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Alright, so let's look at appending items to a list. For this example, we'll allow

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customers to choose which parts they
need when buying a computer. I'll keep it

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simple to start with - we'll just offer
five items for sale. Now we saw how

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to write a simple menu, in the challenge
at the end of the last section.

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This program will start off very similar
to that. So let's start by creating a new

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Python file. I'm going to call it buy
underscore computer.py.

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Alright, so first we need to initialize a couple of
variables. We need a variable to store

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the customers choice, as well as an empty
list to add items to. So on line one,

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current_choice = double
quotes, with a dash character or minus

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character in there. We talked about the
reasons for using that in the last section.

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Line two, computer underscore parts

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equals, and just left and right square
braces, and a comment there, create an empty list.

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Again, that's going to be list to
store the items. Alright, so the code to

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display the menu is the same as we've
used before. It's just the description of

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the items that have changed. So I'm going to start on line four - I'm typing while current

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underscore choice is not equal to, in 
single quotes, 0 colon. Then on the next line,

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if current underscore choice is in and
in double quotes, 1 2 3 4 5 colon print

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parentheses double quotes, adding
left and right curly braces double quotes,

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dot format, and in parentheses, current
underscore choice and two right to

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parentheses to finish the line. Press
enter there, and backspace. We're gonna start on

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line 8, then else : then we're going to
print out our menu option, so print

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parentheses, Please add options from the list below,

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and double quote and right parenthesis.
Next line, print parentheses double

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quotes 1 : computer, double quotes and
right parenthesis. Next line, print

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parentheses 2, and double quotes 2, rather,
monitor. That's the next item there.

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Moving on to the next line, print
parentheses double quotes 3 : keyboard,

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and our double quotes and right parenthesis.
Next line, print parentheses 4 double

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quotes, in 4 rather, or four in double
quotes, mouse, and double quotes and

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parenthesis. Next line, print 5 in
parentheses, sorry in double quotes,

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which is also in parentheses, mouse mat.
There's our 5 options, and the only other

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one we want after entering the double
quote and right parenthesis is our

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option to exit, print. That's on the next
line, parentheses double quotes 0 colon

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to finish. Double quote and right
parenthesis, and then we're going to go

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down to line 17 at the same level as the
if and the else, current underscore

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choice is equal to input, left and right
parentheses. When you're starting out

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learning to program, it's a good idea to
run your code often. That way you can

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spot errors and fix them, without having
hundreds of lines of code to work

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through. I'm going to run the program
that we've got so far, to make sure it

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works as we expect. Let's run it. You can
see there we've got a menu, so that's a

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good start. Let's just start by trying to add
a few options. We're going to start by adding a

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monitor, option 2, and we got the
message there, Adding 2 printed.

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Try adding 4 now, for mouse, Adding 4. So
mouse also works. Now you should test all

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the valid options to make sure you get
the appropriate message printed. I won't

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go through all of them in the video.
Pause the video and check the other

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options yourself. Now we should also test
what happens when we enter something invalid,

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so I'm gonna try entering 8 here,
which obviously, isn't an option there.

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The menu doesn't have an option 8 so
nothing's added. Our code displays the

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menu again, so that's also working fine.
We'll try 0 now to finish, and the

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programs exited.
Alright, so far so good. The next step

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is to add the items to the list. At the
moment, our computer_parts list -

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that's the one we've defined up here on
line two, is empty.

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We've used square brackets with nothing
between them on line two, and that

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creates an empty list, as the comment
said. Inside the if block, we can append

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new items to the list. So let's go ahead
and do that. We're going to add it under

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the print statement on line six. so I'm
going to type if, and indent it at the

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same level, current_choice is
equal to 1, and 1 in single quotes -

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two equal signs there - colon. Then we're
going to do computer_ parts

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dot append, and in parentheses,
computer in double quotes. So the code

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here, if the customer chooses option 1,
we add a computer to the list. We do that

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by calling the list's append method. The
argument is the thing we want to append,

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a computer in this case. That's the text
in double quotes on line eight. Alright,

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so the other items are more of the same.
I'm going to paste the code in here to

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save a bit of time. Alright, so there's
the code that I've pasted in there now.

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Alright, so pause the video while you
enter the remaining code and come back

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when you're ready to continue. Okay, so
those blocks of code use the append

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method to add the items to the list. So
there's a few things wrong with this code.

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We have to duplicate the five
strings, for one thing. The string

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computer, on line eight, also appears in
the code on line 20. Let's take the

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opportunity to fix this up as well. I'm
going to remove that extra line there.

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So you can see the duplication there, again, computer line eight, as well as on line 19 now.

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Adding a new option also
involves a lot of work. We've had to

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change the code in three places. But
let's make sure it works before we start

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improving the code. To make sure it's
working, we need to print out the

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computer_parts list that we're
appending to. So let's do that right at

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the end of the program. So I'm going to
do that right down here, on line 28,

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indented to the left so it's executed
when the program finishes, print

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parentheses computer_parts, and closing right parenthesis there.

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And again, make sure that line
isn't indented. Basically, we want the

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list to be printed out when the loop
exits, as I mentioned. So let's run that.

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Let's choose some items now, to add to
our list. We'll start with option 1, computer.

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Let's do another one,
option 4, which is the mouse.

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Let's exit by selecting zero. As you can see
we've got a list there containing a

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computer and a mouse. So that code's
working fine, but it isn't very clean, though.

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There's a lot of duplication, and
making changes to the menu is a lot

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harder than it should be. Improving the
program is a good way to see some of the

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other things we can do with lists, and
we'll make a start on that in the next

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video. But I'm going to finish this video
first with a mini challenge:

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Change the program to have another
option, option 6, for a HDMI cable.

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So remember that you'll need to make a
change in three places. Have a go,

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test your code thoroughly and I'll go over my
solution to that in the next video.