CSE 311: Foundations of Computing Fall 2013 Lecture 23: Finite state machines and minimization

FSM highlights Finite state machines – States, transitions, start state, final states – Languages recognized by FSMs s0s0 s2s2 s3s3 s1s , s0s0 s2s2 A s1s1 1 R 0

FSM highlights Combining FSMs to check two properties at once New states record states of both FSMs s0s0 s1s1 0,1 2 2 t0t0 t2t2 t1t s0t0s0t0 s1t0s1t0 s1t2s1t2 s0t1s0t1 s0t2s0t2 s1t1s1t

state machines with output InputOutput StateLR s1s1 s1s1 s2s2 Beep s2s2 s1s1 s3s3 s3s3 s2s2 s4s4 s4s4 s3s3 s4s4 S3S3 S4S4 S1S1 S2S2 R L R L R L L R “Tug-of-war”

vending machine Enter 15 cents in dimes or nickels Press S or B for a candy bar

vending machine, v DD N N N, D B, S Basic transitions on N (nickel), D (dime), B (butterfinger), S (snickers)

vending machine, v ’ B Adding output to states: N – Nickel, S – Snickers, B – Butterfinger 15’ N 0 0” S N N N N N B D D D D DB S S

vending machine, v1.0 0’ B Adding additional “unexpected” transitions 15’ N 0 0” S N N N N N B D D D D D B S S 15” D S B B,S N N N D D D

state minimization Many different FSMs (DFAs) for the same problem Take a given FSM and try to reduce its state set by combining states – Algorithm will always produce the unique minimal equivalent machine (up to renaming of states) but we won’t prove this

state minimization algorithm 1.Put states into groups based on their outputs (or whether they are final states or not) 2.Repeat the following until no change happens a.If there is a symbol s so that not all states in a group G agree on which group s leads to, split G into smaller groups based on which group the states go to on s G1G1 G2G2 G3G3 s s s s

state minimization example state transition table present next state output state0123 S0S0S1S2S31 S1S0S3S1S50 S2S1S3S2S41 S3S1S0S4S50 S4S0S1S2S51 S5S1S4S0S S0 [1] S2 [1] S4 [1] S1 [0] S3 [0] S5 [0] 1 Put states into groups based on their outputs (or whether they are final states or not)

state minimization example state transition table present next state output state0123 S0S0S1S2S31 S1S0S3S1S50 S2S1S3S2S41 S3S1S0S4S50 S4S0S1S2S51 S5S1S4S0S S0 [1] S2 [1] S4 [1] S1 [0] S3 [0] S5 [0] 1 Put states into groups based on their outputs (or whether they are final states or not)

state minimization example state transition table present next state output state0123 S0S0S1S2S31 S1S0S3S1S50 S2S1S3S2S41 S3S1S0S4S50 S4S0S1S2S51 S5S1S4S0S S0 [1] S2 [1] S4 [1] S1 [0] S3 [0] S5 [0] 1 Put states into groups based on their outputs (or whether they are final states or not) If there is a symbol s so that not all states in a group G agree on which group s leads to, split G based on which group the states go to on s

state minimization example state transition table present next state output state0123 S0S0S1S2S31 S1S0S3S1S50 S2S1S3S2S41 S3S1S0S4S50 S4S0S1S2S51 S5S1S4S0S S0 [1] S2 [1] S4 [1] S1 [0] S3 [0] S5 [0] 1 Put states into groups based on their outputs (or whether they are final states or not) If there is a symbol s so that not all states in a group G agree on which group s leads to, split G based on which group the states go to on s

state minimization example state transition table present next state output state0123 S0S0S1S2S31 S1S0S3S1S50 S2S1S3S2S41 S3S1S0S4S50 S4S0S1S2S51 S5S1S4S0S S0 [1] S2 [1] S4 [1] S1 [0] S3 [0] S5 [0] 1 Put states into groups based on their outputs (or whether they are final states or not) If there is a symbol s so that not all states in a group G agree on which group s leads to, split G based on which group the states go to on s

state minimization example state transition table present next state output state0123 S0S0S1S2S31 S1S0S3S1S50 S2S1S3S2S41 S3S1S0S4S50 S4S0S1S2S51 S5S1S4S0S S0 [1] S2 [1] S4 [1] S1 [0] S3 [0] S5 [0] 1 Put states into groups based on their outputs (or whether they are final states or not) If there is a symbol s so that not all states in a group G agree on which group s leads to, split G based on which group the states go to on s

state minimization example state transition table present next state output state0123 S0S0S1S2S31 S1S0S3S1S50 S2S1S3S2S41 S3S1S0S4S50 S4S0S1S2S51 S5S1S4S0S S0 [1] S2 [1] S4 [1] S1 [0] S3 [0] S5 [0] 1 Put states into groups based on their outputs (or whether they are final states or not) If there is a symbol s so that not all states in a group G agree on which group s leads to, split G based on which group the states go to on s

state minimization example state transition table present next state output state0123 S0S0S1S2S31 S1S0S3S1S50 S2S1S3S2S41 S3S1S0S4S50 S4S0S1S2S51 S5S1S4S0S S0 [1] S2 [1] S4 [1] S1 [0] S3 [0] S5 [0] 1 Can combine states S0-S4 and S3-S5. In table replace all S4 with S0 and all S5 with S3

minimized machine state transition table present next state output state0123 S0S0S1S2S31 S1S0S3S1S30 S2S1S3S2S01 S3S1S0S0S ,2 S0 [1] S2 [1] S1 [0] S3 [0] 1,3

another way to look at DFAs s0s0 s2s2 s3s3 s1s , Lemma: x is in the language recognized by a DFA iff x labels a path from the start state to some final state Definition: The label of a path in a DFA is the concatenation of all the labels on its edges in order

nondeterministic finite automaton (NFA) Graph with start state, final states, edges labeled by symbols (like DFA) but – Not required to have exactly 1 edge out of each state labeled by each symbol--- can have 0 or >1 – Also can have edges labeled by empty string Definition: x is in the language recognized by an NFA if and only if x labels a path from the start state to some final state s0s0 s2s2 s3s3 s1s ,1

goal: NFA to recognize... binary strings with even # of 1’s that contain the substring 111