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Move from Excel to Python with Pandas Transcripts

Chapter: Appendix: Python language concepts

Lecture: Concept: Slicing

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Python has this really interesting concept called slicing.

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It lets us work with things like lists, here in interesting ways.

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It lets us pull out subsets and subsequences if you will,

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but it doesn't just apply to lists,

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this is a more general concept that can be applied in really interesting way,

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for example some of the database access libraries,

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when you do a query what you pulled back,

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you can actually apply this slicing concept for eliminating the results

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as well as paging and things like that. So let's look at slicing.

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We can index into this list of numbers like so,

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we just go to nums list and we say bracket and we give the index,

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and in Python these are zero-based, so the first one is zero,

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the second one is one and so on.

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This is standard across almost every language.

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However, in Python, you can also have reverse indexes

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so if I want the last one, I can say minus one.

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So this is not slicing, this is just accessing the values.

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But we can take this concept and push it a little farther.

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So if I want the first four, I could say 0:4

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and that will say start at the 0th and go up to but not including the one at index 4.

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So we get 2, 3, 5, 7, out of our list.

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Now, when you are doing these slices,

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any time you are starting at the beginning or finishing at the end,

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you can omit that, so here we could achieve the same goal by just saying :4,

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assuming zero for the starting point.

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So, slicing is like array access but it works for ranges instead of for just individual elements.

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Now if we want to get the middle,

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we can of course say we want to go from the fourth item, so index 3,

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remember zero-based, so 3 and then we want to go up to

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but not including the sixth index value,

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we could say 3:6 and that gives us 7, 11 and 13.

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If we want to access items at the end of the list, it's very much like the beginning,

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we could say we want to go from the sixth element so zero-based,

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that would be 5 up to the end, so 5:9 and it would be 13, 17, 19, 23,

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but like I said, when you are either starting at the beginning or ending at the end,

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you can omit that number, which means you don't have to compute it, that's great,

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so we could say 5: and then it'll get the last one.

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But you still need to know where that starts,

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if we actually wanted 4, so there is a little bit of math there,

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if you just want to think of it starting at the end and give me a certain number of items,

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just like where we got the last prime and that came back as 23 when we gave it a minus one,

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we can do something similar for slicing

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and we could say I'd like to go start 4 in from the back,

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so negative 4 and then go to the end.

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So that's the idea of slicing, it's all about working with subsets of our collection here,

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the example I gave you is about a list,

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but like I said we could apply this to a database query,

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we could apply this to many things in Python

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and you can write classes that extend this concept and make it mean whatever you want,

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so you'll find this is a very useful and common thing to do in Python.