Python lists Understanding the basics of lists in Python
- In this video, I want to introduce you to the idea of a list,
- of a list in python.
- It is one of the most powerful data structures in Python,
- and it really is just a sequence of a bunch of other stuff.
- So a list in Python would look something like this:
- So this would be a list of integers,
- Maybe 1 , -7, 0, 0, 5 and 10
- And we can have some variable refer to it,
- so that we can refer back to it later on in the program,
- so maybe we say a is equal to all of this.
- And if we run this in the interpreter,
- or if we run this in a program,
- now we can refer to the elements of a,
- and you do that by, so let's say I have a
- (let me write in the same pink color)
- You do that by referring to the index of the element inside of the list.
- So, if I say a and then in brackets, I put a zero here.
- This says the 0th element in a,
- 0th elment...
- and the general convention in Python,
- and this is true of most programming languages,
- is what some people would consider the first item in an array, or inside of a list
- you would call it the 0th item, or the index of this item right here, is 0.
- So, if you were to type this in an interpreter,
- or if you were to type it in a program,
- it would evaluate as this item right over here.
- It would be this item, right over there.
- And let me do another example:
- If I were say a of..
- I shouldn't say a of...
- If I were to say the second element...
- in the second element in a
- this would be 0th element, first element, second element.. It would be
- this zero, right over there.
- So, that's how you refer to elements in a list.
- you can maybe already imagine doing interesting things with that.
- But, what is really cool with lists ...
- is that they don't have to all be the same data type.
- They could be all integers,
- or all floats...
- or all strings...
- but you could mix it up.
- So you could... you could...
- Actually, You could take a right over here,
- and you could redefine one of the elements
- so you could say...
- you could say a.....
- you could say let's reset......
- (do it in that same pink color)
- Let's reset the.......the...
- (I don't know...)
- let's say the fourth element..
- Let's reset the fourth element in a
- to be equal to...
- to be equal to.."this text"
- So literally "this text".
- Literally "this text"
- and so If you did that..
- if you set the fourth element be "this text"
- and then you and your program say print..
- print a..print a
- then a... would look like this.
- The 0th element wouldn't change.
- Still be 1
- The first element will stll be -7
- The...the...so this is the 0 first..this...this...this..
- This two index element would still be 0
- This will be the third.
- So this is 0th index.
- First, second, third, fourth.
- to replace the fourth element.
- So the fourth element will now be "this text"
- It will now be...this....will now be "this text"
- ...the second...we wanna try out with Python intrepreter
- and the last element will still be.....
- The last element will still be 10.
- and this will completely valid to do in Python.
- You can mix data type.
- In fact, you could even...if you..after..after you did this,
- you could even say..that..a
- Let's redefine the 0th element of a..
- to be another list.
- To be the list -1, -2.
- and if you did this, then this 0th element over here,
- will in the list, the 0th element will now refer to this thing.
- So the list would now look like this.
- So the first element or the 0th element I should say will be this thing.
- Negative 1, negative 2.
- And then you'd have you're negative seven, zero, zero, this text
- and then you'll have your 10, just like that.
- So you define it this way and literally replaces or the 0th element will now refer to this data structure
- over here and you could maybe already imagine useful ways to use this in programs
- and if you can't don't worry about it. We'll see useful ways.
- Now there is one thing I want to make very clear here
- is that, when I... Let's say that I have... Let me define some new...
- Let's say that I have some list that I have the variable b referred to
- and let's say I make this list: 7, 13, 15.
- So that's my list.
- And let's say that I... the next line in my program or maybe using the python interpetrator.
- I say that c is equal to b.
- And sometimes you might be tempted to do this is, hey I want to have a copy of b.
- And what I wanna make clear is that this point, c and b are actually pointing at the same thing.
- They are referring to the same thing, so if you think about what's happening.
- There are some entity in memory that looks like this: 7, 13, 15.
- And when you execute this, when you execute this first line over here that says
- that look, b is going to definetly refer to that.
- If the variable b refers to this entity and when you say c is equal to b,
- this is actually saying that c is going to refer to the same thing that b is referring to.
- So c is also going to refer to this exact same thing.
- C is not a copy of it, we didn't make a copy of it and make c refer to that.
- And what's relevant here is if we now change c we are also going to change or if we change
- what c is refering to we'll also change what b is referring to.
- And I'll show you that in a second with the interpetator, but let me show you what I'm talking about.
- So this point, I say that c...If I... the next line I say that c, the 0th element in c needs to be equal
- to 1.
