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Welcome back, everyone, to this lecture, we're going to be discussing lists in Python.

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Now, a data structure in Python that can hold elements in a sequence defined by brackets where each

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element is spaced out by a comma.

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So here, for example, we can see a list and this list has five items in it.

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The first one is a string just item.

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Next one is an integer zero.

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The third one is a floating point number ten point two.

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Fourth one is another string, and then the fifth one is actually a variable.

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We don't actually know what this variable is.

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It's just my VAR, but we can imagine it's could be a string and could be a number or something else.

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And typically, when we're organized things in a list as we get better and better, if Python, we'll

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probably just be putting a variables in there to some degree.

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Now we can use these actual lists to organize data in sequence ordering, meaning we can then grab individual

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elements from that list using the same sort of index bracket notation, as well as index slice notation

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that we learned earlier for strings.

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Let's explore the basics of Python lists.

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All right.

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Here I am inside Visual Studio Code Editor.

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If you want to create a list again, you just use square brackets and then commas to separate the items

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in the list.

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So, for example, I can say something like my list is equal to and then square brackets.

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And you can start putting in items.

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So this is a list containing three integers one two three.

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If you had another variable there or something like, let's say my variable is equal to 10, I could

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maybe add in or replace one of these with another variable like this.

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So let's go ahead and print out my list just so I can see what it looks like.

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So I'll say print my list and we should expect to see one two three 10.

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So I'll save that change.

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Let's go ahead and run Python example.

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And there I see it.

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One two three 10.

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OK.

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Now why would we want to use a list?

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A list is really useful when you have a bunch of different types of variables, like either numbers

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or strings or other variables that you want to make sure are organized in a specific order.

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That way, you can actually later on iterate through that list, or you can grab particular elements

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from that list just as we could with a string.

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So just like a string is a sequence of characters.

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A list is just a sequence of items.

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It also starts to indexing at zero.

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So to make it clear, I'm going to say my list and have it be 100, 200 and 300 as the items in the

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list.

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And let's actually go up to 500.

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There we go.

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So I'll save that change.

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Imagine I want to grab the item 100.

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How do I do that?

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I simply say my list.

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And then using the square bracket notation, just like we did four strings, I can pass on a zero,

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which says, grab these zero index item from that list.

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So I should see this print out the number 100 and there it is 100.

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Because again, all I'm saying is grab that first item at index zero.

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Now, let's imagine I actually want to take a slice.

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How does that work?

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Works the exact same way it did for strings.

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You have the starting index, then you say Kolan up to the end index.

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Imagine I wanted to grab 100, 200 and 300.

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How would that work?

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Well, I'm starting index zero and then I go up to, but not including index three.

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So it would be 100 Index zero index one index two.

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And then I wanted to go up to, but not including index three.

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So in order to get 100, 200, 300 and you say go up to index three, save that change.

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Run this and I seen 100, 200 300.

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Is a little more practice.

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Imagine I wanted to grab 200 and 300.

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How would that work?

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Well, to grab 200, I need to start an index one.

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And then to grab up to 300, I need to go up to, but not including index.

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This is index for or, excuse me, three zero one two three.

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Yup.

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The real say that and this should bring back 200 300.

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And there it is.

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If you ever confused on this, I would just encourage you to practice a lot of slicing.

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Just make yourself a really long list.

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Try to get a particular slice and see if you're indexing starts and stops makes sense.

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Now how do we add a new item to a list that already exists?

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For example, let's imagine I wanted to add in my variable into the end of this list.

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So let's go ahead and make it really obvious.

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I'm going to say new as a string, and I want to append it to the list.

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How does that work?

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This is where I get to call a method.

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So I'm going to say my list and I call the append method.

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It's one of the most common methods you call on a list.

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And this allows you to append something at the end of a list, and we'll see how to insert something

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later on anywhere in a list.

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But I can say my list append.

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Go ahead and append that variable, and then I'm going to print out.

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My list.

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And let me put this all kind of line after line here.

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So again, all I'm doing is I'm saying, go ahead and take that variable and append it to the end of

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the list.

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Save that change.

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And when you print out this list, you should now see it as one hundred or three and four to five hundred

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and then the item new has been appended to the list.

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OK, now what if you wanted to insert some item somewhere inside the list that a particular index,

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a pen will always go at the end?

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If you want to insert something at a particular index location, you can use the insert method.

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Open close parentheses and then where do you want to or what do you want to insert and where do you

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want to insert it?

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So it's actually going to take in two arguments.

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You need to know where you want to insert it and what you want to insert.

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So where just means an index location, which means this method actually takes in what is known as arguments.

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You pass in the index location, you want to insert it.

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For example, let's say I want to insert an index location zero, then I'm going to say, Comma, what

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do I want to insert?

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And I want to insert my variable here.

