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For loops in Python are very useful for iterating over container types, and yet

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they are significantly different from for loops in other languages.

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In particular, for loops in other languages tend to have some sort of an index

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variable associated with them, and this can some times be useful. And we don't

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have that in the standard for loop in Python.

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So, let's take a look at how we can get that very easily using the

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enumerate function.

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Go ahead and make a working copy of for.py. I'll call it for-working.py and open

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that working file, and this is our little readlines example. And if we run this,

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You'll notice that we get the lines of text from the lines.txt file, which looks like this.

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They have that extra new line in them. We'll go ahead and get rid of that with end=" ".

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I am going to add something to this here. I am going to add the enumerate

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function, enumerate, and it goes like that.

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Enumerate actually returns two values. The first value is an index, and the

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second the value is the value from the iterator.

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I am going to print the index before the line, and we'll save this and run it, and

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you'll see what we get.

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So, the index is this extra number over here, and you'll notice that it starts at

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0, and that's actually a useful place for it because that's how you index

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containers in Python.

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So, let's take a look at an entirely different example here, and we'll take a

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string, and we'll iterate over that string.

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We'll use the enumerate function, and we will print the index and the character,

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just so you can get an idea of that looks like, save it and run it.

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So, here we have - I am going to scroll back up - each position in the string, with

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its index number and its character.

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Now, let's say that I am just interested in the letter S. So, I want to know

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which of these characters are the letter S. I can say if c='s' like that:

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print ('index'), number is an S, and I'll do a format I, like this.

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That will place the value of the index into this placeholder here.

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So, I'll save that, and run it, and now we know index 3 is an s, index 6 is an s

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and index 10 is an s. So, counting from 0, 1, 2, 3 there is an S, 4, 5, 6 is

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an S, 7, 8, 9 and 10.

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So 3, 6 and 10 are the Ss. So, sometime it's necessary to be able to find at what

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index, in an array type, a particular value is.

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This is a very useful pattern for doing that, and there is actually a lot of uses

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for having this index, and enumerate is how you can get that.

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So, this is how you get indexes and an iterator with for loop.

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Use this pattern here for, index and value and enumerate, which returns those two

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values, and the argument to enumerate is the iterator.

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Then you have the index and the value available inside your for loop.