ELEMENTS(value) NE 0 AND (N_ELEMENTS(omax) EQ 0 OR
N_ELEMENTS(omax) EQ 0) THEN $
message, /ioerror, 'Specifying value requires specifying omax and
omin'
;;;;;;;;; things are ok ;;;;;;;;;;;;;;;;;;;;;;
IF N_ELEMENTS(value) NE 0 THEN BEGIN
;; find which bin has 4 in it
binsize = (omin+omax)/(N_ELEMENTS(histo)-1)
bin = value/binsize
ENDIF
IF bin GE N_ELEMENTS(histo) THEN $
message, /ioerror, 'Bin out of range, [0,N_ELEMENTS(histo)-1]
out = ri[ri[bin]:ri[bin+1]-1]
RETURN, out
END
Brian
------------------------------------------------------------ ---------
Brian A. Larsen
Dept. of Physics
Space Science and Engineering Lab (SSEL)
Montana State University - Bozeman
Bozeman, MT 59717
On Feb 13, 8:25 am, "Conor" <cmanc...@gmail.com> wrote:
> Hmm... I found sample code here:http://www.dfanning.com/code_tips/
> drizzling.html
> that claims to do the job, but that doesn't work. The first problem
> is that n_ind isn't defined. I set that to n_elements(h1)+1 and then
> ran again. Then I got this:
> % SPRSIN: Vector must have 6 elements: <FLOAT Array[3]>
>
> Anyone happen to know what's going on here, or have a better
> suggestion?
>
> On Feb 13, 10:05 am, "Conor" <cmanc...@gmail.com> wrote:
>
>> Hey Everyone,
>> I'm essentially trying to add together separate sections of an
>> array, and I need to do it in a very speedy fashion. Here's the
>> breakdown in IDL. I would want to take an array like this:
>
>> vals = [10, 15, 13, 12, 11, 14]
>
>> and imagine I have a mask (which I can easily make) like this:
>
>> mask = [ 0, 0, 0, 1, 1, 1 ]
>
>> I would then want to add together everything with the same mask value
>> and put it in a new array. So the result would be:
>
>> sums = [38, 37]
>
>> I'm generating images, and for each image I generate this will be done
>> 1000 times, and there will be 1000 different mask values each time it
>> is done. I'll be generating a couple hundred images, so I'll be
>> running this task a couple hundred thounsand times - i.e. execution
>> speed is very important. Any suggestions on how to speed things up
>> would be highly appreciated.
>> Thanks in advanced,
>> Conor
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