Array Variable Verification in Book Chapter 7.2

1. Verification with Array Variables Book: Chapter 7.2

2. The problem Using array variables can lead to complication: {x[1]=1/\x[2]=3} x[x[1]]:=2 {x[x[1]]=2} Why? Because the assignment changes x[1] as well. Now it is also 2, and x[x[1]], which is x[2] is 3 and not 2!

3. What went wrong? Take the postcondition {x[x[1]]=2} and substitute 2 instead of x[x[1]]. We obtain {2=2} (which is equivalent to {true}). Now, (x[1]=1/\x[2]=3) 2=2. So we may wrongly conclude that the above Hoare triple is correct.

4. How to fix this? `Backward substitution’ should be done with arrays as complete elements. Define (x; e1: e2): an array like x, with value at the index e1 changed to e2. (x; e1: e2)[e3]=e2 if e1=e3 x[e3] otherwise (x; e1: e2)[e3]=if(e1=e3, e2, x[e3])

5. Solved the problem? • How to deal with if(φ, e1, e2)? Suppose that formula ψ contains this expression. Replace if(φ, e1, e2) by new variable v in ψ. The original formula ψ is equivalent to: (φ/\ ψ[e1/v])\/(¬φ/\ ψ[e2/v])

6. Returning to our case • Our postcondition is {x[x[1]]=2}. • The assignment x[x[1]]:=2 causes the substitution in the postcondition ofthe (array) variable x by a new array, which is (x; x[1] : 2), resulting in {x[x[1]]=2} (x; x[1] : 2)[(x; x[1] : 2)[1]] = 2

7. Are we done? • Not yet. It remains to • Convert the array form into an if form. • Get rid of the if form. • Will not be done in class. • All we say is that we obtain an expression that is not implied by the precondition x[1]=1/\x[2]=3.