Original subtitles

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Hi again, in the previous lecture we

talked about strings and in this lecture

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again we are going deeper into learning

strings and specifically we're going to add

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indexing. Indexing is a very important

concept not only with strings but

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with all the data type that we'll be

looking at later such as lists,

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tuples and dictionaries, so please follow

this lecture. Yeah, let's talk about

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indexing. Let me open the Python

interactive session. Let me create a

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string, "Hi there!"

exclamation mark and close quote.

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Now when you create a string, every string

that you create Python under the hood

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what it does is it assigns a number to

each or the items of your string so it

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starts from 0 so H will be assigned an

index of 0 and then a 1 for I, 2 for the

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space, 3 for T, five, six

seven, eight.

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So the exclamation mark character will

will have an index of eight, so you would

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ask why is this useful? Well this allows

you to extract certain parts from a

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string and there is a certain notation to

do that you'd want to access the string.

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So in this case that's variable C

holding the string, then you want to use

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square brackets so an opening square

bracket and then a closing square

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bracket and inside those square brackets you

pass the index of the item that you want

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to extract. Let's say I want the T

character so that would be 0 1 2 3.

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I pass number 3 there as an index and that

will extract for me the letter T which is

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actually a string.

So you can check the type of the

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output and you'll see that that's a

string, so a string is made of strings.

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You can say so. If you wanted H you'd

past zero and if you wanted a lost

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character well you might have to count

so one two three four five six seven

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eight.

And so, and sorry including type there.

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Sorry about that!

so c8 would give you the exclamation

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mark, but there's also another indexing

system there that Python uses so in this

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case we had to count from the beginning

to the end of the string but wouldn't it

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be useful if you had another system that

lets you count from the end, so from

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right to left?

Well, such a system exists and it

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starts from minus 1, so the exclamation

mark would be minus 1 now that you can

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extract that with that. Minus 2 would be

E, R would be minus 3 and so on up to H, so we

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have two indexing systems there which

allow you to extract items depending on

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whether the items are in the beginning

or near the end. Now what if you want to

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extract more than one item from a string?

Let's say you want to extract Hi.

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Well there is a notation for that too, so

Hi is expanding from index 0 to index 1.

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So up to here. So let's try 0 to 1.

That will not work a very good, but let's see.

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So as I said this will return only H,

so it doesn't return item with index 1.

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That's because splitting in Python, so we

we're splitting a string here

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splitting in Python is upper bound

exclusive, which means that the upper

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bounds of the split here is not included

in the output and so if you want to

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include I you want to pass the index

after I which is two so this is the string

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just for reference, so we extracted item

with index 0 and item with index 1.

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And then the split starts right there at

index 2 so it doesn't include the space.

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Just hi. If you pass 3 there, that would

include the wide space as well and

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similarly you can pass like one.

That will include the I and the space, there

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are also shortcuts, so if you pass zero

like nothing to 3 but that is the same

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as doing 0 to 3 so it will include 0 1 &

2 and similarly if you want to extract

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there with the exclamation mark

well what you'd want to do is T would

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have an index of 0 1 2 3 so 3 and then

you can just pass a column there and

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execute it and that will give you There

with the exclamation mark. Let's now try

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to extract R and E using negative

indexing, so something to know is that

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even though we are using a negative

indexing which starts from right to left

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the splitting will work from left to

right which means the first index you

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want to pass is that of the first item.

So R we pass minus 3 for R because

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you know we have minus 1, minus 2 and minus 3.

So minus 3 for R and then we want E

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which is minus 2,

but since splitting is upper bounce

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exclusive we want to pass minus one there.

So let's try that, minus one, execute and yeah

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we get the correct output.

That's the idea. If you didn't pass

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anything here you'd get the exclamation

mark as well. So it will take a while

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until we get used to this, but with some

practice you'll be able to remember

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these notations. Yeah, that's what I wanted

to cover in this lecture, so this was

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about string indexing and splitting,

so you split using the index that Python

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uses on the background. Yeah, I hope

you enjoyed this and I'll talk to you

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later, thanks!