WEBVTT

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So now's actually a good time in
the course to actually discuss a very

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important feature of Python.

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And that's actually iterators and
iterable objects.

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Now an iterator is an object that
represents a stream of a data.

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So it actually returns the stream,

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or the actual data in the stream,
one element at a time.

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Now, any object that supports iteration is
called iterable, and we've actually seen

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two iterable objects already so far in
the course, and that's strings and lists.

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So, as we've seen I'm
gonna type in some code.

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We can actually use a for loop to iterate
over each of the characters in a string.

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So let's actually type that out.

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We're gonna use string 1234567890, and

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you see this a few times now in
the course, something similar.

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For char in the string.

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Print char.

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And if we actually run that, we actually
get the results we can see on the screen.

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But what's actually happening behind the
scenes is that there's an iterator that's

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created from the string.

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And the for loop actually uses
that iterator to return each item

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of the string, or
each element in this case.

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1,2, 3, 4, 5, 6, 7, 8, 9, and 0.

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And when there are no more items
the iterator returns an error and the for

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loop actually terminates.

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So, note in this case that the for
loop handles the error, and that's why you

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haven't actually seen an error when
you're actually running the code.

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What we'll do is we'll actually
create an iterator from the string

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rather than letting the for
loop create one implicitly,

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and that's gonna be easier to see
what's going on when we do that.

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So use the iter function to do that, so

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what I'm gonna do is just
comment out the for loop.

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And what we're going to do is put my and

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the school interrator, equals hitter,
and that's the function.

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And then we pass the name, in this case,
the string variable that we want, and

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if we can actually do a print, my
interrator, so if we actually do that and

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we actually run that.

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We actually get a string iterator
object showing on the screen.

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Now this is actually just showing
the object representation,

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supposedly a memory
allocation of where that is,

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but to actually see it in use,
we can do something like this.

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Print next my iterator and then print next

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my iterator the rest you do that and
run it.

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We get the values one and
two as you can see on the screen there so

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it's actually going through one element
at a time and that's what the next did,

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it actually returned the first element.

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Next again and then return the second
element, and so on and so forth.

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So basically each time that we use next,
in my iterator,

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we're passing the iterator
that we created in line 5.

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That's what it's doing,
it's actually going through the list.

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And obviously again getting back to
the for loop, that was doing that for

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us essentially behind the scenes.

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So what we're doing its actually created
the code that more less is happening

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behind the scenes.

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So if we wanted to go right through to the
end we could actually create some more.

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So we've got ten entries, so
if we have another 8, 1, 2, 3, 4, 5, 6,

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7, 8, so that should actually be ten now,
if we actually run that we actually get

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all the elements, 1 through to 0
essentially, so you can see there.

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So that's the exact number of
elements that are in the string.

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But if we actually now,
add actually one more.

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So in other words, we know now that
we're already at the end of the string,

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let's actually see what happens
when we try to go beyond that.

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So we actually run that.

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And you can see, we actually get an error.

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And if we just expand it out so
we can see it.

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You can it manage to go through and count
through to zero successfully and than it

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comes off and basically say, a trace
back call last and stop it's duration.

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And that's basically line 18.

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Which has, as expected, crashed because
there weren't any more elements.

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So, once we've covered error
handling in the future,

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if you have nothing better to do, then you
could actually write all your four loops

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by explicitly requesting an iterator
using the enter function.

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And calling next until you get an error,
and this is,

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again, exactly what the four loop does for
us, it actually does it automatically.

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It keeps using next until it
sees the stop iteration i error

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which is this area here on the screen.

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When it sees that it actually
terminates for us automatically.

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So before it implicitly creates
an iterator from the iterable object

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that we want to iterate over.

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Now of course we don't
have to do all that.

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But it's useful to actually understand how
iterators work, and it'll also become even

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more useful when we come to creating our
own iterable classes later in the course.

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Just to help consort like that,
let's just close down the run we're doing.

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And comment out this code.

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Now paste that code in again.

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I just want you to consider
these two loops here.

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Now let me just get the right
indent levels going.

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Again, very important to get
your indent levels correct.

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So consider these two.

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So the first one's just a standard for
loop.

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The second one, for_char in iter String.

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So I consider both of these scenarios,

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they actually both do
exactly the same thing.

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But this second example here, is what's
actually happening in the first case.

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The for up here,

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it's implicitly creating an iterator
from the iteratable string object.

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So basically if you're just putting for
char in string, then essentially what's

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happening behind the scenes is Python is
actually adding this iter to that string.

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So it was actually creating
that iterator for us.

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So, it can be very useful
again to know that and

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you'll be finding out more about
iterators as we move through the course.

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So, let's just come up
now with a mini challenge

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to help you understand
iterators a bit more.

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I'm gonna top that out now.

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So here is the mini challenge
we've got for the writers.

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So create a list of items and
you can use either strings or

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numbers in the list, it doesn't
really matter, and then from that

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create an iterater using the iter
function that we actually went through.

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Now, next use a for loop to loop n times.

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Now, n is the number of
items in your list and

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each time around the loop is next on
your list to print the next item.

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And just as a hint there,
use the len function,

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we've talked about previously, rather than
counting the number of items in the list.

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So, go away and see if you can figure
that out, and type up some code and

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get that working, and
when you're ready to come back and

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compare your code to mine,
I'll be here waiting.

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So pause the video now, and
I'll see you when you get back.

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So how did you get on?

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Did you come up with a good solution for
that?

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So let's go through with
my implementation, and

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bearing in mind this is
just a implementation.

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There's lot's of different
ways to do these things.

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Let's come up and start by typing
the lists or creating the lists.

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Which will be my_list =, and
I'm gonna use just the days of the week.

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So Monday, Tuesday,

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Wednesday Thursday,

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Friday, Saturday,

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Sunday, like so.

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Next I'm gonna create the iterator.

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That's my iterator equals iter my_list

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the iter function creates that iterator,
as we talked about.

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And then we're gonna go
through using the four loop.

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So I'm gonna do four i in range zero,

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I'm using lan to get the number
of elements from the list.

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Mine is called list.

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Remembering that the lead in this case
on a list actually returns the number of

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elements that are actually
existing in the list itself.

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So, that's the for loop.

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And then we're gonna use next underscore
item equals equals next my iterator so

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we're iterating through one element at
a time, so that's giving us the next

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element, or the next item,
in this case a string from the list.

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And we're going to print
that out print next item.

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So that's the solution, so

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if we actually run that to make sure that
it's working we can see we get Monday,

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Tuesday, Wednesday, Thursday,
Friday, Saturday, Sunday.

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And we can see that it's
actually working fine.

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So the idea of this challenge was
to help you understand iteratives.

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Now of course you wouldn't want
to access the items in the list

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in that way normally.

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You'd probably want to do something a lot
simpler like you've learned earlier

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that in the course, like four i in
minor's called a list Print(i), and

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obviously that would
return the same results.

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Oops, I forgot a call in there.

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And then again,
that obviously returns the same result.

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But the point again here was to teach you
what iterators are and how to use them.

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And as mentioned, later in the course
we'll actually be creating our own classes

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that use iterable objects.

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And you'll be understanding a little
bit more about iterators then.

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So I hope you enjoyed that.

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And then, next video, we're gonna actually
start looking in more detail at ranges,

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which I've used a few times
already in the course.

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So, I'll see you in that next video.