pro get_group,sc,bc,ch,ff,ffe,chl,chh,ctonth=ctonth,fout=fout
;+
;
; calculate the boundary channel numbers for a constant signal to noise ratio
; spectrum
;
; sc - count vector for the source (including background)
; bc - expected background count vector
; ch - mean channel number of the output spectrum bins
; ff, ffe - output spectrum and its erro bars
; chl, chh - the lower and upper channel numbers of the output spectrum bins
; ctonth - the signal to noise threshold; def=3
; fout - the output file name (to be used by grppha for example)
;       which can be used in the program grppha for grouping.
;
; written by wqd, April 28, 2000
;
;-
if n_params() eq 0 then begin
print,'CALLING SEQUENCE -- get_group,sc,bc,ch,ff,ffe,chl,chh,ctonth=ctonth,fout=fout'
return
endif
nch=n_elements(sc)
get_hist_var,lindgen(nch),sc,bc,replicate(1.,nch),ch,chl,chh,ff,ffe,t,cton=ctonth
gl=[-999]
gh=[-999]
chl=fix(chl)+1
ng=n_elements(ch)
chh=[chl(1:ng-1)-1,nch]
chd=chh-chl+1
sel=where(chd ne 1,nsel)
print,'channellow channelhigh groupfactor '
for k=0,nsel-1 do begin
        kk=sel(k)
        print,chl(kk)-1,chh(kk)-1,fix(chd(kk))
        ; the output is to be used in grppha for grouping.
endfor
if n_elements(fout) ne 0 then begin
 openw,un,fout,/get
 for k=0,nsel-1 do begin
        kk=sel(k)
        printf,un,chl(kk)-1,chh(kk)-1,fix(chd(kk))
 endfor
 free_lun,un
endif
; ploterr,ch,ff,ffe
if !debug eq 1 then stop
return
end