20_Lisp_tutorial

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2 .About the Tutorial LISP is the second-oldest high-level programming language after Fortran and has changed a great deal since its early days, and a number of dialects have existed over its history. Today, the most widely known general-purpose LISP dialects are Common LISP and Scheme. This tutorial takes you through features of LISP Programming language by simple and practical approach of learning. Audience This reference has been prepared for the beginners to help them understand the basic to advanced concepts related to LISP Programming language. Prerequisites Before you start doing practice with various types of examples given in this reference, we assume that you are already aware of the fundamentals of computer programming and programming languages. Copyright & Disclaimer  Copyright 2014 by Tutorials Point (I) Pvt. Ltd. All the content and graphics published in this e-book are the property of Tutotorials Point (I) Pvt. Ltd. The user of this e-book is prohibited to reuse, retain, copy, distribute or republish any contents or a part of contents of this e-book in any manner without written consent of the publisher. You strive to update the contents of our website and tutorials as timely and as precisely as possible, however, the contents may contain inaccuracies or errors. Tutorials Point (I) Pvt. Ltd. provides no guarantee regarding the accuracy, timeliness or completeness of our website or its contents including this tutorial. If you discover any errors on our website or in this tutorial, please notify us at contact@tutorialspoint.com i 

3 . LISP Table of Contents About the Tutorial ............................................................................................................................................. i Audience ........................................................................................................................................................... i Prerequisites ..................................................................................................................................................... i Copyright & Disclaimer...................................................................................................................................... i Table of Contents ............................................................................................................................................. ii 1. OVERVIEW ........................................................................................................................................... 1 Features of Common LISP................................................................................................................................. 1 Applications Developed in LISP ........................................................................................................................ 1 2. ENVIRONMENT SETUP ..................................................................................................................... 3 How to Use CLISP ............................................................................................................................................. 3 3. PROGRAM STRUCTURE ................................................................................................................... 4 A Simple LISP Program ..................................................................................................................................... 4 LISP Uses Prefix Notation ................................................................................................................................. 5 Evaluation of LISP Programs ............................................................................................................................. 5 The 'Hello World' Program ............................................................................................................................... 6 4. BASIC SYNTAX .................................................................................................................................... 7 Basic Elements in LISP ...................................................................................................................................... 7 Adding Comments ............................................................................................................................................ 8 Notable Points ................................................................................................................................................. 8 LISP Forms ........................................................................................................................................................ 8 Naming Conventions in LISP ............................................................................................................................. 9 Use of Single Quotation Mark .......................................................................................................................... 9 5. DATA TYPES ..................................................................................................................................... 11 Type Specifiers in LISP .................................................................................................................................... 11 6. MACROS ............................................................................................................................................. 14 ii 

4 . LISP Defining a Macro ............................................................................................................................................ 14 7. VARIABLES ....................................................................................................................................... 15 Global Variables ............................................................................................................................................. 15 Local Variables ............................................................................................................................................... 16 8. CONSTANTS ...................................................................................................................................... 18 9. OPERATORS ..................................................................................................................................... 19 Arithmetic Operations.................................................................................................................................... 19 Comparison Operations ................................................................................................................................. 20 Logical Operations on Boolean Values............................................................................................................ 22 Bitwise Operations on Numbers..................................................................................................................... 24 10. DECISION MAKING ......................................................................................................................... 27 The cond Construct in LISP ............................................................................................................................. 28 The if Construct .............................................................................................................................................. 29 The when Construct ....................................................................................................................................... 30 The case Construct ......................................................................................................................................... 31 11. LOOPS ................................................................................................................................................. 32 The loop Construct ......................................................................................................................................... 33 The loop for Construct .................................................................................................................................... 33 The do Construct ............................................................................................................................................ 35 The dotimes Construct ................................................................................................................................... 36 The dolist Construct ....................................................................................................................................... 37 Exiting Gracefully from a Block ....................................................................................................................... 38 12. FUNCTIONS ....................................................................................................................................... 40 Defining Functions in LISP .............................................................................................................................. 40 Optional Parameters ...................................................................................................................................... 41 Rest Parameters ............................................................................................................................................. 42 iii 

