1. Changing Data: (Continued)
CMSC 11500
Introduction to Computer Programming
November 4, 2002

2. Roadmap Recap: Assignment & Data with state Topical Example: Voting!
Variables to capture state
Multiple state variables: voter types
Actions: Update poll stats (view results)
What’s the same? different?
Encapsulation
Scope
Hand-evaluation

3. Recap: Modeling State Data objects change over time
Bank account, traffic light,
Represent with variable(s) that change
Mechanism: (set! var expr)
Changes value to var to result of eval expr
Supporting mechanism:
(begin exp1 exp2 … expn):
Evaluates all expressions: returns value of last

4. Defining a Poll What state captures a poll? What actions?
Number of voters
Number of voters in each category:
For, against, undecided
What actions?
Add in new votes as come in
Update
Report current levels

5. What’s a vote?
vote-type is
1) ‘for
2) ‘against
3) ‘undecided

6. Make-poll Contract: Purpose:
-> (vote-type number -> (listof number))
Purpose:
;; To setup procedure to update standings
Effect: update voters-for,against, undecided

7. Make-poll (define (make-poll) (let ((voters-for 0) (voters-against 0)
(voters-undecided 0))
(lambda (vote-class new-votes)
(let ((total-votes (+ new-votes voters-for voters-against voters-undecided)))
(cond ((eq? vote-class 'for)
(set! voters-for (+ voters-for new-votes)))
((eq? vote-class 'against)
(set! voters-against (+ voters-against new-votes)))
((eq? vote-class 'undecided)
(set! voters-undecided (+ voters-undecided new-votes)))
(else (error "bad vote type")))
(list (/ voters-for total-votes)
(/ voters-against total-votes)
(/ voters-undecided total-votes))))))

8. Identity (define contest1 (make-poll)) (define contest2 contest1)
Contest1 & contest2 “same”
Use same state variables
Contest3: Different
Also poll – no shared state values

9. Encapsulation Local definition Defining as result shares variables
State variables encapsulated in procedure
Distinct for each invocation of make-poll
Defining as result shares variables
Separate invocation -> separate states

10. Summary
Data objects change over time
Capture in state variables