CSC151.01 2006S, Class 46: Variable-Arity Procedures

Admin:
* Time for questions on HW 16.
* Reading for tomorrow (objects).
  (Please wait until afternoon.)

Overview:
* Definition of arity.
* Why have variable-arity procedures.
* How to write variable-arity procedures.
* Lab.

/Terms/

* "The number of parameters a procedure has": arity
  * "The arity of square root is 1"
  * "The arity of vector-ref is 2" (the vector and the position)
  * "A procedure of arity 3 is substring"
* Add "-ary" to somethi8ng like a number
  * zeroary
  * unary
  * binary
  * trinary
  * arity 4

/Variable-Arity Procedures/

* Some procedures have multiple arities
  * + (plus) can take 0, 1, 2, 10, as many as you want
  * - (minus) can take 1 (negate) 2 or more (subtract all but first from
    first), but not 0
  * Others: 
    * * (multiply): 0 or more
    * / (divide): 1 (reciprocal) 2 or more (divide first by all the rest)
    * list: 0 or more
    * vector: 0 more more
    * string-append: 0 or more
    * append: 0 or more

/Writing Our Own/

* Zero or more
(lambda params
  body)
* One or more
(lambda (first . rest)
  body)
* Two or more
(lambda (first second . rest)
  body)

/Lab!/
(define acronym
  (lambda words
    (list->string
     (map (lambda (word) (string-ref word 0)) words))))

(define acronym
  (lambda words
    (list->string
     (map (r-s string-ref 0) words))))