Original subtitles

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So the code challenge that you're about to do comes from a really interesting phenomenon that you might find

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in London.

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I live in London and it's well known as a big financial district.

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So when you head over to the city or the financial area in London and you go into a restaurant, you

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might see a strange thing happen.

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You might see a whole bunch of people in suits who look very much like the financial banker types. At

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the time when they actually need to pay the bill

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you see everybody pull out their business cards and put them into a bowl.

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So what's actually going on here?

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A friend of mine told me apparently there's this game that the rich banker types play where it's kind

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of like Russian roulette with the bill.

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So everybody puts their business card in and the person's card who gets picked out has to pay for everybody's

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bill, which is kind of crazy.

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But then again, it's finance.

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So in today's challenge, this is what we're going to replicate with code. Head over to the course

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resources and click on the coding exercise to bring it up.

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Now, go ahead and fork your own copy of it so that you can make amendments.

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Now, the idea is that once you've completed the challenge, you should be able to type in a whole bunch

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of names as an input with a comma and a space separating each name.

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And then the code will pick a random name from the one that you've given it and tell you who is going

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to buy the meal.

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Now, there's two lines of code here that you might not have seen before.

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Now this is something called split, and this allows you to split a string into separate components

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based on some sort of divider.

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So in this case, notice how it says hello, from, AskPython.

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Now, if we use the split on this string, then it will divide it up into a list and separate out all

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the words that are divided by this split character, which is the comma.

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Now, similarly, over here, when we give the input, it's going to be in the format like this; everybody's

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names separated by a comma and a space.

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So now if we use the split on a comma and a space, then it will take out the comma and the space and

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it will put everything else as separate items inside a list.

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So what I recommend doing before you get started writing the actual code is to test it out.

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So printout what names actually looks like.

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So if we go ahead and run this code and then we type a bunch of names separated by a comma.

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so Angela, Ben, Jenny and then hit enter,

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then we hit the line where it prints names.

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Now names, as you'll see, is now a list with all the names I typed in.

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And while previously they were separated by a comma and a space, they are now all individual items

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in a list.

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The idea is that you've got all of these items inside a list,

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how can I pick a random one out of it using a random index based on the number of items in the list

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which of course can change?

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And then how can I get a random name as the output and then print so-and-so is going to buy the meal

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today?

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So have a think about it,

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have a pause and then see if you can complete the challenge.

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So now that we've got the list as a starting point stored inside names, how can we go about generating

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a random name and picking it out of the list?

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Well, we know that we can pick a item from the list by adding a set of square brackets and then a index

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number.

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So in this case, it should print the first item out of the list which is going to be just the word

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Angela. And I can run this to prove this to you.

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You can see that names is a list of all the names, Angela, James, Ben. Names at position zero

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is Angela, the first item.

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Now, how can we get a random number to replace that number zero?

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Well, we could use our random generator.

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Let's go ahead and import the random module and we can say random.randint and then we can specify

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the start and end.

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So the start is probably going to be zero because that's where we start counting with our lists.

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But what is the end?

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Well, the end should be the position of the last item.

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But how can we know that position?

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What is that X going to be?

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Well, what if we got hold of the number of items in the list?

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So do remember how previously we learned about the len function?

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Well, the len function can be used to get the number of elements in a list or the number of characters

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in a string.

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And if you take a look at this Stack Overflow question, then you'll see it in action.

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All that we have to do to get hold of the number of items inside this names list is to write len

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(), and then inside it we put names.

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Now we can either store this or print this. Llet's go ahead and first print with the value of this is

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going to be. Let's go ahead and comment out this line of code and then run our code, and then give everybody's

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names separated by a comma, hit enter.

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And you can see that this line of code has printed three.

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So the length of my names list is three.

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So now that we've gotten hold of that, then we can probably use that number that comes from this len

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function inside our random generator, because we know that we can use this to get the total number

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of items in list.

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So in this case, that would be one, two, three, four, five.

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This last item is not at position five, though, right?

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It's at position zero,

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one, two, three, four.

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So we always need one less than the total number of items.

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And the range of random numbers we would want is num_items -1.

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Now, this line of code will allow us to generate random numbers between zero and the last index, and

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I can store that inside a random choice variable and print my random choice.

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So if I run the code again

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and it prints one this time,

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but the next time, it might be zero or two, because those are all the possible choices for a three

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item list. Now we can use that number to actually get hold of a particular item in our list of names.

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So we can say names[], and then we're going to use the random choice to get hold of

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the random item. And then we can go ahead and save this as the person_who_will_pay.

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And we can print

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person_who_will_pay + "is going to buy the meal today."

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So now let's go ahead and run our code again and then give everybody's names separated by a comma, and

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then it says James is going to buy the meal today.

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And now we know that we've completed the challenge.

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Now, you'll notice that in the instructions, I told you that you're not allowed to use the choice

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function.

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And the reason for this is because if you search AskPython and you search for the random module

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and you scroll down, you'll find that you can actually generate random items from a sequence such as

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a list by writing random.choice, and it will actually pick an item from that list.

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So let me demo what our code would look like instead. Instead of needing all of these lines of code,

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all we have to write is random.choice()

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and then we put our names list inside. And now it will do exactly the same as before

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but with far less code. But this, of course, doesn't test whether if you've understood how indices

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work with lists and whether if you're comfortable with the idea that the last index is actually one

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less than the total number of items because we start counting from zero. I've made it a little bit

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harder than it needed to be just to see how good a grasp you have on lists.

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Now, I hope that you managed to complete this challenge.

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If not, don't worry.

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Go back to it and see if you can replicate the logic that we talked about just now.

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And if you really struggle visualizing what these lines of code are doing, then you can always pass it

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through Thonny and use the debugger to step through it one step at a time and see what's happening at

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each stage.

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Now on the next lesson, we're going to see some more advanced parts of lists in action, namely how

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to nest lists inside other lists.

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So for all of that and more, I'll see you on the next lesson.