COMP 320: Programming Languages

Simple Scheme Examples

These are the expressions and functions from the Intro to Functional Programming via Scheme video.

Note:

• The = function can be used to test the equality of numeric expressions.
• The eq? function tests whether 2 items refer to the same object in memory; its behavior with numbers is implementation-dependent.
• The equal? function tests whether 2 items are structurally equivalent with the same values, even if not the exact same object in memory, and so can be used with either atoms or lists.
• The eqv? function does numeric comparisons on numbers (like =), and identity comparisons (like eq?) on anything else.

In the examples below, = or equal? would have been a better choice.

(print "Hello world!")

(* 3 7)
(+ 3 7)
(+ 3 7 1 21 4)
(sqrt 16)

; some relational operators
(< 3 15)
(eq? 8 15)          ;; (= 8 15) would be better test for numbers
(eq? 8 (* 2 4))     ;; (= 8 (* 2 4)) would be better test for numbers
(eq? 8 '(8))        ;; (equal? 8 '(8)) would be a better choice

(max 3 7)
(max 3 7 1 21 4)

; if/then expressions: (if <condition> <then-expr> <else-expr>)
(if (> 3 7) 3 7)

(define (myMax a b) (if (> a b) a b))
(myMax 8 15)
(myMax 82 15)
; myMax only looks at 1st 2 operands
(myMax 82 15 63 4 93)

; simple recursion example showing define, null?, eq?, #t, #f
(define (contains aVal aList)
    (if (null? aList) #f                  ; base case
        (if (eq? aVal (car aList))        ; is 1st element it?
            #t                            ; then true
            (contains aVal (cdr aList)))  ; else check rest of list recursively
    ))

; example of cond (multi-way conditional), null?, list?, zero?
(define (numZeros ls)
    (cond
        ((null? ls) 0)                  ; base case
        ((list? (car ls))               ; is 1st elt a list?
            (+ (numZeros (car ls)) (numZeros (cdr ls))))
        ((zero? (car ls))               ; is 1st elt a 0?
            (+ 1 (numZeros (cdr ls))))
        (else (numZeros (cdr ls)))      ; how many in rest of list?
    ))

; new version of myMax that looks through list, not just 1st 2
(define (myMax2 ls)
    (cond
        ((null? ls) '())                ; base case, return empty list
        ((null? (cdr ls)) (car ls))     ; only 1 elt in list, return it
        ((> (car ls) (myMax2 (cdr ls))) (car ls))   ; 1st elt is max
        (else (myMax2 (cdr ls)))                    ; return max from rest of list
    ))