CSE240 Lecture 23

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CSE 240
Introduction to Programming Languages
Lecture 23: Functions in Lisp
Dr. Javier Gonzalez-Sanchez
[email protected]
javiergs.engineering.asu.edu | javiergs.com
PERALTA 230U
Office Hours: By appointment

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Previously

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Javier Gonzalez-Sanchez | CSE240 | Spring 2018 | 3
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Nested Variable Scope
§ (let ((x 3) (y (+ 4 9))) (* x y))
39
§ (let ((x 3))
(print x)
(let (x)
(print x)
(let ((x "hello"))
(print x))
(print x))
(print x)
(print ”end"))
3
NIL
"hello"
NIL
3
"end"
"end"

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Definition of Functions

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Function Making
§ functions are created by calling a function-making macro. This macro
is called defun.
§ the function returns the value of the last expression.
(defun function-name-symbol
(param1 param2 param3 ...)
expr1
expr2
expr3
...
)

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Function Making
§ (defun do-hello-world ( ) "Hello, World!")
DO-HELLO-WORLD
§ (do-hello-world)
"Hello, World!"
• (defun add-four (x) (+ x 4))
ADD-FOUR
• (add-four 7)
11
• (defun hypotenuse (length width)
(sqrt (+ (* length length)(* width width))))
HYPOTENUSE
• (hypotenuse 7 9)
11.4017

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Function Making
§ (defun first-n-chars (string n reverse-iT)
(if reverse-iT
(subseq (reverse string) 0 n)
(subseq string 0 n)))
FIRST-N-CHARS
§ (first-n-chars "hello world" 5 nil)
"hello"
§ (first-n-chars "hello world" 5 t)
"dlrow"
§ (first-n-chars "hello world" 5 2013)
"dlrow"

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Exercises

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Test Yourselves 1
public static double factorial (double n) {
double sum = 1;
for (double i=0;i < n; i++)
sum = sum * (1 + i);
return sum;
}
public static void main(String [] args) {
System.out.println(factorial(1000)+"");
}

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Answer 1
(defun factorial (n)
(let ((sum 1))
(dotimes (i n)
(setf sum (* sum (1+ i)))
)
sum
)
)
FACTORIAL
(factorial 1000)
result?
… double factorial (double n)
{
double sum = 1;
for (double i=0;i < n; i++)
sum = sum * (1 + i);
return sum;
}
result?

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Test Yourselves 2
public static double factorial (double n) {
if (n<=1)
return 1;
else
return n * factorial (n-1);
}
public static void main(String [] args) {
System.out.println(factorial(1000)+"");
}

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Answer 2
(defun factorial (n)
(if (<= n 0)
1
(* n (factorial (- n 1)))
)
)
FACTORIAL
(factorial 1000)
result?
… double factorial (double n) {
if (n<=1)
return 1;
else
return n * factorial (n-1);
}
result?

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Announcement
§ Homework 05 and Quiz 05 are now open.
§ They are due on November 30.
§ They are all about LISP

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Data Structures

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Data Structures
The only data structure is List
• (quote (hello world 1 2 3))
(HELLO WORLD 1 2 3)
• '(hello world 1 2 3)
(HELLO WORLD 1 2 3)
• (quote (what is (going on) here?))
(WHAT IS (GOING ON) HERE?)
• (quote my-symbol)
MY-SYMBOL
• (quote (+ 4 (* 3 2 9)))
(+ 4 (* 3 2 9))

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Data Structures
§ first returns the first item in a list. The old name of first is car (content of
the address).
§ rest returns a list consisting of everything but the first item. It does not
damage the original list. The old name of rest is cdr (content of the
decrement register).
§ append hooks multiple lists together.
§ cons takes an item and a list, and returns a new list consisting of the
old list with the item tacked on the front.

