Supplying Values for Missing Arguments

The N_PARAMS function returns the number of positional arguments (not keyword arguments) present in a procedure or function call. A frequent use is to call N_PARAMS to determine if all arguments are present and if not, to supply default values for missing parameters. For example:

;Print values of XX and YY. If XX is omitted, print values of YY  
;versus element number. 
PRO XPRINT, XX, YY 
 
   ;Check number of arguments. 
   CASE N_PARAMS() OF  
 
     ;Single-argument case. 
     1: BEGIN  
 
       ;First argument is y values. 
       Y = XX 
 
       ;Create vector of subscript indices. 
       X = INDGEN(N_ELEMENTS(Y)) 
 
       END 
 
     ;Two-argument case. 
     2: BEGIN 
 
       ;Copy parameters to local arguments. 
       Y = YY & X = XX 
 
       END 
 
     ;Print error message. 
     ELSE: MESSAGE, 'Wrong number of arguments' 
 
   ENDCASE 
 
   ;Remainder of procedure. 
   ... 
 
END