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All right.

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So the first thing I want to talk about is dictionaries in Python. Now,

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dictionaries in Python work kind of similarly to dictionaries in real life,

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right? So if you were to look up a word in the dictionary

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say the word code, then you might find the definition as something along the

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lines of program instructions for the computer. And dictionaries are really

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useful because they allow us to group together and tag related pieces of

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information.

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The way I like to think about dictionaries is in the form of a table.

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Every dictionary has two parts to it.

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On the left hand side is the key,

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and that is the equivalent of the word in the dictionary,

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and then it's also got an associated value. That would be the equivalent

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of the actual definition of the word.

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Now let's say that we took this variation simple table of definitions of

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programming words that we've come across so far

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and we go ahead and we try to convert it right into a dictionary,

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how would we do that? Let me firstly

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drop the last two rows and let's just start with the first one.

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The first thing we want to do is we want them to create a dictionary. And to do

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that in Python, this is what the syntax looks like.

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We have a set of curly braces and everything that's inside

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the curly brace is the content of our dictionary.

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The key goes first followed by a colon and then followed by the value. In our

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table we've got this word bug, which is the first key,

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so we can replace that over here in our dictionary. And the value that's 

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associated with this key is the definition for a bug.

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So an error in a program that prevents the program from running as expected,

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that becomes the value and can be replaced here after the colon.

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So now, we've created an actual dictionary using Python code.

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What if you wanted to have more than one entry in your dictionary? Well,

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you would separate each of the key value pairs using a comma,

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and then you can continue adding key and value pairs until you get to the end of

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your dictionary. All right,

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let's take a look at dictionaries in action.

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If you head over to the day nine start code file and go ahead and fork your own

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copy to work alongside me, then you can see that in here

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I've already added the dictionary that we were defining earlier on,

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and I've stored it inside of a variable called programming dictionary. Now,

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currently I've only got two entries in this dictionary,

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the definition for a bug and the definition for function. Now,

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the first thing I want to highlight is when you create a dictionary that has

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more than one element such as in this case,

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then you want to take care to format it properly so that it's more easily

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readable.

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So what you'll see Python programmers do by convention is they will start off

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the dictionary with the open curly brace at the top

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and then every subsequent entry is indented by one indent.

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And then at the very end of that entry, there's a comma

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and then we hit enter so that the next item goes onto the next line

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and finally,

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the last curly brace should go at the very beginning in line with the start of

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the dictionary.

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And another thing that's quite nice to do is to cap off all entries in your

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dictionary or list with a comma.

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This means that if you needed to add more items into the dictionary

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you can simply just hit enter and continue typing the next thing.

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And if we wanted to add another entry,

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it's a simple as adding in the key, a colon and then the value and to cap it

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off again with a comma.

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So now our dictionary represents exactly the same data as we saw in our table

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over here with a bunch of key value pairs and a total of three entries.

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Now,

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the next thing I want to do is what if I wanted to retrieve an item from the

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dictionary, because we know that if we had a list

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what we would do is we would use a set of square brackets

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and then we would give the index of the item that we wanted.

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So the item at index zero or one or two and so on and so forth. Now for

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dictionaries, it's kind of similar in terms of syntax

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but the only difference is that dictionaries have elements which are identified

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by their key. If we wanted this piece of information, for example,

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then all we have to do is tap into the dictionary and then add a set of square

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brackets and inside the square brackets, we're going to provide the key.

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So here the key is a string and it's the string bug.

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So let's go ahead and put bug in here.

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And now if I go ahead and print this,

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then you can see that it is going to give me the value,

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which is 'An error in a program that prevents the program from running as

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expected.' Now it's really,

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really important that you make sure that when you're fetching something out of a

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dictionary by it's key that you actually spell the key correctly.

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A really

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really common error is when you're trying to retrieve something out of a

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dictionary and you've just made a very simple typo. So instead of 'u'

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I'm typing 'o' here and you'll see

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we get an error. And the error tells us that it's a key error referring to this

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particular key and it highlights this line 7 where we're trying to retrieve

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something out of this dictionary by this key.

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Basically it's telling you that this key doesn't actually exist and it can't be

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found. So it doesn't know what it is that you want. Remember how

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when we had lists and when we try to retrieve something that was not inside the

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list. So for example, this particular list at index 4

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doesn't actually exist because this is zero one, two, three,

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and four is actually not a piece of data inside of this list.

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Similarly with dictionaries, if we try to put in a key that doesn't exist,

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then we get this key error.

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Now another common pitfall that students fall down into is they don't actually

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use the correct data type. So for example,

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if we defined this dictionary without putting a string around each of these

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keys, then it's going to error out and it won't even let us run.

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It's going to tell us undefined named bug because it thinks that this is a

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variable that you've declared somewhere,

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but it's not. In fact, what you wanted are these strings for keys.