5 . LISP Keyword Parameters ...................................................................................................................................... 43 Returning Values from a Function .................................................................................................................. 43 Lambda Functions .......................................................................................................................................... 45 Mapping Functions......................................................................................................................................... 45 13. PREDICATES ..................................................................................................................................... 47 14. NUMBERS .......................................................................................................................................... 51 Various Numeric Types in LISP ....................................................................................................................... 52 Number Functions .......................................................................................................................................... 53 15. CHARACTERS ................................................................................................................................... 56 Special Characters .......................................................................................................................................... 56 Character Comparison Functions .................................................................................................................... 57 16. ARRAYS .............................................................................................................................................. 59 17. STRINGS ............................................................................................................................................. 66 String Comparison Functions .......................................................................................................................... 66 Case Controlling Functions ............................................................................................................................. 68 Trimming Strings ............................................................................................................................................ 69 Other String Functions ................................................................................................................................... 70 18. SEQUENCES ....................................................................................................................................... 73 Creating a Sequence ....................................................................................................................................... 73 Generic Functions on Sequences .................................................................................................................... 73 Standard Sequence Function Keyword Arguments ......................................................................................... 76 Finding Length and Element ........................................................................................................................... 76 Modifying Sequences ..................................................................................................................................... 77 Sorting and Merging Sequences ..................................................................................................................... 78 Sequence Predicates ...................................................................................................................................... 79 Mapping Sequences ....................................................................................................................................... 80 iv 

6 . LISP 19. LISTS ................................................................................................................................................... 81 The Cons Record Structure ............................................................................................................................. 81 Creating Lists with list Function in LISP ........................................................................................................... 82 List Manipulating Functions ........................................................................................................................... 83 Concatenation of car and cdr Functions ......................................................................................................... 85 20. SYMBOLS ........................................................................................................................................... 86 Property Lists ................................................................................................................................................. 86 21. VECTORS............................................................................................................................................ 89 Creating Vectors ............................................................................................................................................. 89 Fill Pointer Argument ..................................................................................................................................... 90 22. SET ....................................................................................................................................................... 92 Implementing Sets in LISP .............................................................................................................................. 92 Checking Membership .................................................................................................................................... 93 Set Union ....................................................................................................................................................... 94 Set Intersection .............................................................................................................................................. 95 Set Difference ................................................................................................................................................ 96 23. TREE.................................................................................................................................................... 98 Tree as List of Lists ......................................................................................................................................... 98 Tree Functions in LISP..................................................................................................................................... 98 Building Your Own Tree ................................................................................................................................100 Adding a Child Node into a Tree ....................................................................................................................100 24. HASH TABLE...................................................................................................................................103 Creating Hash Table in LISP ...........................................................................................................................103 Retrieving Items from Hash Table .................................................................................................................104 Adding Items into Hash Table ........................................................................................................................104 Removing an Entry from Hash Table .............................................................................................................105 v 

7 . LISP Applying a Specified Function on Hash Table ................................................................................................106 25. INPUT & OUTPUT .........................................................................................................................107 Input Functions .............................................................................................................................................107 Reading Input from Keyboard .......................................................................................................................108 Output Functions ..........................................................................................................................................110 Formatted Output .........................................................................................................................................113 26. FILE I/O ............................................................................................................................................115 Opening Files.................................................................................................................................................115 Writing to and Reading from Files .................................................................................................................116 Closing a File .................................................................................................................................................118 27. STRUCTURES..................................................................................................................................119 Defining a Structure ......................................................................................................................................119 28. PACKAGES .......................................................................................................................................122 Package Functions in LISP ..............................................................................................................................122 Creating a Package ........................................................................................................................................123 Using a Package.............................................................................................................................................123 Deleting a Package ........................................................................................................................................125 29. ERROR HANDLING .......................................................................................................................127 Signaling a Condition .....................................................................................................................................127 Handling a Condition .....................................................................................................................................127 Restarting or Continuing the Program Execution ...........................................................................................128 Error Signaling Functions in LISP ....................................................................................................................131 30. COMMON LISP OBJECT SYSTEMS ............................................................................................133 Defining Classes ............................................................................................................................................133 Providing Access and Read/Write Control to a Slot .......................................................................................133 Creating Instance of a Class ...........................................................................................................................134 vi 

