# Write a loop invariant

This section describes a systematic approach for writing loop invariants. This special form of loop is used to simulate forward gotos by using exit statements instead. This again can be weakened to give us the invariant. If c is not present in s, then -1 is returned. For this reason, we must pause our study of programming logic for a study of symbolic logic, that is, the algebra of logical assertions.

The phrase reads better like this: The specification of the previous example now looks like this: To understand how it might be used, consider this variation of the previous example. For string, s, s[: If c is not in s, a copy of s is returned, unchanged.

You are welcome to deduce that the program meets its postcondition. Reuse the loop invariant from find in the previous exercise. Worse yet, its presence can lead to false deductions at the point where the function is called! We do it like this: No answer is returned because the precondition is violated and the loop is unable to terminate.

The deduction law for loops guarantees, if the loop terminates, then the postcondition must hold true. There can be silly applications of the loop law.

Consider this faulty program: But the loop body preserves the invariant only because its body, pass, is too timid to make any progress at all towards the goal. So, the loop never terminates.

Now, if the loop would terminate, then the proof shows we will achieve the goal. But, for every argument but 0, the loop will not terminate. Because of this limitation of the loop law, it is called a partial correctness law. To ensure total correctness, that is, to prove the loop must terminate and satisfies its goal, we must use additional reasoning.So, how can we reason about the logic of loops better and write elegant loops?

My answer is to find a good loop invariant and use it to construct the loop. I will review what a loop invariant is and demonstrate the idea using two examples: deleting an entry in a linked list (by Linus) and finding a number that is larger than. Loop invariants. A loop invariant is a condition that is true at the beginning and end of every iteration of a loop.

The concept is similar to a class invariant, which must be true at the beginning and end of every public regardbouddhiste.com you write a loop that works correctly, you are .

Write the loop specifying the guard (loop condition) Insertion sort uses two loops so we will have two loop invariants. As in the previous articles, we will be using while loops in our demonstration because it clearly separates the guard (loop condition) from initialization and loop index, however we can use any other kind of loop if we wish (for example a for loop).

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}.

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 (maintaining the invariant). 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.).

How to Write Loop Invariants — SPARK User's Guide w