Python Jumpstart by Building 10 Apps Transcripts
Chapter: App 5: Real-time weather client
Lecture: Concept: Slicing collections

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0:00 Let's talk about slicing.
0:02 You can slice all sorts of things, you can slice strings,
0:05 you can slice lists, you can even slice results that come back from databases
0:10 you can have that slicing due some kind of paging on the database side on the server,
0:16 it's super powerful and it's one of these concepts
0:19 I would classify as Pythonic meaning that this is a programming idiom
0:23 or style of programming that is somewhat unique to Python.
0:27 So here I have a list of numbers which I've called nums
0:30 and it happens to be the first 9 prime numbers,
0:33 I can do things that are really obvious like get the first prime
0:36 by saying nums [0],
0:39 remember, accessing elements in lists and all sorts of things in Python is zero based
0:44 so it starts counting at 0, 1, 2, 3, so the first one would be nums of 0,
0:47 and you can see on the comment on the right that would return 2,
0:51 something we haven't talked about which is also fairly unique to Python
0:55 is that you can use the negative indexes,
0:57 so instead of having to somehow do some math,
1:00 taking the length of the numbers into account
1:02 and subtracting 1 from them, to get the last item, we can actually use
1:06 here we can use minus 1 and that will give us the last number
1:10 so if we put minus 1 in there we get 23,
1:13 but those are just regular array indexes,
1:15 however if we give it a range separated by colon then we get slices,
1:21 so if we want the lowest four prime numbers
1:24 we would say as you saw in our HTML example 0:4
1:27 and that gives us the first 4 items,
1:29 but there is the shortcut here where we can omit
1:32 the beginning if we are starting at 0,
1:34 or the end, if we are going all the way to the end,
1:36 so these two statements are equivalent
1:39 we can also get the middle ones so if we want the 4th or the 3 index 0 based item
1:45 and then 3 more, we can say 3:6 and that will give us 7 through 13,
1:51 and we can also go to the end just like our sliced one
1:54 from the beginning so we could if we computed the actual link
1:57 that for some reason we knew it because a 5:9
2:00 now in programs this is of course very dynamic
2:03 you rarely have a fixed link you just hard code in there,
2:07 so this would be something like the length of numbers
2:10 minus 4: the length of numbers this is not cool,
2:13 but it does give us the result we are looking for,
2:15 what is more cool is to just say I would like to go from the 5th item onward,
2:20 that still requires you to know the length of numbers
2:23 if you want just the last 4, so we can use this idea
2:26 of a negative index kind of like I talked about
2:29 for indexing here but for ranges,
2:31 and we can say I don't know how long it is
2:33 but I want you to go to the end and then go back for
2:36 and then go all the way to the end,
2:38 so this - 4: nothing will actually give us the last 4 items without
2:43 us caring what the length is.
2:45 It's worth noting that when you are doing these slices
2:47 this type of stuff here you don't get errors
2:52 if you use numbers that are too big or too small.
2:54 So if I said 5:1000 and there is only 10 items,
2:57 it would just give me the last 5
2:59 but up here where I am using array indexes
3:01 if I say give me the 1000th item and there is only 10- boom, that's a crash.
3:06 So these aren't quite treated the same way in that regard,
3:09 but otherwise you can think of using these ranges as indexes
3:13 into the list and what you get back are little smaller lists.