Re: Not where

[Martin Schultz <martin.schultz(at)dkrz.de>]

Alex Schuster wrote:
> 
> David Fanning wrote:
> 
> > Kenneth P. Bowman (bowman@null.edu) writes:
> >
> > > I often find myself using WHERE to divide an array into two parts.  I do
> > > one operation on the first part and a different operation on the second
> > > part.
> > >
> > > It would be nice to have an auxiliary array containing all the indices
> > > that are *not* returned by WHERE in i. [...]
> 
> Easy, yes, maybe not too fast, and not very elegant. Martin Schulz wrote
> (and probably posted) the routine INV_INDEX, which I attached.

I'm listening ;-)


> [...]
> This would add 1 to a[i] and subtract 5 from a[NOT(i)], for example. But
> I admit that
>         j = inv_index( i )
>         a[i] = a[i] + 1
>         a[j] = a[j] - 5
> looks better. (Not cooler, but better.)

It might not work, though ;-( 
1. You *must* supply the second parameter (totaln), e.g. as in
       j = inv_index(i,n_elements(data))
2. I had to fix a bug which was that I used short ints instead of long.

So, please find the new version of this routine attached ...

Cheers,
Martin


-- 
[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[
[[ Dr. Martin Schultz   Max-Planck-Institut fuer Meteorologie    [[
[[                      Bundesstr. 55, 20146 Hamburg             [[
[[                      phone: +49 40 41173-308                  [[
[[                      fax:   +49 40 41173-298                  [[
[[ martin.schultz@dkrz.de                                        [[
[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[
; $Id: inv_index.pro,v 1.11 1999/05/20 16:15:49 mgs Exp $
;-------------------------------------------------------------
;+
; NAME:
;        INV_INDEX
;
; PURPOSE:
;        find the indices that do NOT match a WHERE condition
;
; CATEGORY:
;        array index handling
;
; CALLING SEQUENCE:
;        RESULT = INV_INDEX(INDEX,TOTALN)
;
; INPUTS:
;        INDEX : an index array, e.g. previously generated by a
;              WHERE command (may be -1)
;        TOTALN : the number of elements in the reference data
;              set, i.e. totaln = n_elements(index)+n_elements(result)
;
; KEYWORD PARAMETERS:
;
; OUTPUTS:
;        an integer array with all indices that were NOT in index
;        or -1 if index was complete
;
; SUBROUTINES:
;
; REQUIREMENTS:
;
; NOTES:
;        The function returns -1 if one of the following errors occurs:
;        - invalid number of arguments
;        - index variable is undefined
;        - totaln is less than n_elements(index)
;        - totaln less or equal 1, i.e. no associated data
;        The last error does not produce an error message, since this
;        feature was found to be very useful (in EXPLORE, the widget based
;        interactive data explorer)
;
; EXAMPLE:
;        data = findgen(50)
;        index = where(data ge 25)
;        invers = inv_index(index,n_elements(data))
;        print,invers
;
;        IDL prints numbers 0 through 24
;
; MODIFICATION HISTORY:
;        mgs, 10 May 1997: VERSION 1.00
;        mgs, 18 Aug 1997: - added template and check if n_elements(index) eq 0
;        mgs, 05 Apr 1999: - bug fix: needed to make sure result is type long
;
;-
; Copyright (C) 1997, Martin Schultz, Harvard University
; This software is provided as is without any warranty
; whatsoever. It may be freely used, copied or distributed
; for non-commercial purposes. This copyright notice must be
; kept with any copy of this software. If this software shall
; be used commercially or sold as part of a larger package,
; please contact the author to arrange payment.
; Bugs and comments should be directed to mgs@io.harvard.edu
; with subject "IDL routine inv_index"
;-------------------------------------------------------------


function inv_index,index,totaln

   newindex = -1L     ; default: nothing left

   ; check for errors:
   if (N_Params() ne 2) then begin
      print,'INV_INDEX: wrong number of arguments'
      return,newindex
   endif
   if (n_elements(index) eq 0) then begin
      print,'INV_INDEX: no valid index passed'
      return,newindex
   endif
   if (totaln lt n_elements(index)) then begin
      print,'INV_INDEX: totaln lt n_elements(index)'
      return,newindex
   endif
   if (totaln le 1) then return,newindex    ; no data there

   ; and handle the two situations:
   if (max(index) lt 0) then begin   ; no valid index passed
      newindex = lindgen(totaln)      ; create an integer array
      return,newindex                ; with totaln elements
   endif

   ; else a valid indexarray was passed and we can construct the inverse
   newindex = lindgen(totaln)
   newindex(index) = -1
   i = where(newindex ge 0,count)
   if (count gt 0) then newindex = newindex(i)  $
   else newindex = -1L

   return, newindex
end