# The following piece of code is called Half: x := 0; y := 0; while (x < a) x := x + 2; y := y +… 1 answer below »

The following piece of code is called Half: x := 0; y := 0; while (x < a) x := x + 2; y := y + 1; We wish to use Hoare Logic to show that: {True} Half {x = 2 * y} In the questions below (and your answers), we may refer to the loop code as Loop, the body of the loop (i.e. x:=x+2;y:=y+1;) as Body, and the initialisation assignments (i.e. x:=0;y:=0;) as Init. 1. Given the desired postcondition {x = 2 * y}, what is a suitable invariant for Loop? (Hint: notice that the postcondition is independent of the value of a.) 2. Prove that your answer to the previous question is indeed a loop invariant. That is, if we call your invariant P, show that {P} Body {P}. Be sure to properly justify each step of your proof. 3. Using the previous result and some more proof steps show that {True} Half {x = 2 * y} Be sure to properly justify each step of your proof. 4. To prove total correctness of the program Half, identify and state a suitable variant for the loop. Using the same invariant P as above, the variant E should have the following two properties: it should be = 0 when the loop is entered, i.e. P ? (x < a) ? E = 0 it should decrease every time the loop body is executed, i.e. [P ?(x < a)?E = k] Body [P ? E < k] 1 You just need to state the variant, and do not need to prove the two bullet points above (yet). 5. For the variant E you have identified above, give a proof of the premise of the while-rule for total correctness, i.e. give a Hoare-logic proof of [P ? (x < a) ? E = k] Body [P ? E < k] and argue that P ? (x < a) ? E = 0. Question 3 Counting Modulo 7 [5 + 20 + 10 + 5 credits] Consider the following code fragment that we refer to as Count below, and we refer to the body of the loop (i.e. the two assignments together with the if-statement) as Body. while (y < n) y := y + 1; x := x + 1; if (x = 7) then x := 0 else x := x The goal of the exercise is to show that {x < 7}Count{x < 7} 1. Given the desired postcondition, what is a suitable invariant P for the loop? You just need to state the invariant. 2. Give a Hoare Logic proof of the fact that your invariant above is indeed an invariant, i.e. prove the Hoare-triple {P}Body{P}. 3. Hence, or otherwise, give a Hoare-logic proof of the triple {x < 7}Count{x < 7}. 4. Give an example of a precondition P so that the Hoare-triple {P}Count{x < 7} does not hold and justify your answer briefly.

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