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Prototypes and Fill Items

Every array has an associated prototype which is derived from the array's first item.

If the first item is a number, the prototype is 0. Otherwise, if the first item is a character, the prototype is ' '(space). Otherwise, if the first item is a (ref to) an instance of a Class, the prototype is a ref to that Class.

Otherwise (in the nested case, when the first item is other than a simple scalar), the prototype is defined recursively as the prototype of each of the array's first item.

Examples

Array Prototype
1 2 3.4 0
2 3 5⍴'hello' ' '
99 'b' 66 0
(1 2)(3 4 5) 0 0
((1 2)3)(4 5 6) (0 0)0
'hello' 'world' ' '
⎕NEW MyClass MyClass
(88(⎕NEW MyClass)'X')7 0 MyClass ' '

Fill Items

Fill items for an overtake operation, are derived from the argument's prototype. For each 0 or ' ' in the prototype, there is a corresponding 0 or ' ' in the fill item and for each class reference in the prototype, there is a ref to a (newly constructed and distinct) instance of that class that is initialised by the niladic (default) constructor for that class, if defined.

Examples

      4↑1 2
1 2 0 0
      4↑'ab'
ab  
      4↑(1 2)(3 4 5)
 1 2  3 4 5  0 0  0 0 
      2↑⎕NEW MyClass
 #.[Instance of MyClass]  #.[Instance of MyClass]

In the last example, two distinct instances are constructed (the first by ⎕NEW and the second by the overtake).

Fill items are used in a number of operations including:

  • First (⊃ or ↑) of an empty array
  • Fill-elements for overtake
  • For use with the Each operator on an empty array