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-: Now up to this point, we've learned

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that str,

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is an ordered sequence of characters, right?

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It's essentially a piece of text

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where we can write our names, passwords,

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paragraphs and sentences.

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And strings underneath the hood are stored in memory,

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as I said, as an ordered sequence of characters.

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So for example,

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if I had,

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the string, me, me, me,

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then this will be stored in our computer's memory,

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in an order.

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So M will be stored in a place in memory,

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and after that place in memory,

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E will be stored,

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then the space will be stored,

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and then M, then E, then space, then M, then E.

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So if you think of a bookshelf,

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each one of these is ordered

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in a different part of the bookshelf, 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,

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and then E is stored in location one,

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and then space is stored in location two,

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and so, on and so forth.

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And each shelf, now corresponds

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to,

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a value.

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So that, when we ask the computer,

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"Hey, I want you to grab this string",

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We know that, hey,

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we're 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

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you can do with strings,

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is to access different part of a string

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using its, what we call, an index.

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So for example,

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when you have a string, and let's call this variable,

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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,

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Make sure it's aligned, okay?

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And what I'm going to do here, is I'm going to say

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selfish, and then square brackets right here,

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and then type in zero.

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Now, if I print this,

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and I click run,

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check that out.

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I get the letter M.

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Why is that?

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Well, using the square brackets,

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I'm telling the computer,

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"Hey, grab the variable selfish."

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And then from that variable selfish,

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"grab whatever is an index of zero."

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And what's on that bookshelf of index of zero?

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Well, the letter M.

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What if I do seven?

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What do you think will happen?

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-: If I click run, I get E.

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Because on the bookshelf of number seven, that's E.

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And when I'm getting just selfish,

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I get the entire string.

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It's going to say,

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"all right, I'm gonna grab from the bookshelf,

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index of zero all the way to seven."

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Very, very cool.

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In this concept, we're gonna explore throughout the course,

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and you're going to see

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why it's important very soon.

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But using this knowledge we can do a lot

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of string manipulation.

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And Python has this unique feature,

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that we're gonna talk about right now,

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that allows us to grab different 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,

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is what we call the start.

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The start is, "Hey, where do you want me to look?"

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In our case, we said we wanna look at zero.

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So we got M.

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We wanted to look at seven, we got E.

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

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If we do a colon here,

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we also have the option,

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to say, "Hey, where to stop?"

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What does that mean?

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Well, if I do something,

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and you know what let's change this to

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zero, one, two, three, four, five,

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six, seven,

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just so it's easier to understand.

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If I do, let's say zero,

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colon,

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two,

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and I click run,

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I get start at zero end at two.

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So I get to grab zero,

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to one.

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Because remember, I'm saying start here,

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but then stop as soon as you get to bookshelf two.

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We don't want that.

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So if I do zero to seven,

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what do you think will happen?

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Well, 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 string,

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we have to do zero to eight.

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Because there's actually eight characters here, right?

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Because we start from zero.

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If I click run, there you go.

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Now, there's a third thing that we can add here.

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And this, is called the step over.

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And the step over says,

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"Hey, start here, end here,

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and then step over a few things."

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So the default, is one,

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because we're going one by one,

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through our bookshelf.

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You see that we get the entire string.

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But if I add two here,

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it steps over by two.

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So I grab zero, then I step over,

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one, two, grab two,

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then step over, one, two, grab four,

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step over, and so on and so forth.

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So, let me ask you a couple of tricky questions here.

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What happens if I do this?

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Is this valid?

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If I click run, what will happen?

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Ready to guess?

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I get one, two,

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three, four, five, six, seven.

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This says, "Hey, start at one."

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So that is right here,

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but then afterwards,

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stop, well,

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there's nothing there.

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So the default is gonna say, go all the way to the end.

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I know this is confusing, but hang in there.

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Practice this often enough,

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and this will become second nature.

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Okay, what if I do something like this?

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Where I don't fill the start,

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and do five here.

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What happens now?

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Ready?

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I get zero, one, two, three, four.

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All the way until five.

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So, it starts as default zero, and goes to five.

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Okay, what if I do something, like this?

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Two semicolons, and then one.

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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.

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It ends at whenever the string ends,

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when there's nothing, and then we're stepping over

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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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Hmm, this is a tricky one.

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What will happen, let's find out.

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I get seven.

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In Python, the negative index,

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means, "hey, start at the end of the string."

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So that if I do minus two,

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I get six, If I do minus three,

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I get five.

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'Cause I'm going backwards.

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And a neat trick that you can do here,

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is if we do semicolon, semicolon,

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minus one,

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and this is actually quite a common operation,

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so you just have to memorize this even though

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it looks weird,

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it means start, stop,

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there's no limit here,

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but I wanna 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?

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We get the reverse, of the string.

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So this is a very useful notation,

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if you wanna, let's say, reverse an order.

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And if we wanna, let's say, skip by two,

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I click run, and you see that we're skipping, by two now.

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Now, hopefully your head doesn't hurt

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with all this notation.

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You just have to get used to it.

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It's something quite unique to Python,

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but it's very, very useful.