This document discusses data structures and looping in LISP. It notes that the only data structure in LISP is lists. It describes functions for accessing parts of lists like first, rest, append, and cons. It also demonstrates using dolist to iterate over lists, such as to write a function to reverse a list. Examples are provided to demonstrate applying these functions and looping construct to lists.

1. CSE240 – Introduction to
Programming Languages
Lecture 20:
Programming with LISP| Data Structures
Javier Gonzalez-Sanchez
javiergs@asu.edu
javiergs.engineering.asu.edu
Office Hours: By appointment

2. Javier Gonzalez-Sanchez | CSE 240 | Fall 2017 | 2
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))

3. Javier Gonzalez-Sanchez | CSE 240 | Fall 2017 | 3
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.

4. Javier Gonzalez-Sanchez | CSE 240 | Fall 2017 | 4
Data Structures
• ( setf myList '( (A B) C (D) ) )
((A B) C (D))
• (car (car myList) ) ;( first ( first ) )
A
• (cdr( car myList)) ;( last ( first ) )
(B)
• (car (cdr myList)) ;( first ( rest ) )
C

5. Javier Gonzalez-Sanchez | CSE 240 | Fall 2017 | 5
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))

6. Javier Gonzalez-Sanchez | CSE 240 | Fall 2017 | 6
Loop (dolist)
• (dolist (x '(a b c d e)) (print x))
A
B
C
D
E
NIL
dolist is an iterator with this format
(dolist (var list-to-iterate-over optional-return-val)
expr1
expr2
…)

7. Javier Gonzalez-Sanchez | CSE 240 | Fall 2017 | 7
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)

8. Javier Gonzalez-Sanchez | CSE 240 | Fall 2017 | 8
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)

9. CSE240 – Introduction to Programming Languages
Javier Gonzalez-Sanchez
javiergs@asu.edu
Fall 2017
Disclaimer. These slides can only be used as study material for the class CSE240 at ASU. They cannot be distributed or used for another purpose.