Python for Entrepreneurs Transcripts
Chapter: Appendix: Python language concepts
Lecture: Concept: Slicing

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0:01 Python has this really interesting concept called slicing.
0:03 It lets us work with things like lists, here in interesting ways.
0:07 It lets us pull out subsets and subsequences if you will,
0:10 but it doesn't just apply to lists,
0:13 this is a more general concept that can be applied in really interesting way,
0:17 for example some of the database access libraries,
0:20 when you do a query what you pulled back,
0:23 you can actually apply this slicing concept for eliminating the results
0:27 as well as paging and things like that. So let's look at slicing.
0:31 We can index into this list of numbers like so,
0:34 we just go to nums list and we say bracket and we give the index,
0:37 and in Python these are zero-based, so the first one is zero,
0:40 the second one is one and so on.
0:42 This is standard across almost every language.
0:44 However, in Python, you can also have reverse indexes
0:48 so if I want the last one, I can say minus one.
0:50 So this is not slicing, this is just accessing the values.
0:53 But we can take this concept and push it a little farther.
0:56 So if I want the first four, I could say 0:4
1:01 and that will say start at the 0th and go up to but not including the one at index 4.
1:06 So we get 2, 3, 5, 7, out of our list.
1:09 Now, when you are doing these slices,
1:12 any time you are starting at the beginning or finishing at the end,
1:15 you can omit that, so here we could achieve the same goal by just saying :4,
1:19 assuming zero for the starting point.
1:21 So, slicing is like array access but it works for ranges instead of for just individual elements.
1:27 Now if we want to get the middle,
1:30 we can of course say we want to go from the fourth item, so index 3,
1:34 remember zero-based, so 3 and then we want to go up to
1:37 but not including the sixth index value,
1:40 we could say 3:6 and that gives us 7, 11 and 13.
1:44 If we want to access items at the end of the list, it's very much like the beginning,
1:50 we could say we want to go from the sixth element so zero-based,
1:54 that would be 5 up to the end, so 5:9 and it would be 13, 17, 19, 23,
1:59 but like I said, when you are either starting at the beginning or ending at the end,
2:03 you can omit that number, which means you don't have to compute it, that's great,
2:06 so we could say 5: and then it'll get the last one.
2:09 But you still need to know where that starts,
2:12 if we actually wanted 4, so there is a little bit of math there,
2:15 if you just want to think of it starting at the end and give me a certain number of items,
2:19 just like where we got the last prime and that came back as 23 when we gave it a minus one,
2:23 we can do something similar for slicing
2:26 and we could say I'd like to go start 4 in from the back,
2:29 so negative 4 and then go to the end.
2:32 So that's the idea of slicing, it's all about working with subsets of our collection here,
2:36 the example I gave you is about a list,
2:39 but like I said we could apply this to a database query,
2:42 we could apply this to many things in Python
2:45 and you can write classes that extend this concept and make it mean whatever you want,
2:49 so you'll find this is a very useful and common thing to do in Python.