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Introducing DFA Formal Description Using Peer Instruction Cynthia Bailey Lee UCSD 2011 Theory of Computation Peer Instruction Lecture Slides by Dr. Cynthia Lee, UCSD are licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. Based on a work at www.peerinstruction4cs.org.Dr. Cynthia Lee, UCSDCreative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported Licensewww.peerinstruction4cs.org

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M = (Q, Σ, δ, q 0, F) Learning Objective: DFA Formal Description In order to write proofs, students need to use the formal description of a DFA, not just how to draw the diagram. Students often struggle with the formal description, preferring the more concrete diagram. However, diagrams are not sufficient in proof-writing precisely because concrete examples cause loss of generality. The two sample questions that will be given in the follow slides, although they don’t appear to have anything to do with proof-writing, are actually designed to assist students in building the proof-writing skill of having proficiency with the DFA formal description. This presentation explains how.

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3 M = (Q, Σ, δ, q 0, F) Name of the DFA Set of States Alphabet Transition Function Start State Set of Final (Accept) States Notice that capital letters are used for sets, lowercase letters are used for a function or single item Why students struggle: Formal description is (literally) Greek to them, and there are simply too many parts for them to absorb at once. …except M, the name of the machine…

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TWO SAMPLE QUESTIONS

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Which states should be in F (the set of “final” or “accept” states) so that DFA M 1 recognizes the language L 1 = {w | b’s never appear after a’s in w} (Σ = {a,b}) a)F = {q2} b)F = {q3} c)F = {q1, q2} d)F = {q1, q3} e)F = {q2, q3} M1 :M1 :

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a) s b) s c) s δ: Q x Σ -> Q Transition Table “b’s never appear after a’s” ab q1q2q1 q2 q3 ab q1q2q1 q2q3 q2q3 ab q1 q2 q3q2 q3

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HOW DO THESE TWO SAMPLE QUESTIONS TEACH THE FORMAL DESCRIPTION OF DFAS?

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Which states should be in F (the set of “final” or “accept” states) so that DFA M 1 recognizes the language L 1 = {w | b’s never appear after a’s in w} (Σ = {a,b}) a)F = {q2} b)F = {q3} c)F = {q1, q2} d)F = {q1, q3} e)F = {q2, q3} M1 :M1 : Introduced and defined F Reinforce that F is a set 5 times Final state(s) don’t have to be at the “end”—can even be start state! Introduce sigma as the alphabet

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a) s b) s c) s δ: Q x Σ -> Q Transition Table “b’s never appear after a’s” ab q1q2q1 q2 q3 ab q1q2q1 q2q3 q2q3 ab q1 q2 q3q2 q3 Introduces delta formally Question requires students to practice going back and forth between the concrete state diagram, and the function represented as a table

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