8 . LISP Defining a Class Method................................................................................................................................135 Inheritance ....................................................................................................................................................136 vii 

9 . 1. OVERVIEW LISP stands for LISt Programming. John McCarthy invented LISP in 1958, shortly after the development of FORTRAN. It was first implemented by Steve Russell on an IBM 704 computer. It is particularly suitable for Artificial Intelligence programs, as it processes symbolic information efficiently. Common LISP originated during the decade of 1980 to 1990, in an attempt to unify the work of several implementation groups, as a successor of Maclisp like ZetaLisp and New Implementation of LISP (NIL) etc. It serves as a common language, which can be easily extended for specific implementation. Programs written in Common LISP do not depend on machine-specific characteristics, such as word length etc. Features of Common LISP  It is machine-independent  It uses iterative design methodology  It has easy extensibility  It allows to update the programs dynamically  It provides high level debugging.  It provides advanced object-oriented programming.  It provides convenient macro system.  It provides wide-ranging data types like, objects, structures, lists, vectors, adjustable arrays, hash-tables, and symbols.  It is expression-based.  It provides an object-oriented condition system.  It provides complete I/O library.  It provides extensive control structures. Applications Developed in LISP The following applications are developed in LISP: Large successful applications built in LISP. 1 

10 . LISP  Emacs: It is a cross platform editor with the features of extensibility, customizability, self-document ability, and real-time display.  G2  AutoCad  Igor Engraver  Yahoo Store 2 

11 . LISP 2. ENVIRONMENT SETUP CLISP is the GNU Common LISP multi-architechtural compiler used for setting up LISP in Windows. The Windows version emulates Unix environment using MingW under Windows. The installer takes care of this and automatically adds CLISP to the Windows PATH variable. You can get the latest CLISP for Windows at: http://sourceforge.net/projects/clisp/files/latest/download It creates a shortcut in the Start Menu by default, for the line-by-line interpreter. How to Use CLISP During installation, CLISP is automatically added to your PATH variable if you select the option (RECOMMENDED). It means that you can simply open a new Command window and type "clisp" to bring up the compiler. To run a *.lisp or *.lsp file, simply use: clisp hello.lisp 3 

12 . LISP 3. PROGRAM STRUCTURE LISP expressions are called symbolic expressions or S-expressions. The S-expressions are composed of three valid objects:  Atoms  Lists  Strings Any S-expression is a valid program. LISP programs run either on an interpreter or as compiled code. The interpreter checks the source code in a repeated loop, which is also called the Read-Evaluate-Print Loop (REPL). It reads the program code, evaluates it, and prints the values returned by the program. A Simple LISP Program Let us write an s-expression to find the sum of three numbers 7, 9 and 11. To do this, we can type at the interpreter prompt ->: (+7911) LISP returns the following result: 27 If you would like to execute the same program as a compiled code, then create a LISP source code file named myprog.lisp and type the following code in it: (write(+7911)) When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result is: 27 4 

13 . LISP LISP Uses Prefix Notation In prefix notation, operators are written before their operands. You might have noted that LISP uses prefix notation. In the above program, the ‘+’ symbol works as a function name for the process of summation of the numbers. For example, the following expression, a * ( b + c ) / d is written in LISP as: (/ (* a (+ b c) ) d) Let us take another example. Let us write code for converting Fahrenheit temperature of 60o F to the centigrade scale: The mathematical expression for this conversion is: (60 * 9 / 5) + 32 Create a source code file named main.lisp and type the following code in it: (write(+ (* (/ 9 5) 60) 32)) When you click the Execute button, or type Ctrl+E, MATLAB executes it immediately and the result is: 140 Evaluation of LISP Programs The LISP program has two parts:  Translation of program text into LISP objects by a reader program.  Implementation of the semantics of the language in terms of LSIP objects by an evaluator program. The evaluation program takes the following steps:  The reader translates the strings of characters to LISP objects or s- expressions.  The evaluator defines syntax of LISP forms that are built from s-expressions. 5 

