# Write a loop invariant

Since all the ints are chained together, this technique assembles a proof for every nonnegative int. Thanks to this feature, GNATprove automatically proves the postconditions of both procedures, without the need for loop invariants: The proof is done by mathematical induction: For expressions E and F and a variable x, we use the notation E x: Before giving the first version of our program, we introduce the notion of textual substitution: The quotient q is the counter, and remainder r is what remains after the subtraction.

In this example, it is to find that there could be an imaginary header node. We now need to show that this is actually a loop invariant: A loop invariant is simply something that is true on every iteration of the loop. What matters is that the loop invariant allows proving absence of run-time errors in the subprogram, that the subprogram respects its contract, and that the loop invariant itself holds at each iteration of the loop.

The execution can only reach F directly from A or Ewhere the loop invariant holds. Proof by mathematical induction shows us how: So, knowing this, we can infer the rule for the whole loop: Each time through the loop increased numSorted by one. Assuming the existence of a static method called swap whereby the call swap b,i,j swaps the values of b[i] and b[j], we get this final solution: Testing only reveals that you have them wrong.

Consider this faulty program: Red and Blue take alternating turns, with Red going first. Of course, a repeatable test condition is a kind of invariant, and the above test methodology is the induction step of a mathematical induction proof.

Work is not carried forward to the next pass in complicated, data dependent ways. This will save us a lot of time testing the definition of!.

Does either Red or Blue have a "winning strategy", i. Loop Invariants Overview A loop invariant is a condition that is necessarily true immediately before and immediately after each iteration of a loop.

The guard is the boolean expression that determines whether to execute the body of the loop. In the case that the marbles chosen were of different colors, the number of BLUE marbles remains the same. When the program exits a loop, the loop invariant expresses the intended value s of the variable s at that point.

Blue's goal is to prevent Red from doing so.The loop's invariant is exactly the precondition for executing the loop's body, and it is exactly the postcondition of what is generated by executing the loop's body. Even if you forget all about algebra and proofs, whenever you write a loop, document the loop with its invariant stated in words.

Just one minor correction add x = t; before your while loop – K Mehta Apr 18 '12 at the while loop works just as well as the for loop. Oh, forgot to mention one more correction: you should also add t = h; before you start the while loop.

I've written about writing loop invariants in my blog, see Verifying Loops Part 2. The invariants needed to prove a loop correct typically comprise 2 parts: A generalisation of the state that is intended when the loop terminates. Extra bits needed to ensure that the loop body is well-formed (e.g. array indices in bounds). (2) is straightforward. A Loop invariants: analysis, classiﬁcation, and examples cheri197.com, ETH Zurich BERTRANDMEYER, ETH Zurich, ITMO St. Petersburg, and Eiﬀel Software SERGEYVELDER, ITMO St. Petersburg Software veriﬁcation has emerged as a key concern for ensuring the continued progress of information.

So you basically just change the upper and lower bound of the loop Since h. A loop invariant is a condition that is necessarily true immediately before and immediately after each iteration of a loop. (Note that this says nothing about its truth or falsity part way through an iteration.).

Write a loop invariant
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