Allegro CL Certification Program

Lisp Programming Series Level I

Session 1.1.3

Top Ten Things to Know

1. Defining a Function

(defun plus (a b)

(+ a b))

• Function name

• Argument list

• Function body (sequence of one or more expressions)

Calling a function(plus 3 5)

(defun mean (a b)

(/ (plus a b) 2))

• First: call PLUS to add a to b

• Second: divide the result by 2

2. Printing Output

(defun print-mean (a b)

(format *standard-output*

"~A ~%"

(mean a b)))

• First argument is the stream• Second argument is the “control string”• Third argument is the value to be printed

– ~A takes the argument and prints it– ~% prints a newline– (Analogous to printf in C)

Common Format Control Strings• ~A prints any value (strings w/o double quotes)• ~S prints any value (strings w/ double quotes)• ~5F prints a float in a 5 character wide field• ~% prints a newline• Combine them to print many things:

(format *standard-output* “~A ~5F ~A ~%” 5 pi 10)

5 3.142 10

Printing Hello

(defun hello (name)

(format *standard-output*

" Hello, ~A ~%"

name))

(hello "Linda")

Hello, Linda

3. Using Local Variables(defun mean (a b)

(let* ((sum (+ a b)))

(/ sum 2)))

• LET* creates local variables

• Example creates one named SUM

• Initial value is result of (+ a b)

More Local Variables(defun polynomial (a b c x)

(let* ((y1 (* a (* x x)))

(y2 (* b x))

(y3 c))

(+ y1 y2 y3)))

• Ax2 + Bx + C

• Three local variables

Avoid this Mistake(defun polynomial (a b c x)

(setq y1 (* a (* x x)))

(setq y2 (* b x))

(setq y3 c))

(+ y1 y2 y3))

;;; y1, y2, y3 become globals

;;; compiler will give warnings

4. Using if(defun sign-name (number)

(if (> number 0)

’positive

’not-positive))

• Boolean test returns true or NIL (false)

• (If <test> <then> <else>)

Using if, cont’d (defun sign-name (number)

(if (> number 0)

“positive”

(if (= number 0)

"zero"

"negative")))

Compound Tests

• NOT: (not (> x 3))

• AND: (and (> x 3) (< x 10))

• OR: (or (> x 3) (< x 0) (= y 7) (< (+ x y) 5 ))

Other Types of Tests

• Numeric comparisons: >, >=, <, <=, =

• Equality of objects: EQ, EQL, EQUAL

• Equality of strings: string=, string-equal– (string-equal "Radar" "RADAR")

• Type tests:– (typep x 'integer)

5. Global Variables and Constants

• (defvar *website* "www.whitehouse.gov")– *website* is the symbol– Naming convention is to use asterisk in names

of global variables• (defconstant +timeout+ 60.0)

– +timeout+ is the symbol– Naming convention is to use + in names of

program constants

6. Assigning Values to Variables

• In Lisp we use setq or setf instead of =, := etc

(setq *todays-temp* 101.5)

*todays-temp* 101.5

7. Using Symbols

(defun color-name (action)

(if (eq action 'stop)

'red

(if (eq action 'go)

'green

'yellow)))

8. Using Lists

(defvar *abbreviations* '((bos “Boston”) (lga “NY/LaGuardia”) (sfo “San Francisco”)))

(defun decode-abbreviation (symbol) (second (assoc symbol *abbreviations*)))

(defun add-abbreviation (symbol str) (push (list symbol str) *abbreviations*))

9. Iteration(defun count (N)

(dotimes (I N)

(format *standard-output* "~A " N)))

(defun enumerate (list)

(dolist (element list)

(format *standard-output* "~A"

element)))

Iteration(defun sum (list)

(let* ((result 0))

(dolist (item list)

(setq result (+ result item)))

result))

(sum '(3 5 2 4)) 14

10. Packages• A Lisp package is a namespace for related

functionality– COMMON-LISP: a package for ANSI

Common Lisp; you can’t add to it or change it– USER: a package for you to play around in (it’s

also the default or initial package)– CG-USER

• Large applications should use their own package

Package Example(defpackage math

(:use common-lisp))

(in-package :math)

(defun square (x) (* x x))

• The symbol SQUARE now exists in the MATH package

• The math package “uses” or inherits from the common-lisp package

Packages

• Every symbols exists in a package– *package* will return the current package– in-package will change the package

• Default package in the IDE is

“COMMON-GRAPHICS-USER”