Python has lists. Let's see how they work. The empty list, Is just empty bracket. A list with one element, let's say a letter a in it, would look like this. A list with two letters would look like this. A list can have numbers. Like this. A list can have strings. Like that. And it can have all of these things. Hello, 3.14. A number. Let's define a list of letters. Let me just execute this line, and it gives us this list named lis. And it's equal to the letters a through f. If I type lis, you'll see that I get that list back. Let's talk about how the items of a list are accessed. This is item zero, this is item one, this is item two, and so on. And notice that Python starts with item zero, not item one. So zero, one, two, three, four, five. That's the way these are accessed. Let's take a look at how to pull an element out. If you want item 0, you type lis[0]. If you want item 1, you type lis[1]. If you want to know how many items are in the list, you can use the len function, the length function. And it'll tell you that there's six, there's three, four, five, six. Now, what if we wanted to get the last element of the list? Well you might be tempted to ask for the following. lis[6]. But let's see if that would work. This is item zero, one, two, three, four, five so the last item of this list is item five, not item six. So this won't work, but I can say item five and get it. That worked just fine. There is a simpler way and that is that I can type -1. That says count from the end of the list so this is first one at the end of the list. And the next to the last one will be lis[-2]. And so on, let me repeat the list. Now I might want to get items two through four back into it this way. Now what that will tell me is, items two and three but it will not print four. Zero, one, two so item two is a c, so it'll start there and it'll do three but it will not do four, which is the e. Let's see that, zero, one, two. There's the two. Three, there's the third one. Four is not there. I want to get everything from item three on to the end, I can do that. And not put an ending spot. I want to get everything up to, but not including three, I can do that. Zero, one, two, here's three. It's d, it's not there. If I want to add onto the end of the list, I can use a method of lis called append and I can, in this case, add g on in this manner. You see g is now added on to the end of the list. We can ask whether something is in the list. So I'm going to ask if d in lis. I can make that assertion. Python will tell me whether that's true or false. It is true. If I make the assertion that x is in the list, it'll tell me that it's false. So I can tell whether something is in the list by doing an assertion and see whether I get true or false. Now let's look at a function that makes use of the list. First of all I'll go over here and click on, in that cell, and execute it. Who_is_there is the name of the function. And I'll give it a simple list with just a lion in it. And it prints, there's a lion. There's no horse in the list and the list has one item. Let's take a look at the code If bear's in the list, it'll do something. But there's no bear in the list. There's only lions in the list. If lion is in the list, then it'll print there's a lion. And it does that. If daisy or iris is in the list, it'll print there are flowers, but they're not there. If they both are there, it'll print something. If donkey's in the list, it'll print there's a donkey, but that's not in the list. And now let's look at this horse not in list. The assertion horse not in list is true, only a lion's in the list, so this is true. If this is true, then it'll print there's no horse in the list, and it does. The last line prints, the list has a length of list items, and there's one item. Let's execute it, and let's say iris. Put that in the list. Now again the lion's in the list, so it prints there is a lion. But now, it'll see this line, if daisy in list or iris in list then, there are flowers. Well, iris is in the list so it'll print flowers. If it had seen daisy it would of print flowers too. Either or. There's still no horse in the list so it prints that, and now there are two items. Let's put daisy in the list. Now the lion is still there. So it's still checks the other line and prints there is a lion. If daisy is in the list, and it is, it'll print there are flowers. It would've also printed it because iris is also in the list. Since daisy is in the list and iris is in the list, there are at least two flowers and it prints that. There's still no horse in the list, and now the length of the list has three items. If we put horse in the list. Then there is no horse disappears because horse not in the list is no longer true, horse is in fact in the list. So this if statement is not true, so we'll not write there is no horse in the list. You should make up some lists and pass them to this function and make sure you understand how all those ifs work. Let me point out that we didn't have to type the list directly into the argument. Let me find a few lists here, I'll show what I mean. We've got a list called alion, we got another list ld, has a lion and a daisy in it, and lbf, that list has a lion, a bear, and an iris. So we could have passed these into the function who_is_there(lbf) and it would say, there is a bear, there is a lion, and there are flowers, because iris is there. There's no horse in the list, and the list has three items.
