Introduction	LISP	LISP Programs	Expert Systems
LISP ( LISt Processing)

LISP was invented by John McCarthy in the late 1950's.

The basic building blocks of LISP are Atoms,List and Strings.

• An Atom is a number or string of contigous characters,including numbers and special characters.Example: bill, 1000, a123
• A List is a sequence of atoms and / or other lists enclosed within parentheses.Example: (this is a list), (a(ab)c def)
• A String is a group of characters enclosed in double quotation marks.Example: "this is a string"

List may contain atoms as well as other lists.The basic unit members of a list are called the Top elements.Atoms,lists & strings are the only valid objects in LISP.They are called Symbloic-Expressions or S-Expressions.

LISP uses Prefix notation.

Predefined Numeric Functions
Function Call	Value Returned	Remarks
(+ 3 5 8 4)	20	+ takes zero or more arguments.The sum of zero arguments is 0
(- 10 12)	-2	- takes two arguments
(* 2 3 4)	24	* takes zero or more arguments.The product of no arguments is 1,and the product of 1 argument is the value of the argument.
(/ 25 2)	12.5	/ takes two arguments.

Basic List Manipulation Functions
Function Call	Value Returned	Remarks
( car '(a b c))	A	Car takes one argument, a list & returns the first element.
( cdr '(a b c))	(b c)	Cdr takes one argument, a list & returns a list with the first element removed.
(cons 'a '(b c))	(a b c)	Cons takes 2 arguments, an element & a list and returns a list with the element inserted at the beginning.
(list 'a '(b c))	(a (b c))	List take any number of arguments and returns a list with the arguments as elements.

Additional List Manipulation Functions
Function Call	Value Returned	Remarks
(append �(a) �(bc))	(a b c)	Merges 2 or more lists into a single list.
(last �(a b c d))	(d)	Returns a list containing the last element.
(member �b �(a b d))	(b d)	Returns remainder of second argument list starting with element matching first argument.
(reverse �(a (b c) d))	( d (b c) a)	Returns list with top elements in reverse order.

Defining Functions

General Format : (defun name (parm1 parm2 ���.) body)

Defun requires 3 arguments

• The new function name
• The parameters for the function
• The function body or LISP code

Example :

->(defun averagethree (n1 n2 n3)

(/ (+ n1 n2 n3) 3) )

AVERAGETHREE

->

To call averagethree,

->( averagethree 10 20 30)

20

->

Predicate Functions

Predicates are functions that test their arguments for some specific condition.

Function Call	Value Returned	Remarks
( atom 'aabb)	T	aabb is a valid atom
( equal 'a ( car '(a b))	T	a equals a but note that ( equal 1 1.0) returns nil
( evenp 3)	Nil	3 is not an even number
(numberp 20ab)	Nil	20ab is not a number
(oddp 5)	T	5 is an odd number
(zerop 00010)	Nil	Argument is not zero
(greaterp 2 4 27)	T	Arguments are succeedingly larger from left to right
(lessp 5 3 1 2)	Nil	Arguments are not succeedingly smaller from left to right.
(listp '(a))	T	(a) is a valid list
(null nil)	T	Nil is an empty list

The Conditional Cond

Cond is like if�then�else

General Format:

(cond (<test1> <action1>)

(<test2> <action2>)

(<testk> <actionk>) )

• If <test1> evaluates to non nil,then <action1> is evaluated,its value returned and the remaining clauses are skipped over.
• If <test1> evaluates to nil,control passes to the second clause without evaluating <action1> and the procedure is repeated.
• If all tests evaluates to nil,cond returns nil.

Common LISP provides a conventional if�then�else conditional

General Format:

(it test <then-action> <else-action>)

Logical Functions

Basic logical operations are and , or & not

Input Output

• Read takes no arguments.When read appears in a procedure, processing halts until a single s-expression is entered from the keyboard.The s-expression is then returned as the value of read and processing continues.

Example :

->(+ 5 (read))

6

11

->

• Print takes one argument.It prints the argument as it received and then returns the argument.
• Printl is the same as print except that the new line characters and space are not provided.
• Printc is the same as printl except it does not print the unwanted quotation marks.
• Terpri takes no arguments.It introduces a new line (carriage return and line feed) whereever it appears and then returns nil.
• The format function permits us to create a cleaner output.It has the general form : ( format <destination> <string> arg1 arg2 ���.). Destination specifies where the o/pis to be directed (monitor or external file).String is the desaired o/p string.Format directives which are intermixed with the string specify how each argument is to be represented.

Local Variables

The parameters named as arguments in a function definition are local in scope.

The let and prog constructs permit the creation of local variables.

General Format:

( let ( (var1 val1) (var2 val2) ����) <s-expression> )

where each var i is a different variable name and val i is an initial value assigned to each var i respectively.The s-expression which follow are evaluated in order.

The prog function is similar to let in that the first arguments following it are a list of local variable where each element is either a variable name or a list containing a variable name and its initial value.This is followed by the body of the prog,and any number of s-expression.

Prog executes list s-expression in sequence and returns nil unless it encounters a function call named return.Prog also permits the use of unconditional go statements and labels(atom labels) to identify the go-to transfer locations.

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