Keyword and Default Parameters

Keyword Parameters

Keyword Parameters

• One declaration for Ada.Integer_Text_IO.Put is similar to this (but see below):

   package Ada.Integer_Text_IO is
      ...
      procedure Put(Item: Num; Width: Field; Base: Number_Base);
   

• Put has three formal parameters:
• Item - the integer to print
• Width - how many spaces to use to print the integer
• Base - the base to print (eg base 2 or base 16)


• Parameter passing can be positional - matched by position:

   put(i, 1, 2);    -- Positional: item i, width 1, base 2  
   

• Parameter passing can be by Keyword!
• Matched by parameter name
• Can be in any order

   put(i, 1, 2);    -- Positional: item i, width 1, base 2  

   put(Item => i, Width => 1, Base => 2); -- Keyword

   put(Width => 1, Base => 2, Item => i);  -- Any order
   

• Can mix positional and Keyword
• Positional must be first

   put(i, Width => 1, Base => 2);
   put(i, Base => 2, Width => 1);
   put(i, 1, Base => 2);

   --  put(Width => 1, i);               -- Error.  What's wrong?
   --  put(Width => 1, i, Base => 2); -- Error
   --  put(2, Item => i, Width => 1); -- Error
   

Keyword Parameters - Benefits

• Easier to write (don't have to know order)


• Easier to read (don't have to know order)
• Easier to read (avoid looking up parameters)


• More reliable (can't pass parameters in wrong order)
• More reliable (all of the above)

Default Parameters

Default Parameters

• A more complete declaration for Ada.Integer_Text_IO.Put:

   procedure Put(Item  : Num;
                 Width : Field := Default_Width;
                 Base  : Number_Base := Default_Base);
   

• The Width parameter has the default value Default_Width
• If put is called without a width, then the default value is used
• Default_Width is a constant defined in package Ada.Integer_Text_IO.
• What is the default width for an integer? Why?

   -- Check the value
   put(default_width);  -- 11
   

• The Base parameter has the default value Default_Base
• What is the default base?

   put(default_base);   -- 10
   

• So the following are all the same:

   put(i, 11, 10);    -- width 11, base 10
   put(i, 11);        -- width 11, base 10
   put(i);            -- width 11, base 10
   

• Consider the following:

   put(i);
   put(i, 1);       -- width 1
   put(i, 1, 2);    -- width 1, base 2

   put(Item => i);
   put(Item => i, Width => 1); 
   put(Item => i, Width => 1, Base => 2);

   put(Width => 1, Item => i); 
   put(Width => 1, Base => 2, Item => i);

   put(i, Width => 1);               

   --  put(Width => 1, Base => 2); --  What's wrong?
   

OLD NOTES ON Keyword, Positional, and Default Parameters

Motivation

• Concern for programmer: easier to read and write
• Reliability: mistakes less likely

Keyword Parameters

• Ada.Integer_Text_IO.put is declared something like this:

          procedure put(Item: Integer; width: Natural) ...  

• Positional parameters are matched by order
• put(n, 5); -- n matched with parameter item, 5 with width


• Keyword parameters are matched by name
• put(item => n, width => 5);
• put(item => n, base => 2, width => 5);
• put(width => 5, item => n);
• put(item = > x, fore= > 5, aft= > 2, exp= > 0);


• Can mix positional and keyword: positional must be first
• put(n, width => 5);

Default Parameters

• Parameters can have default values


• Example: for new_line, the default is 1

        procedure new_line(Count: Positive := 1) ...  



• For integer_text_io, the default width is 11 (big enough for 2 billion and a sign)

        procedure put(Item: Integer; width: Natural := 11) ...