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I remember each time I saved the script, I am essentially kind of overwriting everything I've done

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in the past, so we shouldn't expect it to have new still at the end of that list.

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Remember, I'm essentially redefining that list again from 100 to 500 here.

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So let's save this and see how this works.

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And now I see new at the beginning of that list.

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Does that make sense?

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Hopefully it does.

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What I'm saying is go ahead and take my list insert at index location zero.

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This new variable?

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Let's go and experiment what happens if I insert it to index one?

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Then I run this the way this works is you should expect the final output result to have this variable

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at this index.

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Index one, which is true 100 Index zero now and Mew is an index one.

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So that's how you can add new items to a list append if you just want to add it to the end of the the

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list.

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That's really useful when you keep iterating and keep adding items and don't want to upend the order

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that is already in your list.

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And then you have insert when you maybe want to insert in a very specific position.

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That kind of implies that you know the list really well and know a lot of specifics about the current

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items in the list and where indices should go.

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That's why you actually don't see insert use that often compared to Append, because again, Insert

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implies, Hey, I know the list really well, and I know this very specific variable has to go to this

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very specific index versus append.

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You just say, OK, just keep adding it to the end of that list.

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And now I have kind of almost like a history of what I added to the list as I keep adding, adding more

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and more things.

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Now you may be wondering, how do I actually remove an item from a list?

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The way this works is with the pop method.

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So I'm going to say item removed as a new variable and then I'm going to set it equal to and the way

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you remove an item from a list as you pop it out, you can say my list dot pop.

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And then if you just say dot pop by itself, it's going to remove the very last item in the list.

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So what I'm going to do is, in fact, we don't actually need my variable anymore, since we won't be

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using it.

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I'm going to say my list is equal to these numbers.

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Then item removed is equal to my list dot pop.

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Notice that pop is actually going to return in item and then I'm going to print out my list and then

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I'm also going to print out the item that was removed.

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So I save those changes.

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I run this, and now I see the following.

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So I get 100, 200, 300, 400 and then I see 500.

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So by calling Pop, it removed the last item in this list and then it actually gave it to be assigned

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to this variable.

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Now, do you actually need to assign this to a variable like item removed?

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No, you do not.

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If you really wanted to, you could just say my list that pop.

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And now notice Visual Studio Code is warning us, Hey, by the way, this isn't defined anywhere.

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So that's OK.

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If I going to comment this out to put a comment in Python, it's just this hash tag symbol so you can

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put a hashtag in front of that.

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That's only a comment.

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Go ahead and save those changes.

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And what's going to happen is you will still see the actual pop occur.

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So five hundred is no longer in that list.

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You just decided not to save it to any particular variable name.

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Depending on the situation, you may just be popping things off a list.

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Now, let's imagine you want to actually remove at a specific index like index zero.

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You just pass in the index, you want to remove it.

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So if I say pop zero, that says go to index zero this list and just remove that item.

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So now when I run this, I should see 200 through 500, and there I see it 200 through 500.

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The last thing I want to point out is you do have the ability to reverse the order of a list by calling

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the reverse method, so you can say my list.

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Reverse open, close parentheses.

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And now look what's going to happen when I prints out my list.

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It is now in reverse order 500, 400, 300 200, et cetera.

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There is also the ability to sort a list depending on what items you have in your list.

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It may make sense to sort them now if you have a list of a bunch of different mixed data types like

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numbers and characters and strings and letters.

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It may not make too much sense to sort them, but if you have a bunch of numbers, then it could make

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sense to order them or sort them in ascending or descending order.

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It could also make sense to organize things in alphabetical order.

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That's pretty much the two use cases.

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You see this when you call that sort open close parentheses.

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If you're super advanced, well, maybe not super advanced, but if you're getting more advanced than

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Python, eventually you realize you can actually sort by a particular key, which means you can have

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a custom definition of how you want to sort things.

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We're just going to ignore that for right now.

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And instead, I'm going to put in a bunch of random numbers here.

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So go ahead and just kind of input.

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A bunch of random numbers doesn't matter.

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They don't need to be in order.

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And then what we're going to do here is call my list that sort.

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And then print my list and let's see the effects.

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And there you have it now, it's sorted in ascending order, starting from the east all the way to the

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greatest.

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Now the very last thing I want to point out before and this lecture, that's sometimes a common question

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is the default way of sorting is ascending.

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But what if I want things in descending order?

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How can I actually do that?

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Well, there are many different ways of doing that, but one way to combine two ideas that we already

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thought about is to actually call one method right after the other.

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I could first sort my list and then I could come here and call my list and then call reverse on it.

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So then I'm sorting in ascending order and then simply putting it in reverse.

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So I'm going from random number order, sorting it out and then reversing it, and let's go ahead and

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see the results there.

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And there you have it.

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Fifty nine and then onwards down to one.

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OK.

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That's it for the basics of lists.

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I'll see you at the next lecture.