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Unformatted text preview: Arithmetic Expression Summary for n I ( n , ) 7 eval ( n ) for x Loc ( x , ) 7 ( x ) ( a , ) 7 k ( a 1 , ) 7 k 1 where k = k + k 1 ( a + a 1 , ) 7 k ( a , ) 7 k ( a 1 , ) 7 k 1 where k = k k 1 ( a a 1 , ) 7 k ( a , ) 7 k ( a 1 , ) 7 k 1 where k = k k 1 ( a a 1 , ) 7 k ( a , ) 7 k (( a ) , ) 7 k with k , k , k 1 I , a , a , a 1 Aexp , and . Expression Equivalence Our notion of semantic value for expressions leads to a natural equivalence relation between arithmetic expressions: a a 1 iff , n I . ( a , ) 7 n ( a 1 , ) 7 n , where a , a 1 Aexp . Two expressions are equivalent if and only if they evaluate to the same semantic value in all possible states. (You should convince yourself that this is indeed an equivalence relation, i.e., check that the relation is reexive, symmetric, and transitive.) Expression Equivalence Problem: Let a 2 3 and a 1 3 + 3, show that a a 1 . Expression Equivalence Proof : We need to show that (2 3 , ) 7 k and (3 + 3 , ) 7 k for all states and some k I . Let be any state, then (2 , ) 7 2 (3 , ) 7 3 (2 3 , ) 7 6 and (3 , ) 7 3 (3 , ) 7 3 (3 + 3 , ) 7 6 which shows that regardless of the state, the two expressions will always produce the same semantics value, namely the integer 6. This concludes the proof. 2 Expression Equivalence Problem: Show that the + operator is commutative. Expression Equivalence...
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 Spring '09
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