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# hw2 - Homework 2 CSC 7101 Spring 2011 Due 28 February 2011...

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Homework 2 CSC 7101, Spring 2011 Due: 28 February 2011 1. (5 pts) What set of states does the formula ‘ x.x = 3’ characterize? What set of states does the formula ‘ x.x = x ’ characterize? Explain your answers. 2. (10 pts) Which of the following Hoare triples are valid: (a) { true } x := 2 { true } (b) { true } x := x { false } (c) { false } x := 2 { true } (d) { false } x := 2 { false } (e) { true } while true do x := 2 end { false } (f) { true } x := x + 1 { x = x + 1 } (g) { x = y } t := x; x := y; y := t { x = y } (h) { x >= 0 } x := y { y >= 0 } 3. (5 pts) The following axiom for the assignment isn’t sound: { true } x := e { x = e } Find an example that demonstrates the unsoundness of the rule, i.e., a Hoare triple that can be proved with this rule but that isn’t operationally valid. 4. (10 pts) Give a good (i.e., sound and complete) rule for the assignment statement where the post- condition is determined by the precondition. In other words, fill in the question mark in the following proof rule. Explain why your rule is good. { P } x := e { ? } Hint: make sure you don’t lose the information contained in P . 1

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5. (10 pts)
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