Write a loop invariant

Our still rather sketchy solution has advanced to: The selected integer is swapped with the integer in location i The first comment describes the invariant in English: We could as easily put it at the start of the loop.

Remember, for the moment, we pretend this! There does not exist on the grid a pair of red line segments that form an upper right corner. To finish, we repeat the mathematical-induction proof law: Check that the Loop Invariant is setup true by the initialization of nextToCheck and smallestSoFar; Check that each time through the loop, assuming the Loop Invariant true going into the loop, it is true going out of the loop; Check that headway towards termination is being made in this case, nextToCheck is always incremented in the loop body.

So, the formula works correctly for the starting case, when n is 0. Test the loop's body, not the entire loop. Programming language support[ edit ] Eiffel[ edit ] The Eiffel programming language provides native support for loop invariants.

To cover all the cases and prove our goal, we write two proofs: The second step is the inductive step, in which it is shown that, if, for any positive number n, P holds the n-th time execution reaches the critical spot i.

Are you ever certain that you have tested the program sufficiently? A proof written in this style is just a proof by mathematical induction.

It generates two medium messages, one corresponding to a possible run-time check failure, and one corresponding to a possible failure of the postcondition: We set the Loop Invariant true before the loop by setting: For this reason, we must pause our study of programming logic for a study of symbolic logic, that is, the algebra of logical assertions.

This is not always the case. The technique of abstract interpretation can be used to detect loop invariant of given code automatically.

Loop invariant

To understand how it might be used, consider this variation of the previous example. Formal statement of the mathematical induction proof law The mathematical induction proof law was formulated almost the same time as the counting numbers nonnegative ints0, 1, 2, In our example, the variant is a.

Loop-invariant code motion A loop invariant loop-invariant property is to be distinguished from loop-invariant code; note "loop invariant" noun versus "loop-invariant" adjective.

But multiplication is associative, meaning that we have proved the result in this case, too: As a first example, here is a variant of the search algorithm described in SPARK Tutorialwhich returns whether a collection contains a desired value, and if so, at which index.

For example, consider the subprograms Init and Sum below: There is an important principle hiding in here, and once we understand it we will realize there are only two test cases that matter: Does either player have a winning strategy?

Formal statement of the mathematical induction proof law The mathematical induction proof law was formulated almost the same time as the counting numbers nonnegative ints0, 1, 2, The only information that GNATprove knows about the value of variables that are modified in the loop, at each loop iteration, is the information provided in the loop invariant.

Taking into account the possibility that there are no such indexes in A if either Left or Right are at the boundaries of the array, we can express it as follows: Here is what we want to prove: Here is the Loop Invariant: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.

Loop invariants help. A loop invariant is a formal statement about the relationship between variables in your program which holds true just before the loop is ever run (establishing the invariant) and is true again at the bottom of the loop, each time through the loop.

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.). 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. A Loop invariants: analysis, classification, and examples dfaduke.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.

Loop invariant

Loop Invariants in Python. up vote 6 down vote favorite. 1. A loop invariant is simply something that is true on every iteration of the loop. For example, take a really trivial while loop: So we can actually write out the full result as: {A} while cond: code {A ∧ ¬cond}.

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Write a loop invariant
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