Compute the weakest precondition

Question

Compute the weakest precondition for each of the following assignment statements and postconditions:

a = a + 2 * b - 1 {a > 1}

attempt:

a is on both sides

0 = 2b -1

1/2 = b is the weakest precon

answer: b > 1 - a/2

how do you get this answer?

Here's one way: Start with `a + 2 * b - 1 > 1` and solve for `b`. — melpomene, Feb 26 '18 at 23:05
@melpomene Can you post your answer? Some people (including me) looks for the first-sight green check symbol. — ndrwnaguib, Nov 10 '18 at 17:57

score 3 · Answer 1 · edited Dec 06 '19 at 06:50

3

Here is answer :

a = a + 2 * b - 1 {a > 1}

We replace a in {a > 1} by a + 2 * b - 1

a + 2 * b - 1 > 1
a + 2 * b > 2
2 * b > 2 - a
b > 1 - a / 2`

I think you found the answer since but for the people who might get here looking for this answer.

edited Dec 06 '19 at 06:50

Avinash Dalvi

answered Dec 06 '19 at 02:02

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1 Answers1