- And if I were to tell you then print c you would get.
- You would get 1, 13 and 15.
- But if you were to say print b you would also get 1, 13 and 15.
- And that's because this call right over here or this statement right over here
- it changes what the 0th element in this list refers to.
- So it won't refer to 7 anymore it will refer to 1.
- So now if you say b of 0, this will be equal to 1 and not 7.
- So you might say:" Well Sal okay, I understand that, so if set a variable equal to another variable
- that refers to a list they are actually refering to the same list and if I change one of the items in
- the list that this is referring to is actually changing what this is referring to.
- So how could I copy lists?"
- How could I actually copy it.
- And there you use the notion that can actually and I'll write right over here if you actually want to
- copy things you would do this.
- You would write c is equal to b and this might look a little funny for you
- but you put this colon sign over here and what this does is it tells you copy everything in b
- from the beginning of b all the way to the end of the b.
- So you could write c is equal to b and then you could say 0 all the way up to but not including
- the second element.
- So this right over here, this would create a new list that is the 0th and only the first element.
- It won't include the second element.
- So in this case, what you do... So this one right over here would create a new list that looks like this.
- That creates... That looks like... If we assume we haven't changed b and if we assume this was
- our definition of b, then it will create a new list that looks like this and c will now refer to this
- new list.
- So c will now refer to this new list.
- So it created a copy of the first two elements, 0th element and first, not the seconth element.
- You would have to put 3 here if you wanted the second element,
- so up to but not including the second element.
- If you wanted all of the elements to copy you would do this.
- So this is simply saying from the beginning all the way to the end.
- So it would create... It would create a separate list...
- It would create a separate list that looks just like b, but it is a separate one.
- And then c could refer to that and now if you change c you won't change b.
- So enough of just talking about it, let's actually play with it on our interpetrator.
- Cuz this is the fun stuff right over here.
- So let's say that we have... Let's say that... Let me get rid some of this text that I have here
- So you don't get confused.
- So let's say I define some list.
- A is equal to 1, 2 -- I'll write this way -- 1, 2, negative 7, 9, 11.
- So I've defined and could even verify that.
- Print a, you can see it right there.
- We can change one of the elements in a.
- We can say... Let's change the first element and remember, when we say we have an index 1
- we are not talking about this one here this is the 0th element.
- So we are changing this one right over here.
- So let's change that.
- Let's just for fun... Let's change that to a string.
- So "Sal's String*.
- And now let's see what a looks like. Print a.
- So notice, it took this, this is the index one, this is the first, 0th element, 1st element.
- It changed the first element to "Sal's String" and we see it right there.
- Now let's do something interesting.
- Let's set b equal to a.
- And let's print... Let's print b. So it looks like a.
- And let me do something else.
- Let me... Let c equal to a and then do what I talked about, so let me make c equal to a copy of a.
- So let's print c. Print c.
- So b and c looked the same, but they are two different things.
- C is a copy of a or c is a copy of the thing that a is referring to.
- B is the thing that a is referring to and to see that, let's change the first element
- of both b and c.
- so if you change the first element of b.
- So b of... the 0th element in b. Let's just make that, I don't know. Let's make that equal to...
- Let's make it equal to 0.
- Now if we print b you see the first element is 0.
- And what you'll also see is that if you print a, the first element will be 0 now.
- Cuz they were referring to the same thing.
- So it said, whatever b is referring to make it, the very 0th element, make that equal to 0.
- Well a was referring to the same thing so it also became 0.
- But c is referring to a copy so c should not have changed.
- C did not change. The 0th element here did not change a bit.
- So I'll leave you there. Lists are super powerful, there's a ton of functions you can do
- with lists, In fact, I'll expose you to a few right here.
- So that is a, you can add elements to a.
- So you could say a dot append and you can add something to the end of a.
- So you could add "new element" and if you do that it should add it to the end of a.
- So let's look at a.
- It added that element.
- Well b is referring to the same thing so let's see what b looks like.
- It has a new element.
- C is referring to a copy of the old a, so let's see what that looks like.
- Didn't add it there, cuz it didn't add it to the copy.
- It added only to what a was referring to which is the same thing b is referring to
- and there is a ton of useful functions that we'll explore them as we do more and more
- programs, more more example problems,
- but there is a bunch of useful functions or methods that lists have to be able to manipulate them
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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When naming a variable, it is okay to use most letters, but some are reserved, like 'e', which represents the value 2.7831...
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