Let us revisit Loops. So, we saw that we have two types of loops in Python. The first is a 'for loop' where the value ranges over a list in general or say the output of the range sequence; so for i in l will repeat the body for each item in the list l. Sometimes we do not know how many times we need to repeat in advance, so then we use the while forms. While takes a condition and it keeps repeating the body until the condition becomes false. But the main thing to notice in both cases is that how long the loop takes is determined in advance either we have a fixed number of iterations depending on number of values in l for a for loop or we keep going until the condition becomes false. We cannot stop before that.

Now it does turn out that sometimes we have natural situations where we want to cut short the loop and not proceed till the very end. Suppose, we want to write a function findpos which finds the first position of a value v in the list l. We had seen that there is a built-in function called index which pretty much does this, so l.index(v) will report the first position where v occurs in the list l. However, the index function will report an error if there is no occurrence of v in l. So, we want to change that slightly and what we want to say is that if there is no v in l report minus 1. So either we get a value between 0 to n minus 1, where n is the length of the list or we get the value minus 1 saying that v is not in the list.

Here is a while loop implementation. We use a name found as the name suggests to indicate whether the value have been found or not. Initially we have not seen any values in l, so v is, the found - is false. And we use i as a name to go through the positions, so i is initially 0 and we will increment i until we hit the length of l. So long as, while i is less than the length of the l, if we see the value we are looking for then we update found to be true and we mark the position that we have found it to be pos.

Now the issue is why we have to wastefully go through the entire list even though after we find the very first position of v in l we can safely return and report that position. So, a more natural strategy suggests the following; we scan the list and we stop the scan as soon as we find the value. So, whenever we find the value for the first time we report that position and we get out.

We could think of doing this using a 'for loop'. So, we go through every value in l using a variable x. So, x is the name which goes through the first element, second element, these are not positions now these are the actual values. We check whether the value x that we currently pick up in the list is indeed the value v we are looking for; if so we exit and we report the position of this x.

At the end of this loop if you have not found it, pos has not been reset so it remains minus 1. On the other hand if we have found it without looking at all the remaining elements we have set it to the first position we found it and we have taken a break statement to get out of the loop so we do not unnecessarily scan.

Python is one of the few languages which actually provide way to do this. So it allows something that looks a bit odd because of the name; it allows something called else which we saw with if. It allows an else for a loop as well. The idea is that else this part will be executed if there is no break if the loop terminated normally, so do not worry about the fact that else does not mean this in English.

To summarize, it is very often useful to be able to break out of a loop prematurely. We can do this for both for and while we can execute the break statement and get out of a loop before its natural termination. Sometimes it is important to know why we terminate it, we terminate it because the loop ended naturally or because we used the break statement. This can be done by supplying the optional else.