CSC151, Class 39: Variable-Arity Procedures

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

Notes:
* How was last night's talk?
* Are there questions on the project proposal?
* Reminder: Class Friday in Saints' Rest.  Please arrive early if
  you can.  We'll discuss exam 2.
* Extra credit for attending cool Friday noon talk on mathematics
  education (even for those planning a career in the subject who
  probably have to attend anyway).
* Thinking ahead to preregistration: Future courses in CS

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Definitions:
* Parameter: Something a procedure takes as input
* Two ways to think about parameters:
  + The things in the procedure definition
  + The things in the procedure call
(define foo (lambda 
  (x) 
(+ x 
			2)))
(foo 2)

* The number of parameters a procedure expects is the
  "arity" of a procedure.

foo has arity 1
foo is unary

(define bar
  (lambda (x y) ...))

bar has arity 2
bar is binary

What is the arity of +?
+ "As many as you want"
  0 params: 0

Are there other procedures that take differing numbers of
parameters?  yes
* String-append: 0 or more
  0 params: Empty string
* -: 1 or more

Are there procedures we might want to write (or, more precisely,
that you expect that I might assign) that take a variable number
of parameters?

Test 1: Write a markup procedure

(markup "hr")
"<hr>"
(markup "p" "hello")
"<p>hello</p>"

Interesting combinations of lists
(intersect lst1 lst2 ... lstn)

How to write a procedure of zero or more parameters

(define proc
  (lambda paramlist
     body))

How to write a procedure of one or more parameters

(define proc
  (lambda (firstparam . restofparams)
     body))