14 . LISP  This second level of evaluation defines a syntax that determines which s- expressions are LISP forms.  The evaluator works as a function that takes a valid LISP form as an argument and returns a value. This is the reason why we put the LISP expression in parenthesis, because we are sending the entire expression/form to the evaluator as argument. The 'Hello World' Program Learning a new programming language does not really take off until you learn how to greet the entire world in that language, right ? Let us create new source code file named main.lisp and type the following code in it: (write-line "Hello World") (write-line "I am at 'Tutorials Point'! Learning LISP") When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result is: Hello World I am at 'Tutorials Point'! Learning LISP 6 

15 . LISP 4. BASIC SYNTAX This chapter introduces you to basic syntax structure in LISP. Basic Elements in LISP LISP programs are made up of three basic elements:  atom  list  string An atom is a number or string of contiguous characters. It includes numbers and special characters. The following examples show some valid atoms: hello-from-tutorials-point name 123008907 *hello* Block#221 abc123 A list is a sequence of atoms and/or other lists enclosed in parentheses. The following examples show some valid lists: ( i am a list) (a ( a b c) d e fgh) (father tom ( susan bill joe)) (sun mon tue wed thur fri sat) ( ) A string is a group of characters enclosed in double quotation marks. The following examples show some valid strings: " I am a string" "a ba c d efg #$%^&!" "Please enter the following details:" 7 

16 . LISP "Hello from 'Tutorials Point'! " Adding Comments The semicolon symbol (;) is used for indicating a comment line. Example (write-line "Hello World") ; greet the world ; tell them your whereabouts (write-line "I am at 'Tutorials Point'! Learning LISP") When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result returned is: Hello World I am at 'Tutorials Point'! Learning LISP Notable Points The following important points are notable:  The basic numeric operations in LISP are +, -, *, and /  LISP represents a function call f(x) as (f x), for example cos(45) is written as cos 45  LISP expressions are not case-sensitive. Means, cos 45 or COS 45 are same.  LISP tries to evaluate everything, including the arguments of a function. Only three types of elements are constants and always return their own value: o Numbers o The letter t, that stands for logical true o The value nil, that stands for logical false, as well as an empty list. LISP Forms In the previous chapter, we mentioned that the evaluation process of LISP code takes the following steps:  The reader translates the strings of characters to LISP objects or s- expressions. 8 

17 . LISP  The evaluator defines syntax of LISP forms that are built from s-expressions. This second level of evaluation defines a syntax that determines which s- expressions are LISP forms. A LISP form can be:  An atom  An empty list or non-list  Any list that has a symbol as its first element The evaluator works as a function that takes a valid LISP form as an argument and returns a value. This is the reason why we put the LISP expression in parenthesis, because we are sending the entire expression/form to the evaluator as argument. Naming Conventions in LISP Name or symbols can consist of any number of alphanumeric characters other than whitespace, open and closing parentheses, double and single quotes, backslash, comma, colon, semicolon and vertical bar. To use these characters in a name, you need to use escape character (\). A name can have digits but must not be made of only digits, because then it would be read as a number. Similarly a name can have periods, but cannot be entirely made of periods. Use of Single Quotation Mark LISP evaluates everything including the function arguments and list members. At times, we need to take atoms or lists literally and do not want them evaluated or treated as function calls. To do this, we need to precede the atom or the list with a single quotation mark. The following example demonstrates this: Create a file named main.lisp and type the following code into it: (write-line "single quote used, it inhibits evaluation") (write '(* 2 3)) (write-line " ") (write-line "single quote not used, so expression evaluated") (write (* 2 3)) 9 

18 . LISP When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result is: single quote used, it inhibits evaluation (* 2 3) single quote not used, so expression evaluated 6 10 

19 . LISP 5. DATA TYPES LISP data types can be categorized as: Scalar types - numbers, characters, symbols etc. Data structures - lists, vectors, bit-vectors, and strings. Any variable can take any LISP object as its value, unless you declare it explicitly. Although, it is not necessary to specify a data type for a LISP variable, however, it helps in certain loop expansions, in method declarations and some other situations that we will discuss in later chapters. The data types are arranged into a hierarchy. A data type is a set of LISP objects and many objects may belong to one such set. The typep predicate is used for finding whether an object belongs to a specific type. The type-of function returns the data type of a given object. Type Specifiers in LISP Type specifiers are system-defined symbols for data types. Array fixnum package simple-string Atom float pathname simple-vector Bignum function random-state single-float Bit hash-table Ratio standard-char bit-vector integer Rational stream Character keyword readtable string [common] list sequence [string-char] compiled-function long-float short-float symbol Complex nill signed-byte t 11 

