2.9 Lists (Deep-Dive) - IP

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Time
2 hours 57 minutes
Difficulty
Beginner
CEU/CPE
3
Video Transcription
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>> Hello everyone and welcome back to Cybrary On Demand.
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I am your instructor Joe Perry and
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you are watching Intro to Python.
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This is Lesson 8,
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where we're going to be doing a deep dive on lists.
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In this lesson we're going to learn two things.
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The first is we're going to learn some list methods
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as we talked about string methods,
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going to do the same thing with lists and then
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we're going to start applying slicing to list.
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We're just going to go over
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the same things we did with strings,
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but this time we're going to apply them to lists.
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To do that we're going to generate our first list,
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which we're going to call l1 and that's going
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to consist of all the numbers between
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one and 10. l1,
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as you can see exists,
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contains the numbers we want it to exist.
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Let's look at the possible functions
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we can call against that list.
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We have append, clear, copy, count, extend,
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index, insert, pop, remove, reverse and sort.
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Those are all of the built-in methods for lists
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we're going to discuss most of them pretty quickly.
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First let's look at l1.append,
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and we can do that by examining it with this first help.
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We can see here that append is a method of
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the built-in list instance we
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do append and then we give it some object argument,
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and it appends the object to the end of the list.
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Fair enough, let's see what that looks like practically.
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We'll do l1.append the number 11.
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Mixing up my commands here, there we go.
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L1, again, and we can see here that the number
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11 has been added to our list.
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Fantastic. Now we know how to add things to the list.
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What other functions did we have?
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We have clear l1.clear.
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Let's see what that does.
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Well, pretty straightforward it clears the list out.
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We're not going to quite go item by item through this,
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but I am going to just demonstrate
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a few of these methods for your real fast.
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There's that. We talked about append,
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the other one that we really want to talk
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about right away is pop.
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Pop is a function that will take
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the last item off of the list and will
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print it to your screen or will
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return it to
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your variable depending on what you're doing.
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If I were to do x equals l1.pop and then print
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x and print l1
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you'll see that the list no longer
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contains the number 10.
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It is now stored in the variable at x.
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It's a really easy way when you use
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things like pop and append,
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what you're actually doing is
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creating what we call a stack.
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The idea of a last-in, first-out structure.
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When you're implementing stacks in Python,
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you actually will generally be using
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the append and pop functions.
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Very, very useful tools.
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Looking back at our list of options,
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we have two more that really want to examine.
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First is reverse.
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Reverse is going to do something pretty
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similar to what we've seen before,
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but what's cool about it is that it's not actually
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going to return anything
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and that's why I wanted to address this is.
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What's actually happening with this list is
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happening inside the list
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without ever actually returning any code.
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You're not creating a new list,
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you are reversing the existing list in place.
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Then what's useful about
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that is that it lets us
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demonstrate the next function that
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I want to talk about real fast, which is sort.
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The last list function.
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Sorting lists is a challenge
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in basically every programming language.
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It's something that you'll see in a lot of Java reviews,
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is a task you'll have pretty often.
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In Python you can very easily do it just by
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running that sort command and real
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fast let's look at sort under health as well.
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You see it has two arguments,
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key and reverse, stable sort in place.
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Basically what it's saying is if you have the key,
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you have some mechanism by which you want to sort it,
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you can give that as an argument,
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you can also reverse sort it.
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But we've sorted our list here and we
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can see that l1 is back to being 1,
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2, 3, 4, 5, 6, 7, 8, 9.
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There's some other list methods and
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in our lab you'll get to play with those a lot.
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I definitely recommend you check them out,
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but for now that we understand
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what the methods are and how to apply them and
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how to find them and find information about them
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we're going to move on to the second half of this lesson,
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which is applying slicing to lists.
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Now if you remember, we can have, for example,
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our string and we can do
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slicing in that of
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reversing the list or reversing the string.
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With negative 1,
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we can do x is two through five into.
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We can do x
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is the entire list but only every second letter.
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All that slicing works exactly the same way with
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lists and I actually
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accidentally said list a couple of times in there,
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I think because this methodology is interchangeable.
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We can do, for example, we have our l1,
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we could do l1 and we only want the fifth item.
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This by the way is why I really should stop using L,
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because I keep mixing up L and one,
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but I'm going to keep doing it because,
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well, there's no one here to tell me I can't.
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But here we see the index of five is six,
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makes sense the fifth number or
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the sixth number rather is 6,
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0, 1, 2, 3, 4, 5.
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We can also do l1 reversed.
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Now, the thing that's interesting
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here and the reason why I wanted to address it
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and why I brought reverse up earlier is
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that l1 by itself actually is still the same.
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So l1 with the negative 1 index step
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and l1.reverse are not actually the same thing.
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Reverse is sorting that or reversing that list in place,
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it changes the list itself whereas just using
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the indexes is something that you can do
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to reverse it without ever actually modifying the list.
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Additionally, if you wanted to copy l1,
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you wouldn't actually do something like l2 equals
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l1 because then you're using the same list.
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What you would do and to
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demonstrate that we've got l2 and l1.
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You can see that by modifying l2,
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we modified l1 because it's the same list,
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so that's one of the dangers of using lists.
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The way you would copy a list and first resort l1.
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You would copy list is actually by
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using slicing like we just talked about.
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You would just do all of
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l1 with no step and actually you can get rid of
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the second colon there because you're not
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using a step so all of
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l1 and then you would just assign that to l2.
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Now we can do l2.reverse
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and we see that l1 stays the same.
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When you're trying to copy a list,
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you want to use string
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slicing or list slicing rather to do that.
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String slicing and list slicing
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the reason why I keep using them interchangeably,
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it is they function in essentially the exact same way.
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Everything that we've done with strings
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you'll also get to do with lists.
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In the lab while you're working on those exercises,
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which I highly recommend you do.
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That's going to be all there is for this lesson.
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Again, in this lesson we learned how to apply and
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use some list methods and then we took
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string slicing and we turned it into
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list slicing. I hope you enjoyed it.
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I hope you found it informative as always.
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I have been your instructor Joe Perry,
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and thank you for watching intro to
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Python on Cybrary On Demand.
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