FUNCTION centertrack,data,dim,average = average, pretrack = pretrack

;+
; NAME:
;	centertrack
; PURPOSE:
;	Returns center-of-mass coordinates of a track or pretrack array.
;	Centering is done by default for each individual timestamp.
; CALLING SEQUENCE:
;	ctrack=centertrack(track,dimension,average=average,pretrack=pretrack)
; INPUTS:
;   track: a two- or three-dimensional track or pretrack array
;   dim:  the spatial dimension of the data
; KEYWORDS:
;   average: specify /average to return coordinates relative to the
;   time-averaged center-of-mass
;   pretrack:  specify /pretrack if data is a pretrack array
; OUTPUTS:
;	ctrack: an array one-to-one with the initial track file, with
;	centered coordinates
; SIDE EFFECTS:
;   Sweating has been documented in rare cases.
; MODIFICATION HISTORY:
;   Written by Gary L. Hunter, 2009
;-


blah = data
d = dim
nc = n_elements(blah[*,0])

IF keyword_set(pretrack) THEN BEGIN
tloc = nc-1
ENDIF ELSE BEGIN
tloc = nc-2
ENDELSE

IF keyword_set(average) THEN BEGIN

FOR i = 0, d-1 DO blah[i, *] = blah[i, *]-mean(blah[i, *])

ENDIF ELSE begin

tlist = unique(blah[tloc,sort(blah[tloc,*])])
nts = n_elements(tlist)

FOR i = 0L,nts-1 DO BEGIN

IF i MOD 1e2 EQ 0 THEN BEGIN
 print,n2s(i)+' of '+n2s(nts-1)
endif

wt = where(blah[tloc,*] EQ tlist[i])

FOR j = 0, d-1 DO BEGIN
me = mean(blah[j, wt])
blah[j, wt] = blah[j, wt]-me
endfor


ENDFOR



ENDELSE



return,blah
end