20 . LISP Cons null simple-array unsigned-byte double-float number simple-bit-vector vector Apart from these system-defined types, you can create your own data types. When a structure type is defined using defstruct function, the name of the structure type becomes a valid type symbol.>/p> Example 1 Create new source code file named main.lisp and type the following code in it: (setq x 10) (setq y 34.567) (setq ch nil) (setq n 123.78) (setq bg 11.0e+4) (setq r 124/2) (print x) (print y) (print n) (print ch) (print bg) (print r) When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result returned is: 10 34.567 123.78 NIL 110000.0 62 12 

21 . LISP Example 2 Next let us check the types of the variables used in the previous example. Create new source code file named main.lisp and type the following code in it: (setq x 10) (setq y 34.567) (setq ch nil) (setq n 123.78) (setq bg 11.0e+4) (setq r 124/2) (print (type-of x)) (print (type-of y)) (print (type-of n)) (print (type-of ch)) (print (type-of bg)) (print (type-of r)) When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the resultis: (INTEGER 0 281474976710655) SINGLE-FLOAT SINGLE-FLOAT NULL SINGLE-FLOAT (INTEGER 0 281474976710655) 13 

22 . LISP 6. MACROS This chapter introduces you about macros in LISP. A macro is a function that takes an s-expression as arguments and returns a LISP form, which is then evaluated. Macros allow you to extend the syntax of standard LISP. Defining a Macro In LISP, a named macro is defined using another macro named defmacro. Syntax for defining a macro is: (defmacro macro-name (parameter-list) "Optional documentation string." body-form) The macro definition consists of the name of the macro, a parameter list, an optional documentation string, and a body of LISP expressions that defines the job to be performed by the macro. Example Let us write a simple macro named setTo10, which takes a number and sets its value to 10. Create new source code file named main.lisp and type the following code in it: defmacro setTo10(num) (setq num 10)(print num)) (setq x 25) (print x) (setTo10 x) When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result is: 25 10 14 

23 . LISP 7. VARIABLES In LISP, each variable is represented by a symbol. The name of the variable is the name of the symbol and it is stored in the storage cell of the symbol. Global Variables Global variables are generally declared using the defvar construct. Global variables have permanent values throughout the LISP system and remain in effect until new values are specified. Example (defvar x 234) (write x) When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result is: 234 As there is no type declaration for variables in LISP, you need to specify a value for a symbol directly with the setq construct. Example ->(setq x 10) The above expression assigns the value 10 to the variable x. You can refer to the variable using the symbol itself as an expression. The symbol-value function allows you to extract the value stored at the symbol storage place. Example Create new source code file named main.lisp and type the following code in it: (setq x 10) (setq y 20) (format t "x = ~2d y = ~2d ~%" x y) (setq x 100) (setq y 200) 15 

24 . LISP (format t "x = ~2d y = ~2d" x y) When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result is: x = 10 y = 20 x = 100 y = 200 Local Variables Local variables are defined within a given procedure. The parameters named as arguments within a function definition are also local variables. Local variables are accessible only within the respective function. Like the global variables, local variables can also be created using the setq construct. There are two other constructs - let and prog for creating local variables. The let construct has the following syntax: (let ((var1 val1) (var2 val2).. (varn valn))<s-expressions>) Where var1, var2,…,varn are variable names and val1, val2,…, valn are the initial values assigned to the respective variables. When let is executed, each variable is assigned the respective value and at last, the s- expression is evaluated. The value of the last expression evaluated is returned. If you do not include an initial value for a variable, the variable is assigned to nil. Example Create new source code file named main.lisp and type the following code in it: (let ((x 'a) (y 'b) (z 'c)) (format t "x = ~a y = ~a z = ~a" x y z)) When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result is: x = A y = B z = C The prog construct also has the list of local variables as its first argument, which is followed by the body of the prog, and any number of s-expressions. 16 

