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Now, up to this point, we've learned that Esther.

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Is an ordered sequence of characters, right?

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It's essentially a piece of text where we can write our names, passwords, paragraphs and sentences

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and strings underneath the hood are stored in memory, as I said, as an ordered sequence of characters.

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So, for example, if I had the string mi mi mi.

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Then this will be stored in our computers memory in an order so and will be stored in a place in memory,

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and after that place and memory, it will be stored, then the space will be stored and then M, then

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e the space than the need.

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So if you think of a bookshelf, each one of these is ordered in a different part of the bookshelf,

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but one after another.

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And the way we can think about this is like this.

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The M is stored in location zero and then E is stored in location one, and then space is stored in

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location two and so on and so forth.

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And each shelf now corresponds.

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To a value.

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So that when we ask the computer, hey, I want you to grab this string, we know that, hey, we're

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going to look in the shelf zero to seven.

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Now, why is this important?

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Because one of the most useful things you can do with strings is to access different part of a string

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using it's what we call an index.

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So, for example, when you have a string and let's call this variable or this piece of string selfish.

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And selfish has me, me, me, me, me.

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And I'm going to comment this out.

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Just like this, make sure it's aligned.

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OK?

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And what I'm going to do here is I'm going to say selfish and then square brackets right here and then

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type in zero.

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Now, if I print this.

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Check that out, I get the letter M, why is that wall using the square brackets?

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I'm telling the computer, hey, grab the variable selfish.

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And then from that variable selfish grab whatever is an index of zero and what's on that bookshelf of

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index of zero?

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Well, the letter and what if I do seven?

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What do you think will happen if I click run?

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I get it, because on the bookshelf of number seven, that's it, and when I am getting just selfish,

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I get the entire string.

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It's going to say, all right, I'm going to grab from the bookshelf index of zero all the way to seven.

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Very, very cool, and this concept we're going to explore throughout the course and you're going to

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see why it's important very soon, but using this knowledge, we can do a lot of string manipulation.

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And Python has this unique feature that we're going to talk about right now that allows us to grab different

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pieces of text.

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So let's have a look here.

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The rule here is when we use square brackets in Python.

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The first item that we put between the square brackets is what we call the start.

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The start is, hey, where do you want me to look in our case, we said we want to look at zero.

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So we got in, we want to look at seven.

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We got it.

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But then there's an extra thing that we can do if we do a.

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Colin here.

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We also have the option to say, hey, where to stop?

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What does that mean?

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Well, if I do something and you know what, let's change this to.

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Zero, one, two, three, four, five, six, seven, just so it's easier to understand if I do, let's

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say the zero.

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Colin to and I click run, I get start at zero and at two, so I get to grab zero.

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To one, because remember, I'm saying start here, but then stop as soon as you get to bookshelf,

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too.

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We don't want that.

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So if I do zero to seven, what do you think will happen while I click run?

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And I get all the way to six.

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So in order for us to grab the full strength, we have to do zero to eight because there's actually

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a character here, right?

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Because we start from zero, in fact, like Ron.

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Now, there's a third thing that we can add here, and this is called the step over.

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And a step over says, hey, start here and here and then step over a few things.

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So the default is one because we're going one by one through our bookshelf.

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You see that we get the entire string, but if I add to here, it steps over by two, so I grab zero,

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then I step over one to grab to, then step over one to grab for step over and so on and so forth.

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So let me ask you a couple of tricky questions here, what happens if I do this?

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Is this valid, if I click run, what will happen, ready to guess?

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One, two, three, four, five, six, seven.

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This says, hey, start at 1:00.

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So that is right here, but then afterwards, stop, well, there's nothing there.

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So the default is going to say go all the way to the end.

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I know this is confusing, but hang in there, practice this often enough, and this will become second

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nature.

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OK, what if I do something like this where I don't fill the start and do five here, what happens now?

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I get zero one, two, three, four all the way until five, so it starts as default zero and goes to

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five.

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OK, what if I do something like this to semicolons, and then what if I click run?

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All right, I get the default behavior.

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Because it's starting at zero when there's nothing it and whenever the string ends, when there's nothing,

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and then we're stepping over through it once.

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All right, but what if we do something like this?

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What if I do minus one here?

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This is a tricky one.

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What will happen?

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Let's find out.

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I get seven in Python, the negative index means, hey, start at the end of the string so that if I

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do minus two.

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I get six if I do minus three.

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I get five because I'm going backwards and a neat trick that you can do here is if we do semicolon,

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semicolon minus one, and this is actually quite a common operation.

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So you just have to memorize this, even though it looks weird, it means start, stop.

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But there's no limit here.

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But I want to step over from the back.

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So what do you think will happen here?

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If I click, run.

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Do you see that we get the reverse of the string, so this is a very useful notation if you want to,

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let's say, reverse an order.

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And if we want to, let's say skip by two, I click run and you see that we're skipping by two now.

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Now, hopefully your head doesn't hurt with all this notation, you just have to get used to it.

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It's something quite unique to Python, but it's very, very useful.