;+
; NAME:
;  SetDefaultValue
;
; PURPOSE:
;
;   This procedure sets default values for positional and keyword arguments to
;   IDL procedures and functions.
;
; AUTHOR:
;
;   FANNING SOFTWARE CONSULTING
;   David Fanning, Ph.D.
;   1645 Sheely Drive
;   Fort Collins, CO 80526 USA
;   Phone: 970-221-0438
;   E-mail: davidf@dfanning.com
;   Coyote's Guide to IDL Programming: http://www.dfanning.com/
;
; CATEGORY:
;
;   Utilities
;
; CALLING SEQUENCE:
;
;   SetDefaultValue, argument, defaultValue
;
; ARGUMENTS:
;
;  argument:       The augument variable you are setting the default value of. If this variable
;                  is undefined, the defaultValue will be assigned to it. Otherwise, the argument
;                  variable will not change.
;                  
;  defaultValue:   The default value that will be assigned to the argument variable ONLY if the argument
;                  variable is undefined. If this variable is undefined, the argument variable will
;                  be treated as if the BOOLEAN keyword had been set.
;
; KEYWORDS:
;
;  BOOLEAN:        If this keyword is set, the argument value will always be forced to return with a 
;                  value of 0 or 1.
;
; EXAMPLE:
;
;  FUNCTION Action, arg1, arg2, MULTIPLY=multiply
;  
;     SetDefaultValue, arg1, 1
;     SetDefaultValue, arg2, 2
;     SetDefaultValue, multiply, 1, /BOOLEAN 
;     
;     IF multiply THEN RETURN, arg1 * arg2 ELSE RETURN, arg1 + arg2
;     
;  END
;
; MODIFICATION HISTORY:
;
;  Written by: David W. Fanning, November 26, 2008, from suggestion by Carsten Lechte on
;     IDL newsgroup on this date.
;  Made a change to the way the BOOLEAN keyword works. Now argument is set to BOOLEAN before
;     return, if required. 3 Dec 2008. DWF.
;-
;******************************************************************************************;
;  Copyright (c) 2008, by Fanning Software Consulting, Inc.                                ;
;  All rights reserved.                                                                    ;
;                                                                                          ;
;  Redistribution and use in source and binary forms, with or without                      ;
;  modification, are permitted provided that the following conditions are met:             ;
;                                                                                          ;
;      * Redistributions of source code must retain the above copyright                    ;
;        notice, this list of conditions and the following disclaimer.                     ;
;      * Redistributions in binary form must reproduce the above copyright                 ;
;        notice, this list of conditions and the following disclaimer in the               ;
;        documentation and/or other materials provided with the distribution.              ;
;      * Neither the name of Fanning Software Consulting, Inc. nor the names of its        ;
;        contributors may be used to endorse or promote products derived from this         ;
;        software without specific prior written permission.                               ;
;                                                                                          ;
;  THIS SOFTWARE IS PROVIDED BY FANNING SOFTWARE CONSULTING, INC. ''AS IS'' AND ANY        ;
;  EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES    ;
;  OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT     ;
;  SHALL FANNING SOFTWARE CONSULTING, INC. BE LIABLE FOR ANY DIRECT, INDIRECT,             ;
;  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED    ;
;  TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;         ;
;  LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND             ;
;  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT              ;
;  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS           ;
;  SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.                            ;
;******************************************************************************************;
PRO SetDefaultValue, argument, defaultValue, BOOLEAN=boolean

        ; We only need change if the argument is undefined.
        IF N_Elements(argument) EQ 0 THEN BEGIN
        
            ; If the default value is undefined, treat as BOOLEAN.
            ; Otherwise, assign default value to the argument.
            IF N_Elements(defaultValue) EQ 0 THEN BEGIN
                argument = Keyword_Set(argument)
            ENDIF ELSE BEGIN
                argument = defaultValue
            ENDELSE
            
        ENDIF
        
        ; Require boolean.
        IF Keyword_Set(boolean) THEN argument = Keyword_Set(argument)    
END ;-----------------------------------------------------------------------------------------