Processing Lists Part V

1
Processing ListsPart V
2
Outline

• Prerequisites
• Normal list processing
• Case I second list constant
• Case II lists in lock-step
• Case III list indexing
• Case IV independent lists
• Objectives
• List equality

3
Background

• We know how to process lists several ways
• What happens when we want to determine whether
  two lists are identical?
• This is simple in principle, but we have to cover
  a number of special cases

4
Program Requirements

• Objective
• Given two lists
• Return true if each item on each list is
  identical
• Otherwise, return false

5
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 1
jean
jean
sally
sue
juan
juan
carl
carl
lien
6
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 1
jean
jean
sally
sue
juan
juan
carl
carl
lien
7
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 1
jean
jean
sally
sue
juan
juan
carl
carl
lien
8
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 2
jean
jean
sally
sally
juan
juan
carl
carl
lien
9
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 2
jean
jean
sally
sally
juan
juan
carl
carl
lien
10
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 2
jean
jean
sally
sally
juan
juan
carl
carl
lien
11
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 2
jean
jean
sally
sally
juan
juan
carl
carl
lien
12
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 2
jean
jean
sally
sally
juan
juan
carl
carl
lien
13
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 2
jean
jean
sally
sally
juan
juan
carl
carl
lien
14
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 3
jean
jean
sally
sally
juan
juan
carl
carl
15
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 3
jean
jean
sally
sally
juan
juan
carl
carl
16
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 3
jean
jean
sally
sally
juan
juan
carl
carl
17
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 3
jean
jean
sally
sally
juan
juan
carl
carl
18
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 3
jean
jean
sally
sally
juan
juan
carl
carl
19
Program Design

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical

List 1
List 2
fred
fred
Case 3
jean
jean
sally
sally
juan
juan
carl
carl
20
Program Design

• Cases

list-1 empty
-gt (empty? list-2)
list-2 empty
-gt return false
both list-1 and list-2 are lists
-gt return (and ( (first list-1)
(first list-2))) (list?
(rest list-1) (rest
list-2)))
21
Short-Circuit Evaluation
Evaluating (and ( (first list-1)(first
list-2))) (list? (rest list-1)
(rest list-2)))
22
Short-Circuit Evaluation

• When we evaluate (and expr-1 expr-2),
• as soon as either expression evaluates to false,
  the total result can no longer be true.
• In the particular case of evaluating
• (and ( (first list-1)(first list-2)))
• (list? (rest list-1)
• (rest list-2)))
• expr-1 looks at the head of each list
• As long as this result is true, the recursive
  evaluation of (list? ) continues
• As soon as the items are not equal, the recursive
  part is not evaluated
• Recursion stops as you would expect as soon as a
  mis-match is detected.

23
(No Transcript)
24
Code Used

• list? list-1 list-2 -gt boolean
• determine whether two lists are
• identical
• (define (list? list-1 list-2)
• (cond (empty? list-1) (empty? list-2)
• (empty? list-2) false
• (and (cons? list-1) (cons? list-2))
• (and ( (first list-1)
• (first list-2))
• (list? (rest list-1)
• (rest list-2)))
• else "error - ill-formed problem"))

25
Questions?
26
Summary

• You should now know
• List equality

27
(No Transcript)