LISP Functions and Control Structures, Slides of Programming Languages

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Functions

> ( reverse L) ; reverses a list (C B A)

> ( length L) ; returns the length of list L 3

List has elements (A B C)

Predicates

• A special function which returns NIL if

the predicate is false, T or anything

other than NIL, otherwise

• We can use the following operators to

build the predicates

• = , > , < , >= , <= for numeric values
• eq or equal for others
• atom : test if x is an atom
• listp : test if x is a list
• Also number , symbolp , null

> ( cond (<test-1> <action-1>)

... (<test-k> <action-k>))

• Each (<test-i> <action-i>) pair is called a clause
• If test-i (where i=1) returns T (or anything other than NIL), this function returns the value of action-i; otherwise it will go to the next clause
• Usually the last test is T, which always holds (default case)
• cond can be nested and may contain other conditional statements (action-i may contain (cond ...))

Control Flow

> (setf operation ‘area L 100 W 50)

> 50

> (cond ((eq operation ‘perimeter) (* 2 (+ L W)))

(eq operation ‘area) (* L W)) (t ‘i-do-not-understand-this) ) )

> 5000

Defining LISP functions

( defun func-name (arg-1 ... Arg-n) func-body)

Example:

> (defun y-plus (x) (+ x y)) ;definition of y-plus > (setq y 2) > (y-plus 23) 25

• Local and global variables: local variables are

defined in function body

• Function returns the value of last expression in

its body

Example:

(defun power (x y)

(if (= y 0) 1

(* x (power x (1- y)))

• There is a limit to the depth of recursion,

and it depends on the version of LISP

Recursion