The best answers to the question “What is a loop invariant?” in the category Dev.

__QUESTION__:

I’m reading “Introduction to Algorithm” by CLRS. In chapter 2, the authors mention “loop invariants”. What is a loop invariant?

__ANSWER__:

I like this very simple definition: (source)

A loop invariant is a condition [among program variables] 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.)

By itself, a loop invariant doesn’t do much. However, given an appropriate invariant, it can be used to help prove the correctness of an algorithm. The simple example in CLRS probably has to do with sorting. For example, let your loop invariant be something like, at the start of the loop, the first `i`

entries of this array are sorted. If you can prove that this is indeed a loop invariant (i.e. that it holds before and after every loop iteration), you can use this to prove the correctness of a sorting algorithm: at the termination of the loop, the loop invariant is still satisfied, and the counter `i`

is the length of the array. Therefore, the first `i`

entries are sorted means the entire array is sorted.

An even simpler example: Loops Invariants, Correctness, and Program Derivation.

The way I understand a loop invariant is as a systematic, formal tool to reason about programs. We make a single statement that we focus on proving true, and we call it the loop invariant. This organizes our logic. While we can just as well argue informally about the correctness of some algorithm, using a loop invariant forces us to think very carefully and ensures our reasoning is airtight.

__ANSWER__:

In simple words, a loop invariant is some predicate (condition) that holds for every iteration of the loop. For example, let’s look at a simple `for`

loop that looks like this:

```
int j = 9;
for(int i=0; i<10; i++)
j--;
```

In this example it is true (for every iteration) that `i + j == 9`

. A weaker invariant that is also true is that

`i >= 0 && i <= 10`

.

__ANSWER__:

Previous answers have defined a loop invariant in a very good way.

Following is how authors of CLRS used loop invariant to prove **correctness** of Insertion Sort.

**Insertion Sort algorithm**(as given in Book):

```
INSERTION-SORT(A)
for j ← 2 to length[A]
do key ← A[j]
// Insert A[j] into the sorted sequence A[1..j-1].
i ← j - 1
while i > 0 and A[i] > key
do A[i + 1] ← A[i]
i ← i - 1
A[i + 1] ← key
```

Loop Invariant in this case:

**Sub-array[1 to j-1] is always sorted.**

Now let us check this and prove that algorithm is correct.

**Initialization**: Before the first iteration j=2. So sub-array [1:1] is the array to be tested. As it has only one element so it is sorted. Thus invariant is satisfied.

**Maintenance**: This can be easily verified by checking the invariant after each iteration. In this case it is satisfied.

**Termination**: *This is the step where we will prove the correctness of the algorithm.*

When the loop terminates then value of j=n+1. Again loop invariant is satisfied. This means that Sub-array[1 to n] should be sorted.

This is what we want to do with our algorithm. Thus our algorithm is correct.

__ANSWER__:

There is one thing that many people don’t realize right away when dealing with loops and invariants. They get confused between the loop invariant, and the loop conditional ( the condition which controls termination of the loop ).

As people point out, the loop invariant must be true

- before the loop starts
- before each iteration of the loop
- after the loop terminates

( although it can temporarily be false during the body of the loop ). *On the other hand the loop conditional* **must** be false after the loop terminates, otherwise the loop would never terminate.

Thus the loop invariant and the loop conditional **must** be different conditions.

A good example of a complex loop invariant is for binary search.

```
bsearch(type A[], type a) {
start = 1, end = length(A)
while ( start <= end ) {
mid = floor(start + end / 2)
if ( A[mid] == a ) return mid
if ( A[mid] > a ) end = mid - 1
if ( A[mid] < a ) start = mid + 1
}
return -1
}
```

So the loop conditional *seems* pretty straight forward – when start > end the loop terminates. But why is the loop correct? What is the loop invariant which proves it’s correctness?

The invariant is the logical statement:

```
if ( A[mid] == a ) then ( start <= mid <= end )
```

This statement is a logical tautology – it is always true *in the context of the specific loop / algorithm we are trying to prove*. And it provides useful information about the correctness of the loop after it terminates.

If we return because we found the element in the array then the statement is clearly true, since if `A[mid] == a`

then `a`

is in the array and `mid`

must be between start and end. And if the loop terminates because `start > end`

then there can be no number such that `start <= mid`

**and** `mid <= end`

and therefore we know that the statement `A[mid] == a`

must be false. However, as a result the overall logical statement is still true in the null sense. ( In logic the statement if ( false ) then ( something ) is always true. )

Now what about what I said about the loop conditional necessarily being false when the loop terminates? It looks like when the element is found in the array then the loop conditional is true when the loop terminates!? It’s actually not, because the implied loop conditional is really `while ( A[mid] != a && start <= end )`

but we shorten the actual test since the first part is implied. This conditional is clearly false after the loop regardless of how the loop terminates.