;+
; @file_comments
; extract elements of a structure to constitute a new structure.
;
; @categories
; Utilities
;
; @param STRU {in}{required}
; A structure
;
; @param LISTE {in}{required}{type=vector}
; A vector of string including names of STRU to be deleted
; (by default) or to be kept (if KEEP is activated).
;
; @keyword KEEP
; Specify that the given liste concern elements of STRU to be kept.
;
; @keyword DELETE
; Specify  that the given liste concern elements of STRU to be deleted.
; This keyword is activated by default.
;
; @returns
; A structure or -1 in case of problem
;
; @examples
;
;   IDL> extra=get_extra(/ok, year=1999, age_capitaine=35 )
;   IDL> help, extra,/struct
;    ** Structure <83e66bc>, 3 tags, length=6, refs=1:
;       AGE_CAPITAINE   INT             35
;       OK              INT              1
;       YEAR            INT           1999
;   IDL> help, extractstru(extra,['ok','hhuihi','YEAR']),/stru
;    ** Structure <831afac>, 1 tags, length=2, refs=1:
;       AGE_CAPITAINE   INT             35
;   IDL> help, extractstru(extra,['ok','hhuihi','YEAR'],/keep),/stru
;    ** Structure <834bbc4>, 2 tags, length=4, refs=1:
;       OK              INT              1
;       YEAR            INT           1999
;
; @history
; Sebastien Masson (smasson\@lodyc.jussieu.fr)
;                      8/10/1999
;
; @version
; $Id$
;
;-
FUNCTION extractstru, stru, liste, KEEP=keep, DELETE=delete
;
  compile_opt idl2, strictarrsubs
;
   if size(stru, /type) NE 8 then return,  -1
   if size(liste, /type) NE 7 then return,  -1
; checking for keep and vire keywords
   keep = keyword_set(keep)*(1-keyword_set(delete))
   delete = keyword_set(delete)*(1-keyword_set(keep)) +(keyword_set(delete) EQ keep)
;
   tname = tag_names(stru)
   index = make_selection(tname, strupcase(liste), /only_valid, /quiet)
;
   if keep then BEGIN ; We just keep the list
      if index[0] EQ -1 then return,  -1
      if n_elements(index) EQ n_elements(tname) then return, stru
      res = create_struct(tname[index[0]], stru.(index[0]))
      if n_elements(index) GT 1 then for i = 1, n_elements(index)-1 do $
            res = create_struct(res, tname[index[i]], stru.(index[i]))
   ENDIF ELSE BEGIN ; We delete the list
      if n_elements(index) EQ n_elements(tname) then return,  -1
      if index[0] EQ -1 then return, stru
; We take the complementary one of index to obtain indexes we keep.
      index = different(indgen(n_elements(tname)), index)
      res = create_struct(tname[index[0]], stru.(index[0]))
      if n_elements(index) GT 1 then for i = 1, n_elements(index)-1 do $
            res = create_struct(res, tname[index[i]], stru.(index[i]))
   ENDELSE

   return, res
end