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And so when you have a key that is a string,

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when you're trying to retrieve the data from that key,

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you also have to make sure that you provide the key in its actual data type.

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So for example, if this was just a number, say one, two, three,

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then, of course, all you have to write in here is just one, two, three,

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and it would know that this piece of data is what you wanted.

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So that's how you retrieve items from a dictionary by adding a square bracket

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and then giving it the key.

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And it will look for the key inside the dictionary and give you back the

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value. Now,

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what if you wanted to add a piece of data such as that loop that we had earlier

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on, but you want to do it programmatically.

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So instead of doing it

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when you were defining the dictionary at the beginning,

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what if at some later stage in your program you needed to add a new entry?

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Well, to do this, it's also really simple.

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All you have to do is to tap into the dictionary,

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which is called programming dictionary in our case and again,

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using square brackets, we define the key.

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The key I'm going to add is our loop.

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And then after a equal sign, I get to assign the value.

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So in my case, the value is going to be the definition for a loop.

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And now when this line of code is executed and we go ahead and just print the

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programming dictionary after this has happened,

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then we'll see that it's actually different from what we had before.

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So let's hit run and you can see that previously we had a programming dictionary

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that had only two items, bug and function.

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And after this line 10, where we print our programming dictionary again,

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you can see it's now got three items, bug, function, and loop.

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Now very often when you're writing code,

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it can be really helpful to start out with a empty dictionary. Just as you saw

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previously,

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you create an empty list by simply having a set of square brackets with nothing

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inside.

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You can also create an empty dictionary by simply creating a set of curly braces

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with nothing inside. And then at a later stage,

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you can add to your dictionary by using this method that you saw here.

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Now, on the other hand,

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you might actually want to wipe an entire dictionary. Here,

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I'm creating a new empty dictionary by creating this pair of curly braces with

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nothing inside.

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But I can also wipe an existing dictionary by simply doing the same thing.

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We know that this dictionary, programming_dictionary, has three items in it.

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But on line 17,

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I'm going to say programming dictionary equals empty dictionary.

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And now if I move this print statement down here,

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then you can see that when it prints out,

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it's actually going to be completely empty.

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That can be really useful if you wanted to clear out a user's progress

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or for example, if a game restarts,

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then all the scores and stats will probably have to be wiped empty.

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So this is one way that you could do that.

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Now this method of tapping into a dictionary,

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using the key to fetch the relevant item from it

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and then doing something with it goes beyond just adding.

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You can also use this to edit an item in a dictionary.

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For example, let me go ahead and comment out these two lines of code

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so we don't wipe our programming dictionary and instead I'm going to tap into

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the programming dictionary and I'm going to fetch the item that has a key of bug

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and I'm going to redefine its value.

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So currently it should be an error in a program that prevents the program from

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running as expected.

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And I can prove that if I just wrap this around a print statement.

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If instead I wanted this to be a different value,

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then I can simply use the same syntax as I did for adding new items

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but in this case

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I'm actually editing this entry because it's going to look through the

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dictionary,

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find a value with this key and then assign it to whatever I put on the right

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hand side of the equal sign. Now, on the other hand,

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if it finds nothing with that key,

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then it's going to create a new entry with the value again

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on the right hand side of the equal sign.

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Now let's say that a bug is instead 'A moth in your computer.'

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Now, if I go and print my programming dictionary once more,

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then you can see that the definition for our bug has now just been changed.

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The final thing with regards to dictionaries that I think is really,

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really useful is how you loop through a dictionary. And using your knowledge from

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lists you might think that to loop through a dictionary, you would,

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let's say we are using a for loop. And we say for, um,

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thing in programming dictionary,

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let's go ahead and print this thing each time.

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Here's a good moment to play computer and think,

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what do you expect to be printed?

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So let's comment out all of the other print statements

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so we don't get confused. And when I click run,

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what do you think will be printed?

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Right?

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All right. Is that what you expected? Because it certainly

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wasn't what I expected when I first learned Python.

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I thought it would give me each of the items in the dictionary with this key and

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its value.

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But instead this code actually just gives you the keys.

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Now, of course, once you do have access to the key, instead of thing,

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I should actually really say for key in programming_dictionary.

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If I wanted to print the key, then of course I could just write print key.

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But if I wanted to get hold of the value,

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I could equally just as easily tap into my dictionary,

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use the square brackets and pass in that key.

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So now when I hit run, you can see it's giving me first

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the key from this line and then secondly

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the value based on this line.

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I'm using that retrieval code in order to get the value.

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Of course,

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remember that you can get access to all the code that I'm writing here by

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looking at the end code of the day

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that you'll find on the course resources list.

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And if you want to, have a quick review of everything that we've gone through so

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far,

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because a lot of this knowledge is going to be tested in the next lesson where

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we do a coding exercise that will really solidify everything you've learned in

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