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So far, we have discussed numbers and strings, but the reality in an actual program, we often want

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to collect our results and maybe store all of them in one variable.

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So of course, we could do this by writing something like, I don't know, a one is equal to one and

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then make another one.

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A two is equal to five and a three is equal to 10.

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But you can see this is really tedious.

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We have to continue and we have to define many of these variables, and we'll be a bit difficult to

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assess the individual variables because we always have to remember what we named them.

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So much better would be to just make a list.

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And this is, of course, possible with Python by, for example, defining a variable called List one,

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which will be these square brackets.

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And then you can just write to individual elements of the list.

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For example, these square numbers and what you can also do is you can combine several data types in

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a single list.

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For example, you have one, then a string, which is a and then another number.

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So now we have two lists.

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What we can do now is we can select individual elements of this list.

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We can, for example, start with list one and then one.

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So maybe you are a bit surprised now if you're new to Python, because the result is four, which is

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actually the second element of this list.

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And the reason for this is that these indices in Python start counting from zero.

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So if we write zero, then we actually mean the first element.

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If we write one, then we mean the second element.

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So this is different in other programming languages, and it can be a bit confusing, but you just have

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to get used to it.

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This is the way of Python works.

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So if we want to have to second element write one now, we can also select multiple elements of the

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list at once.

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For example, our writing list one.

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And then we do not write here only a single value, but two values.

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So for example, if we want to have the first two values, you could think that we would have to write

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zero to one.

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But here it's again a bit confusing, to be honest.

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We have to write a zero to two because this means something like start from the index zero, which is

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the first element and go until the index is smaller than two, which means only use zero and one and

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zero and one correspond to the first and second element.

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So I agree this is a bit confusing, but this is what it is.

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You have to get used to it, unfortunately.

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But once you realize it, it's really not that difficult.

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Just have to remember start counting at zero.

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And when you want to select several elements, then you have to go one number higher than you actually

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want to go.

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So the next thing that we can, of course, do is we can manipulate these lists because it would be

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very tedious to always redefine the whole list if we want to change something so we can just manipulate

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the list.

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For example, we could write this one.

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And then there's a function called Append.

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So we write list one dots append, and then we just write what we want to add to the list.

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And the cool thing is that this number is already added to the list.

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So we do not have to write something like this one equals list, one appends.

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But just this, this is already sufficient and you see the number three has been added to the list.

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We can, of course, also remove something from lists.

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For example, this time, let's take our lists two.

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And let's remove the eight.

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So we write list two dots to remove a.

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And then I check what it will be.

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The result around this and this will give us the results.

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The only thing you need to be careful about is that when you run these cells multiple times, for example,

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by accident, you run this cell twice and then you have already added the number three twice at the

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end of the list.

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What we can also do is we can sort the lists by using the command sort and once again, this running

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this command will already manipulate the list and sort has some optional arguments that we are not going

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to use.

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For example, you could select that you want to store that number of First Order.

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But we want to we don't want to use any of the optional arguments.

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So you just write.

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It open bracket close and then it will work, but you need to remember that these brackets here and

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then the result will be not one four nine three, but one three four nine.

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All right.

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So one more thing that we can do when we want to manipulate our lists is to change a single value of

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the list.

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And what I want to do now is I want to change the number nine to number 10.

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So I just write list one.

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I want to select the fourth element, which will mean the index has to be three.

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Since we're starting counting from zero and this should be 10, then the result will be one three four

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10.

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Multidimensional lists are also possible, so we can create a new list list three.

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And here I just tried to square brackets one to three as an example.

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And then here another list which we could use here to square numbers one four nine.

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So you see, this is a list consisting of two lists, and now I can run this.

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And of course, now I can select individual values of the list.

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For example, I could write, I want to take only the zeros element or else that the first element,

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which would be this list here.

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And then I could also say from this list, I want to take the first element.

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So we get the one, this one here, that's all possible.

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So now I want to show you why lists are not always perfect because there are some problems with these,

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especially if you want to do mathematical operations.

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So if you remember all of this one, we, for example, could have the problem or we want to multiply

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all the values in the list by a number.

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For example, we could then write list times too, and we would think that this would give us two six,

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eight 20.

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However, if we run this, it doesn't work.

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It gives us twice the list and they are merged to a single list.

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And the same happens if we

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are not the same.

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A similar problem happens when we want to add a number.

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So here we even get an error because plus like four strings means concatenate, so basically merge lists.

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So we tried to merge here this list with a number which is not working because one of these properties

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is the list and the other one is just a number.

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So it would work if we would write something like this.

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But then we would just add another element to our list.

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So this is the same as using the append commands.

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And this is not really what I had in mind here by adding a number.

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So the thing is, lists are really nice to quickly store some, some some variables, some numbers,

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some strings, but it's very difficult to do mathematical operations with them and also storing things

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in lists requires a whole lot of memory.

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So most of the times in our for our purposes lists are not really ideal only if we have a very few elements

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that we do not want to do any mathematical calculations with them.

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So what we are going to use instead are arrays and these are we'll discuss in the next lecture.