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Python Jumpstart by Building 10 Apps Transcripts

Chapter: App 5: Real-time weather client

Lecture: Concept: Slicing collections

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Let's talk about slicing.

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You can slice all sorts of things, you can slice strings,

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you can slice lists, you can even slice results that come back from databases

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you can have that slicing due some kind of paging on the database side on the server,

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it's super powerful and it's one of these concepts

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I would classify as Pythonic meaning that this is a programming idiom

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or style of programming that is somewhat unique to Python.

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So here I have a list of numbers which I've called nums

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and it happens to be the first 9 prime numbers,

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I can do things that are really obvious like get the first prime

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by saying nums [0],

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remember, accessing elements in lists and all sorts of things in Python is zero based

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so it starts counting at 0, 1, 2, 3, so the first one would be nums of 0,

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and you can see on the comment on the right that would return 2,

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something we haven't talked about which is also fairly unique to Python

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is that you can use the negative indexes,

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so instead of having to somehow do some math,

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taking the length of the numbers into account

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and subtracting 1 from them, to get the last item, we can actually use

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here we can use minus 1 and that will give us the last number

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so if we put minus 1 in there we get 23,

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but those are just regular array indexes,

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however if we give it a range separated by colon then we get slices,

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so if we want the lowest four prime numbers

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we would say as you saw in our HTML example 0:4

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and that gives us the first 4 items,

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but there is the shortcut here where we can omit

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the beginning if we are starting at 0,

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or the end, if we are going all the way to the end.

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So these two statements are equivalent

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we can also get the middle ones so if we want the 4th or the 3 index 0 based item

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and then 3 more, we can say 3:6 and that will give us 7 through 13,

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and we can also go to the end just like our sliced one

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from the beginning so we could if we computed the actual link

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that for some reason we knew it because a 5:9

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now in programs this is of course very dynamic

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you rarely have a fixed link you just hard code in there,

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so this would be something like the length of numbers

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minus 4: the length of numbers this is not cool,

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but it does give us the result we are looking for.

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What is more cool is to just say I would like to go from the 5th item onward,

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that still requires you to know the length of numbers

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if you want just the last 4, so we can use this idea

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of a negative index kind of like I talked about

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for indexing here but for ranges,

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and we can say I don't know how long it is

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but I want you to go to the end and then go back for

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and then go all the way to the end,

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so this -4: nothing will actually give us the last 4 items without

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us caring what the length is.

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It's worth noting that when you are doing these slices

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this type of stuff here you don't get errors

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if you use numbers that are too big or too small.

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So if I said 5:1000 and there is only 10 items,

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it would just give me the last 5

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but up here where I am using array indexes

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if I say give me the 1000th item and there is only 10- boom, that's a crash.

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So these aren't quite treated the same way in that regard,

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but otherwise you can think of using these ranges as indexes

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into the list and what you get back are little smaller lists.