ITEC 380 2008fall ibarland

home—info—lectures—exams—archive

lect02a
scheme functions; structs

We already saw
  (define fave-color "teal")
  (define class-size 17)
`define` attaches a value to a place-holder(constant).
We also saw calling functions, prefix notation (and, images as values):
  (sqrt 25)
  (define circ1 (circle 25 'solid 'orange))
  (overlay circ1 (rectangle 15 20 'outline 'black))


define for functions
New: We can also define functions:

  ; intensify: string -> string
  ; Given an adjective phrase, return a more intense version.
  ;
  (define (intensify adj)
     (string-append "very, very " adj))


Note how 'define' has two flavors: 
   associate a name with a value;
   associate a name with a function.
   Wait, these are the same!  We'll see more in a second.

There is a built-in function "if":

  ; pluralize: (-> number string string)
  ;
  (define (pluralize num noun)
    (if (= num 1)
      (string-append (number->string num) noun)
      (string-append (number->string num) noun) "s"))

Compare to the Java version.
Another way: remember that 'if' is a function:
  
  ; pluralize: (-> number string string)
  ;
  (define (pluralize num noun)
    (string-append (number->string num) noun (if (= num 1) "" "s")))


Q: Java's "if"? is a statment, but Java does have an expression version.
   What is it?
A: ?:    (Though this infix notation suddenly interacts with precedence.)
         2 + y ? 3 : 5 + 4
         (Exercise: come up with an expression of nested ?: which is
            difficult to parse w/o knowing precedence.)


lambda
We can create a function, without naming it:
(lambda (x) (+ x x x))
= (lambda (x) ...)

((lambda (x) (+ x x x)) 7)
= 21

Is this a mere curiosity?  No, it's very handy; we'll see it used later!
  (For now: 
  reflect on java's Comparable interface, or the command pattern in general.)

Note that
  (define (f x) ...)
is syntactic sugar for
  (define f (lambda (x) ...))
And we are back to a single conceptual use of 'define': attach
a name to a placeholder.  (The name 'f' is a constant.  Cf. Java.)

home—info—lectures—exams—archive

©2008, Ian Barland, Radford University Last modified 2008.Sep.10 (Wed)

Please mail any suggestions (incl. typos, broken links) to iba�rlandrad�ford.edu