1. Scheme A “small” variant of Lisp

2. Basics • Why Lisp? • Learning lisp in the study of AI is like learning French if you are going to France (Charniak & McDermot) • Interpreted language • Prefix operator notation, with parentheses • Functional style

3. Overview • Dr Scheme • Atoms & Lists • Functions: Math & predicates • Constants: Quoting • More functions: List construction • Defining variables • Flow of control • Defining functions

4. Dr. Scheme • See web site for downloads • Has different language levels: • “Language” • “Choose Language” • “Full Scheme” • Two windows • Top: program editing (“definitions”) window • Define functions, variables, comments • Bottom: interactive evaluation window: interpreted • Evaluate functions, test code

5. Atoms & Lists • Characters are: A…Z, a…z, 0…9, ! % $ * + - / = ? • Atom • String of one or more characters with no spaces between; Examples: 123, 1xyz, fred • #\ used for character constants; e.g., #\newline • #t, #f (true, false) • Upper/lower case only distinguished in constants

6. Math • Form of function application is prefix Syntax: operatorvaluevalue • Expressions are parenthesized, e.g., (+ 1 2) • Math operators: + - / *; other examples: • Other examples: abs, sqrt, modulo, remainder • Nested expressions often used: (abs (* (- 2 3) (- 0 1)))

7. “Interactions” window > (+ 3 4) 7 > > (+ (- 6 3) 10) WARNING: Interactions window is out of sync with the definitions window. Click Execute. 13 • Notation: (+ (- 6 3) 10)  13

8. Predicates: Functions returning boolean values • Often named ending with “?” equal? – test for equality (works with lists) boolean? – test for boolean type object integer? – test for integer type object (equal? 10 3)  #f (equal? 4 4)  #t (boolean? #f)  #t

9. Relational operators and functions (> op1 op2) – test if op1 > op2 (< op1 op2) – test if op1 < op2 (= op1 op2) – numeric equality test (>= op1 op2) (<= op1 op2) (not op1) – negate the boolean operand (and op1op2) – boolean conjunction (or op1op2) – boolean disjunction

10. Defining & using variables (define x 1) • defines variable x to have value 1 > (define a 4) > a 4 a  4 (+ a 6)  10

11. Lists • Lists () is the empty list Lists are fully parenthesized and contain lists and/or sequences of atoms • Often need to quote lists to pass as parameters to functions • E.g., test if two lists “(+ 1 2)” and “(+ 2 1)” are equal (equal? (+ 1 2) (+ 2 1))  #t • Quote the lists so that they are not evaluated (equal? ‘(+ 1 2) ‘(+ 2 1))  #f • Quotes just need single quote in front of list

12. Functions for lists (member op1op2) – if op1 is a list element of op2, return sublist of op2 starting with op1; else return #f (member ‘1 ‘(+ 1 2))  (1 2) (member ‘- ‘(+ 1 2))  #f (member ‘1 ‘((1 2) (2 3)))  #f

13. List construction functions-1 (list op1op2 …) - form a list from a sequence of items (append op1op2 …) – put elements of lists in a new list (car op) - return the head element of a list op (cdr op) - return the tail of a list op, as a list (cons op1op2) - make new list with op1 on front of op2 (list ‘a ‘b ‘c)  (a b c) (append '(a) '(b))  (a b) (car ‘(a b))  a (cdr ‘(a b))  (b) (cons ‘a ‘(b c))  (a b c)

14. Exercise-1 (list ‘(a) ‘b))  (append ‘(a b c) ‘(1 2 3))  (car (list ‘a ‘b ‘c))  (cdr ‘((a b) c))  (cdr ‘(a))  (cons ‘a ‘((b)))  (cons ‘(a) ‘()) 

15. List construction functions-2 • More list functions & predicates caar: (car (car x)) cadr: (car (cdr x)) caddr: (car (cdr (cdr x))) etc (up to 4 operators in a sequence) (null? ‘())  #t (null? ‘(a b c))  #f (null? (cdr ‘(a)))  #t

16. Defining list variables (define y ‘(a b c)) y  (a b c) (define init-state ‘((blank 5 4) (6 1 8) (7 3 2))) (car init-state)  (blank 5 4)

17. Define-struct: Dr. Scheme Syntax: (define-struct name (field1field2 …)) Make new structs: make-name Get elements: name-field2, name-field2… (define-struct row (left middle right)) (define row1 (make-row ‘blank 1 8)) (row-left row1)  blank (row-middle row1)  1 (row-right row1)  8

18. Conditional evaluation Syntax: (cond [bool-condition1expr1] [bool-condition2expr2] … [else default-expr] ; optional ) (define init-state ‘((blank 5 4) (6 1 8) (7 3 2))) (cond [(null? init-state) init-state] [else (car init-state)] )  (blank 5 4)

19. Functions-1 • Use “lambda expressions” • Syntax: (lambda (op1op2 …) (expression)) (lambda (init-state) (cond [(null? init-state) init-state] [else (car init-state)] ) ) #<procedure> >

20. Functions-2: Supplying parameters (list ‘(a)) ; typical function usage ( list ; the function ‘(a) ; the parameter ) ( (lambda (init-state) (cond [(null? init-state) init-state] [else (car init-state)] ) ) ; the function '(a) ; the parameter )

21. Functions-3 • Usually want to name our functions • These have been “anonymous” functions (define ourFun (lambda (arg) (cond [(null? arg) arg] [else (car arg)] ) ; end cond ) ; end lambda ) ; end define (ourFun '(a))  a

22. Exercise-2 • Write a function to return –1, 0, +1 depending on two parameters a and b a < b return –1 a > b return +1 a = b return 0

23. Recursion: Control structure in functional programming • Count the number of elements in a list (define list-length (lambda (myList) (cond [(null? myList) 0] [else (+ 1 (list-length (cdr myList)) ) ] ) ; end cond ) ; end lambda ) ; end list-length

24. Recursive list reverse (define list-reverse (lambda (myList) (cond [(null? myList) myList] [else (append (list-reverse (cdr myList)) (list (car myList)) ) ] ; end else ) ; end cond ) ; end lambda ) ; end list-reverse

25. Non-negative integer multiply through repeated addition (define int-mult (lambda (x y) (cond [(equal? y 0) 0] [(equal? x 0) 0] [(equal? y 1) x] [else (+ x (int-mult x (- y 1)) ) ] ; end else ) ; end cond ) ; end lambda ) ; end int-mult