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Before we move on, I'll use the next few
videos to practice what we've learnt so far.

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So I'm going to start with another
menu program. In fact, we'll implement

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our buy_computer program from
the Improving our code lecture,

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in the Lists and Tuples section. And you can
see that I've put that into this project.

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Oh not another menu! Now look, note we're
not learning how to write menus here.

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We're looking at how to access and
add items to Python data structures.

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We've already seen how to do that with a list, now
we're doing the same thing with the dictionary.

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"But which one should I use?" Well, that's a question
new programmers often ask. There are so many options,

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and it can be confusing working out what you
should use, and when. We're going to write the same

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program using a dictionary instead of a list.
We can then talk about the advantages that

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a dictionary gives us, and the disadvantages.
When you understand the pros and cons of each,

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you'll be in a position to decide which one to use,
for your particular application. Understanding the

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different capabilities of these data structures
is the key here. Once you understand the different

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ways that a dictionary works, compared to a list,
then choosing between them becomes much easier.

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Alright, so you can see that I've got
the buy_computer.py program here. 

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Alright, so I'm going to start by creating a new Python file.
Let's go ahead and do that. We'll call this one

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buy_computer_dict.

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Now I'm going to paste in the dictionary
literal - the literal definition of our dictionary.

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What I'm going to do is grab
it from the old file - available_parts.

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Take a copy of that, then we'll make some
changes to it, so it's in the right format.

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Now obviously, that's a list. We're going to
change that, so it's a dictionary. So I'm going to

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change it to curly braces here, left and right.
Then we also need to add the key, so "1"

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in double quotes, colon and a space. We'll do the same
for the rest, and I'll just speed this part up.

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You saw there wasn't much editing needed.
The original list used square brackets,

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but I changed those to curly braces on lines
one through seven. Then I also added the keys

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at the start of each line. The keys, as
you saw, are all strings - the characters

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one to six. I added that in double quotes, and I
separated the keys and the values with a colon.

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Alright, so let's start there by displaying
the menu, and allowing the user to choose items.

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So to start, on line 9, by typing current_choice is equal to None,

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then on line 10, while current_choice is
not equal to zero in double quotes and a colon.

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Now in the earlier example, if we go
back to the original buy_computer,

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we initialize current choice, down here on line 13,
to a minus character. We have to give it a value,

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otherwise we'll get an error when we check the
value, down here on line 16. It doesn't matter what

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value we bind the variable to, as long as it's not
a valid choice. But of course, it also mustn't be zero

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otherwise we wouldn't enter the while loop. We use
the minus character here, but back in buy_computer_dict,

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on line 9 you can see that I've
initialized it to None. It doesn't really

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matter what you use. What's important is that we
want to initialize it to something, and we don't

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use a valid choice or zero. Alright, so next
we need to decide if the user's choice is valid.

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If it is, we print a message that we're adding
the part, otherwise we'll display the menu. 

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So let's start with the code for a valid choice. So on line
11, if current_choice is in available_parts colon,

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then it's chosen_part is equal to available_parts

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square bracket current_choice,
then closing square bracket.

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Then we'll print out a message: print
parentheses f, in double quotes, Adding, then

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left and right curly braces, chosen_part
in between those left and right curly braces,

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and then the end of the line. Alright, so
that's our if, and then I'm going to, at the

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same indentation level as the if but this time
on line 15. we'll type current_choice is

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equal to input, parentheses and in double quotes,
I'm going to put a greater than sign and a space,

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and then end the line. Right, so you can see that
I've used an f-string on line 13. Up until now

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we've been using dot format. f-strings are more
compact and now that Python 3.5 has reached end of life,

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all supported versions of Python can now use
f-strings. If you're still using Python 3.5, 

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then you'll need to change line 13 to use dot format
instead. Review the video on f-strings in section 3,

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if you're not sure how to use them. They're very
similar to placeholders with dot format, but you

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include the values inside the placeholder curly
braces, instead of passing them to the dot format

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method. And you can see that I've done that on line
13. Alright, so let's have a look at what we've done here.

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We're going to run the program, and we'll
see what it does before looking at how it does it.

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Now we haven't added the code to print the
menu yet, so you can see we just get an input

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prompt - the greater than. Our program is waiting
for us to make a choice, so I'm going to choose 2

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to add a monitor here, and you can see we've
got the message there - Adding monitor. So experiment

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with a few other options, and also enter something
invalid. So I'm going to add another option here:

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5 to add an hdmi cable. I'm going to do
7, which we know is an invalid option. 

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We get nothing added. We just get the prompt again. So how
does that work? Well, looking at the code - we've got

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a test there, on line 11. We're checking to see
if current_choice is in available_parts, 

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in other words, in the dictionary. available_parts
is obviously line one, we've defined.

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Now that's something that dictionaries allow you
to do, just like you can with lists and strings.

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There is a difference, though. When you use in
with a dictionary, you're only checking the keys.

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Now we can confirm that, in looking
at our list up there, by entering mouse.

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So mouse isn't a key in the dictionary, so
we get nothing added. We're back with an input

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prompt again. Now that might seem strange because
looking at line four, mouse is in the dictionary.

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Right, so on the left here,
we're using a dictionary. in

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is checking the keys, and mouse isn't a key in our
dictionary. So therefore line 9 is False. The key 4

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is in the dictionary so we get True on line 10.
On the right, we're working with a list. The value

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mouse is in the list, so we get True on line 9. The
value 4 isn't in the list and line 10 prints False.

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So remember that: when you use in with a list it checks the items
in a list. When you use in with a dictionary it checks the keys
in the dictionary not, the values.

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Alright, so back to our code,

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I'm going to enter 0 now to terminate the program,
and let's have a challenge to end the video.

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So you want to add an else block to our code, and
print the menu when the user's choice isn't in

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the dictionary. Now if you're not sure how to do
that, review the video Iterating over a dictionary.

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The loop in your else block will
be very similar to the code we used

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to iterate over the dictionary's
keys and values.

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Alright, so that's the challenge, and I'll go
over my solution to it in the next video.