LISP

1
LISP

• Symbolic Expressions (S Expressions)
• Syntax
• Opening and Closing parenthesis having elements
  in between.
• List represented in LISP
• (A B2 C3 Shahid)
• (A B (C D E) F G)

2
Internal Representation
nil
B2
C3
Shahid
A
nil
F
G
A B
D
E
C
nil
3
Functional Programming
Defining Functions Syntax ( defunFunction
Name ( parameters procedure or body
of the function ) ) Setting Values to
variables (setq A 2) or (set A 2)
4
Arithmetic Operations

• ( A B) means AB
• ( 7 7) means 7749
• (- C D) means C-D
• (- (10 20) 7) means ((1020)- 7 23

5
Tree Diagram for Evaluation

• ( ( 10 20) ( 2 3))

( 10 20)
( 2 3)


10
20
2
3
6
Built In Commands

• Quote returns the list as it is.
• (quote(a b c)) or use instead of quote.
• Returns (a b c)
• List load the argument as a list
• (list(1 2 3 4))
• Returns (1 2 3 4)
• Eval evaluates the list or quoted list.
• (eval ( quote ( 2 3)))
• Returns 5
• ( quote ( 2 3))
• Returns (2 3)
• Nth n returns the nth value of the list
• (nth 0 (1 2 3 4 5 6)) returns 1
• (nth 3 (1 2 3 4 5 6)) returns 4
• (nth 5 (1 2 3 4 5 6)) returns 6

7
Examples

• ( ( 5 1 ) 6 )
• returns t
• ( gt ( 5 6) ( 4 5))
• returns t
• Other List Operators
• (length (1 2 3 4))
• returns 4
• (member 8 (1 4 2 8 7))
• returns t and if not present then returns nil
• (null ())
• returns t

8
Assignments Statements

• (setq a0)
• returns 0
• (let ((a 3) b) local assignment
• (setq b 4) if you go outside let a goes to 0
• ( a b))
• 7
• a
• 0
• b
• error b not bounded at the top level

9
Sample Programme
Programme to add three numbers ( defun
ADDNUMBERS (num1 num2 num3) (setq sum 0)
initialise the sum to zero (setq sum ( sum
num1) add first number (setq sum ( sum
num2)) (setq sum ( sum num3)) )
10
Conditionals

• Command cond
• Syntax
• ( cond (ltcondition 1gtltaction 1gt)
• (ltcondition 2gtltaction 2gt)
• ------------------------
• ------------------------
• (ltcondition ngtltaction ngt)
• )

11
Example

• ( defun absolute_values(x)
• (cond ( ( lt x 0) (-x))
• (t x)))
• This programme returns the value of x as it is if
  xgt0 otherwise returns -x

12
List Manipulation

• Command cons
• (cons a (b c)) returns (a b c)
• Command append does the same operation as cons
• Command car
• Returns the first element of the list
• Command cdr
• Returns the list excluding the first element

13
Examples

• (cdr ((a b) (c d)))
• returns ((c d))
• (car (cdr (car list1)))
• ca d ar (cadar x)
• (car (car(x)) (caar x)
• (car(car(car x))) (caaar x)
• (cdr(car(cdr x))) (cdadr x)

14
Recursion

• Write a programme that finds the factorial of a
  number
• Syntax (defun factorial(x)
• -----------------
• (recursive call)) ?
• recursive call ( x ( factorial ( - x 1) )
  )