25 . LISP The prog function executes the list of s-expressions in sequence and returns nil unless it encounters a function call named return. Then the argument of the return function is evaluated and returned. Example Create new source code file named main.lisp and type the following code in it: (prog ((x '(a b c)) (y '(1 2 3)) (z '(p q 10))) (format t "x = ~a y = ~a z = ~a" x y z)) When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result is: x = (A B C) y = (1 2 3) z = (P Q 10) 17 

26 . LISP 8. CONSTANTS In LISP, constants are variables that never change their values during program execution. Constants are declared using the defconstant construct. Example The following example shows declaring a global constant PI and later using this value inside a function named area-circle that calculates the area of a circle. The defun construct is used for defining a function, we will look into it in the 'Functions' chapter. Create a new source code file named main.lisp and type the following code in it: (defconstant PI 3.141592) (defun area-circle(rad) (terpri) (format t "Radius: ~5f" rad) (format t "~%Area: ~10f" (* PI rad rad))) (area-circle 10) When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result is: Radius: 10.0 Area: 314.1592 18 

27 . LISP 9. OPERATORS An operator is a symbol that tells the compiler to perform specific mathematical or logical manipulations. LISP allows numerous operations on data, supported by various functions, macros and other constructs. The operations allowed on data could be categorized as:  Arithmetic Operations  Comparison Operations  Logical Operations  Bitwise Operations Arithmetic Operations The following table shows all the arithmetic operators supported by LISP. Assume variable A = 10 and variable B = 20 then: Operator Description Example + Adds two operands (+ A B) gives 30 - Subtracts second operand from the first (- A B) gives-10 * Multiplies both operands (* A B) gives 200 / Divides numerator by de-numerator (/ B A) gives 2 Modulus Operator and remainder of after mod,rem (mod B A )gives 0 an integer division Increments operator increases integer incf (incf A 3) gives 13 value by the second argument specified Decrements operator decreases integer decf (decf A 4) gives 9 value by the second argument specified 19 

28 . LISP Example Create a new source code file named main.lisp and type the following code in it: (setq a 10) (setq b 20) (format t "~% A + B = ~d" (+ a b)) (format t "~% A - B = ~d" (- a b)) (format t "~% A x B = ~d" (* a b)) (format t "~% B / A = ~d" (/ b a)) (format t "~% Increment A by 3 = ~d" (incf a 3)) (format t "~% Decrement A by 4 = ~d" (decf a 4)) When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result is: A + B = 30 A - B = -10 A x B = 200 B / A = 2 Increment A by 3 = 13 Decrement A by 4 = 9 Comparison Operations Following table shows all the relational operators supported by LISP that compares between numbers. However unlike relational operators in other languages, LISP comparison operators may take more than two operands and they work on numbers only. Assume variable A = 10 and variable B = 20, then: Operator Description Example Checks if the values of the operands are all equal = (= A B) is not true. or not, if yes then condition becomes true. Checks if the values of the operands are all /= different or not, if values are not equal then (/= A B) is true. condition becomes true. 20 

29 . LISP Checks if the values of the operands are > (> A B) is not true. monotonically decreasing. Checks if the values of the operands are < (< A B) is true. monotonically increasing. Checks if the value of any left operand is greater (>= A B) is not >= than or equal to the value of next right operand, true. if yes then condition becomes true. Checks if the value of any left operand is less <= than or equal to the value of its right operand, if (<= A B) is true. yes then condition becomes true. It compares two or more arguments and returns (max A B) returns max the maximum value. 20 It compares two or more arguments and returns (min A B) returns min the minimum value. 20 Example Create a new source code file named main.lisp and type the following code in it: (setq a 10) (setq b 20) (format t "~% A = B is ~a" (= a b)) (format t "~% A /= B is ~a" (/= a b)) (format t "~% A > B is ~a" (> a b)) (format t "~% A < B is ~a" (< a b)) (format t "~% A >= B is ~a" (>= a b)) (format t "~% A <= B is ~a" (<= a b)) (format t "~% Max of A and B is ~d" (max a b)) (format t "~% Min of A and B is ~d" (min a b)) When you click the Execute button, or type Ctrl+E, LISP executes it immediately and the result is: A = B is NIL A /= B is T 21