;+
; NAME:
;  struct_assignfield
; PURPOSE: (one line)
;  assign a field to a value in an existing structure
; DESCRIPTION:
;  assign a field to a value in an existing structure, changing type
;  to type of passed variable.  This allows us to 
; CATEGORY:
;  util
; CALLING SEQUENCE:
;  new = struct_assignfield(old, tag, new_structname = new_structname)
; INPUTS:
;  old - existing structure
;  tag - name of field to delete (string)
;  val - value for tag
; OPTIONAL INPUT PARAMETERS:
;  none
; KEYWORD INPUT PARAMETERS:
;  new_structname - if the structure is to be named (not anonymous),
;                   the name
; KEYWORD OUTPUT PARAMETERS:
;  none
; OUTPUTS:
;  new - new structure with field added
; COMMON BLOCKS:
;  None
; SIDE EFFECTS:
;  none
; RESTRICTIONS:
; PROCEDURE:
;  calls struct_delfield and struct_addfield
; USAGE
; MODIFICATION HISTORY:
;  2005 Dec 31 Leslie Young
;-

function struct_assignfield, old, tag, val, new_structname = new_structname 

    n = n_elements(old)

    if n eq 1 then begin
        old_names = tag_names(old)
        match = where( strcmp(old_names, tag, /fold_case) eq 1, count )
        if count ne 0 then begin
            tmp = struct_delfield(old, tag)
            new = struct_addfield(tmp, tag, val, $
                                  new_structname = new_structname)
        endif else begin
            new = struct_addfield(tmp, tag, val, $
                                  new_structname = new_structname)
        endelse
        
        return, new
    endif

    ; recursive call for old = array of structures
    new1 = struct_assignfield(old[0], tag, val[0], $
                              new_structname = new_structname)
    new = replicate(new1, n)
    for i=1, n-1 do begin
        new[i] = struct_assignfield(old[i], tag, val[i], $
                              new_structname = new_structname)
    endfor
    return, new

end