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Data Structures
§ ( setf myList '( (A B) C (D) ) )
((A B) C (D))
§ (car (car myList) ) ;( first ( first ) )
A
§ (cdr( car myList)) ;( rest ( first ) )
(B)
§ (car (cdr myList)) ;( first ( rest ) )
C

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Data Structures
§ ( setf myList2 '(A (B () (C () () )) (D (E () () ) () ) ) )
(A (B NIL (C NIL NIL)) (D (E NIL NIL) NIL))
§ (car (car myList2)) ;( first ( first (L))
Error: Cannot take CAR of A.
§ (cdr( car myList2)) ;( rest ( first (L))
Error: Cannot take CDR of A.
§ (car (cdr myList2)) ;( first ( rest (L))
(B NIL (C NIL NIL))

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Loop (dolist)
dolist is an iterator with this format
(dolist (var list-to-iterate-over optional-return-val)
expr1
expr2
…)
§ (dolist (x '(a b c d e)) (print x))
A
B
C
D
E
NIL

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Loop (dolist)
• (defun my-reverse (list)
(let (new-list) ; empty list
(dolist (x list)
(setf new-list (cons x new-list))
)
new-list)
)
MY-REVERSE
• (my-reverse '(a b c d e f g))
(G F E D C B A)

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Loop (dolist)
• (defun my-reverse (list)
(let (new-list) ; empty list
(dolist (x list)
(setf new-list (cons x new-list))
)
new-list)
)
MY-REVERSE
• (my-reverse '(a b c d e f g))
(G F E D C B A)
• (print (first (my-reverse '(a b c d e f g))))
G
• (print (last (my-reverse '(a b c d e f g))))
(A)

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More Examples

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Example 1
; A function that return the leftmost even member."
(defun find-even (L)
(if (null L)
nil
(if (evenp (first L))
(first L)
(find-even (rest L))
)
)
)

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Example 2
; develop recursive functions that traverse a list
; and count its elements
(defun recursive-list-length (L)
(if (null L)
0
(1+ (recursive-list-length (rest L)))
)
)

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Example 3
; defines a function (get N L) that returns the N member
; of list L –assuming that the elements are numbered from
; zero onwards
(defun get (N L)
(if (null L)
nil
(if (zerop N)
(first L)
(get (1- N) (rest L))
)
)
)

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Test Yourselves

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Questions
1. Create a function that receives a list and return the lowerst value
2. Create a function that receives a list and an integer and return “true” if the number
exist in the list
3. Create a function that receives a list and return “true” if the list contains a
palindrome. such as (a b c d e d c b a)
4. Create a variable in Lisp to store a tic-tac-toe, such as
x E x
0 X 0
E x E

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Questions
5. Create a function that receives the variable with the tic-tac-toe and return
“true” if there is an empty space (an “E” letter).
6. The following list represent a tree
'(a (b () ()) (c (d () () ) (e () ()) ) )
Create a function (recursive) that traverse the tree and print all the elements
as A B C D E.
7. What is printed by the following code?
(setf route '(boston cambridge lincoln concord))
(print (first (rest route)))

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Questions
8. What is the output of the following code?
(let ((x '(1 2)) (y '(9 10))) (print (+ (first x) (first y)) ) )
9. What is printed by the following code?
(defun fun(n)
(if (= n 0) () (fun (- n 1)) )
n
)
(print (fun 5))
10. What is equivalent code in C++ for the following LISP statement
(if (< 1 2) (+ 3 4) (- 5 6) )

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Solve 5 of the following problems
1. A function test-min that returns the lower value in a List of numbers
(test-min L)
2. A function test-average that returns the average of all values in a List of
numbers
(test-average L)
3. A function test-fibonacci that prints the first n elements of the fibonacci series
(test-fibonacci n)
4. A recursive function that returns the leftmost even element in a list of
numbers
(test-even L)
5. A functions that traverse a list and returns how many of its elements are 0
(test-count-zeros L)
6. A recursive function that return the N member of a list L (assuming that the
elements are numbered from zero onwards):
(test-get n L)
7. A recursive function that returns non-NIL if x is a member of the list L.
(test-is-in-list x L)

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Questions

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CSE 240 Introduction to Programming Languages
Javier Gonzalez-Sanchez, Ph.D.
[email protected]
Fall 2021
Copyright. These slides can only be used as study material for the class CSE240 at Arizona State University.
They cannot be distributed or used for another purpose.

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CSE240 Lecture 23

CSE240 Lecture 23

Javier Gonzalez-Sanchez
PRO

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CSE240 Lecture 23

CSE240 Lecture 23

Javier Gonzalez-Sanchez PRO

More Decks by Javier Gonzalez-Sanchez

Other Decks in Programming

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Transcript

Javier Gonzalez-Sanchez
PRO