1. Lecture 6
CSE 331
Sep 9, 2013

2. Homeworks HW 1 posted online: see piazza
Pickup graded HW 0 in TA OHs/recitations

3. Comments on feedback
70% on exams and 4% on group project seems uneven (unfair?)
Distribution proportional to time spent preparing for it.

4. Aside: Do your HWs!
Fall 2012
Fall 2011

5. Another comment Discomfort with proofs
I will not cover proof basics in class
Please come and talk to us in person if you need help

6. Lecture pace
Mid-term

7. Online Office Hours
Tomorrow: 9:30pm to 10:30pm

8. Questions/Comments?

9. Gale-Shapley Algorithm
Intially all men and women are free
While there exists a free woman who can propose
Let w be such a woman and m be the best man she has not proposed to
w proposes to m
If m is free
(m,w) get engaged
Else (m,w’) are engaged
If m prefers w’ to w
w remains free
Else
(m,w) get engaged and w’ is free
Output the set S of engaged pairs as the final output

10. Today’s agenda Prove correctness of the GS algorithm
Run of GS algorithm on an instance
Prove correctness of the GS algorithm

11. Preferences
Mal
Wash
Simon
Inara
Zoe
Kaylee

12. GS algorithm: Firefly Edition
Mal
Inara 1 2 3 4 5 6
Wash
Zoe 1
Simon
Kaylee

13. Once a man gets engaged, he remains engaged (to “better” women)
Observation 1
Intially all men and women are free
While there exists a free woman who can propose
Let w be such a woman and m be the best man she has not proposed to
w proposes to m
If m is free
(m,w) get engaged
Once a man gets engaged, he remains engaged (to “better” women)
Else (m,w’) are engaged
If m prefers w’ to w
w remains free
Else
(m,w) get engaged and w’ is free
Output the engaged pairs as the final output

14. If w proposes to m after m’, then she prefers m’ to m
Observation 2
Intially all men and women are free
While there exists a free woman who can propose
Let w be such a woman and m be the best man she has not proposed to
w proposes to m
If m is free
If w proposes to m after m’, then she prefers m’ to m
(m,w) get engaged
Else (m,w’) are engaged
If m prefers w’ to w
w remains free
Else
(m,w) get engaged and w’ is free
Output the engaged pairs as the final output

15. Questions/Comments?

16. Why bother proving correctness?
Consider a variant where any free man or free woman can propose
Is this variant any different? Can you prove it?

17. GS’ does not output a stable marriage