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Lisp

Lisp. Functional Language or Applicative Language Achieves its effect by applying functions, either recursively or through composition Powerful, expressive, and semantically elegant language http://www.youtube.com/watch?v=D5kIq5dyb6o&NR=1. Pure Functions. Pure functions avoid “side effects”

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Lisp

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  1. Lisp • Functional Language or Applicative Language • Achieves its effect by applying functions, either recursively or through composition • Powerful, expressive, and semantically elegant language http://www.youtube.com/watch?v=D5kIq5dyb6o&NR=1

  2. Pure Functions • Pure functions avoid “side effects” • No use of global variables • Do not change parameters • Return one thing • Obeys principle of referential transparency “Function using same arguments will always produce same value”

  3. Function examples plustwo(x) ::= plusone (plusone(x)) where plusone(x) ::= x+1 fact(n) ::= if n = 0 then 1 else n * fact(n-1)

  4. Lisp’s base language • Pure functions, operations, functional composition, recursion, and conditional expression • Basics of Lisp • Domain language operates on S-expr • S-expr constructed out of • atoms • and parentheses (list of atoms)

  5. Basics… • Atom (primitive data structure) • Indivisible • Numeric, non-numeric (some versions string) • Sequence of alphabetic, numeric characters • 876 -250 ATOM four • Equality is only built-in operation • S-expr • Atom: 876 four • List: (ADD 2 PLUS 3) ((Ruth a) (Edward b)) (TIMES Y Z)

  6. Lists and Operators • Lists can be versatile • Can be composed of constants, variables, and operators • Built- in binary functions for arithmetic exist (plus 2 3) or (+ 2 3) (+ 2 3 7 8 9 )

  7. Conditions (cond ( (null x) 0) ( (eq x y) (f x) ) ( T (g y) ) ) if null (x) then 0 elseif x == y then f(x) else g(y) endif;

  8. How to construct a data Structure • Only one – the list (to be or not to be) has 6 atoms • cons – can put atoms to atoms together to make a list • car – extract atoms from a list

  9. Primitive Functions • car – returns the first S-expr in a list car (A)  A car (A B)  A car ((A B) (C D))  (A B) car ATOM  undefined • cdr – returns a list containing everything but the first S-expr in a list cdr (A)  NIL cdr (A B)  (B) cdr ((A B) (C D))  ((C D)) cdr ATOM  undefined

  10. Composition of Functions L = ( (A B) (C) (D E F)) car (car (cdr (L)))  car(car( (C) (D E F)))  car ( C)  C

  11. Another Primitive Functions • cons – construct (or concatenate) cons (A; (B C) )  (A B C) cons ( (A); (B C) )  ( (A) B C) • construct s-expr z, such that car (z) = x and cdr (z) = y

  12. User-defined functions • Everything is a function in LISP and program functions are lists • Use defun • Call function “defun” with 3 arguments • Function name, formal parameter list, body (defun double (N) (double 5)  10 (* N 2) )

  13. (make-table text nil) Q: Is this 3 atoms or a function call with two parameters? A: function call assuming defined function make-table If you want it to be treated as 3 atoms use a single-quote ‘(make-table text nil)

  14. mapcar • Code iterative functions • mapcar takes 2 arguments • Function name • A list • Returns a list • Applies a function to each item in a list and returns resulting values in a new list

  15. (defun double-list (lis) (mapcar (function double) lis)) or (defun double-list (lis) (mapcar ‘double lis) --the function name is an argument to mapcar function

  16. mapcar… • if lis = (5 2 7 4) before (double-list lis) or (double ‘(5 2 7 4)) • then lis = (10 4 14 8) after

  17. lambda expression • Serve the same purpose as a “nested function” • Anonymous functions • Define a function within a function • But function has no name

  18. Do not evaluate within mapcar, DEFINE (defun double-list (lis) (mapcar ‘(lambda (N) (* N 2 ) ) lis) ) • Almost identical to “helping function” or subfunction • defun and function name replaced by lambda, which has a parameter list and function body • However appears in middle of another function; • No way to call lambda function from any function except from one in which it is embeded

  19. More examples (lambda (x) (+ x 5)) (lambda (x y) (+ (* 2 x ) (* 3 y))) (lambda (x y z) (cond (eq x NIL) y) (T z) ) ) • Variables x, y and z are bound variables – local to lambda expression

  20. May have unbound variables occurring in lambda expressions (lambda (x) (+ x z)) • Here z is an unbound variable Q: Where does it get its value when lambda expression is evaluated? A: outer function that lambda is nested inside

  21. (defun MultList (z lis) (mapcar #’(lambda (x) (+ x z) ) lis ) ) Treat variables like z inside lambda expression as local variables from definition

  22. Other Statements • Reading (read) returns whatever typed in (be it a list or an atom) (setq list (read)) I’ll type in (A B C D) or (setq Item (read)) I’ll type in A

  23. Local variables in a function? • Use let (defun fnName (parameters) (let ( (list nil) … (n 1) ( ) )

  24. Set members • Member of a set (setq oddset ‘(1 3 5 7 9) ) (member 5 oddset)  returns T or F

  25. Append and Reverse (append list-A list-B)  returns a list (reverse (concatenate ‘string “ABC” “DEF”)

  26. Setup and Running • Download http://sourceforge.net/projects/clisp/ • CLISP - Brief introduction to install and setup of an artificially intelligent environment (YouTube video) • Demo

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