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Welcome back everyone in this lecture.

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We're going to talk about dictionaries on dictionaries are ordered mappings for storing objects.

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Previously we saw how store objects in an ordered sequence dictionaries instead are going to use a key

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value pair system.

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Basically this key value pair allows users to quickly grab objects without needing to own index location

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beforehand.

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The actual syntax is that dictionaries use curly braces and colons to signify the keys and their associated

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values.

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So here we have an example of a python dictionary with one key value pair key one in value one and then

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another key value pair key to add value to.

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So we have curly braces on the outside and then each individual key value pair.

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We have a string key colon and then its corresponding value and its value doesn't need to be a string.

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It could be a number a list something else.

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Boolean.

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Lots of other options there and it can even be another dictionary.

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But the main idea here is if I'm looking for value one then all I need to do is pass in Q1 and will

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bring me back that value 1.

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Or if I'm looking for a value to I can pasand key 2 and I'll bring it back value to and we're actually

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coding this out and the Adam text editor will see some examples that kind of clarify the use cases for

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a dictionary but often beginners have the question while why how is this different than a list.

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Why should I choose a list and when should I choose a dictionary.

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That's often a point of confusion.

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The basic idea here is that dictionaries those objects are values are retrieved by the key name.

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So if I'm interested in value one all I need to know is that its associated key is key one unlike a

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list where I would have had to have known the index location beforehand.

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Now the tradeoff here is that because the objects are retrieved by a key name that means there's no

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real order to them so you can't really sort a dictionary.

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It's instead the key value pair mappings.

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Now list those objects are retrieved by location.

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And what's nice about that is that they can be indexed or sliced because it's an ordered sequence.

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The con there is that if you actually want to grab something an individual element from the list you

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would need to know its actual index location beforehand.

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So essentially that's a tradeoff.

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We're going to see some more examples of this idea of dictionaries and the key value pair mapping right

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now by hopping over to the text editor and working through some examples.

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So let's head over to that now.

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All right what I'm going to start off with is just creating a very simple dictionary.

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So we'll say D is the variable and we're going to set that equal to key one and value 1 and the other

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key is going to be key to and then value 2.

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So then let's print out the dictionary and see what that looks like.

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There we go so we have one that value 1 and key to value 2.

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Now let's imagine I wanted to actually grab a corresponding key value pair.

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What I can do is the following.

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I can say the square brackets.

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But now instead of passing an index location like it would for a list or a string I pass in the Keitt.

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So I'll say key one and then let's print the that response.

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Will say print out d of key one save that change.

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Run this again and now I see value 1 is the output as well as that dictionary from before.

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So that's the way you can grab values.

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Now let's go ahead and show you a more realistic example.

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Let's say I have a list called salaries or excuse me a dictionary called salaries.

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So this is a really great idea for storing information if a dictionary instead of a list.

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So I'm going to create a salary's dictionary and the key is going to be someone's name and then the

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actual value is going to be how much their salary is let's say we'll do it by our.

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So John earns $20 an hour.

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Notice here I'm just passing in a number and then we'll say Sally earns $30 an hour and let's say Sammy

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he's going to earn $15 an hour.

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And now the question arises Well how much does John earn per hour how much does Sammy earn per hour.

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Now it's really readable because all I need to do is say salaries and then just pass in the name of

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whose salary I'm interested in.

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So then I can say Sallie and then if I just print this out so print out salaries for Sallie and let

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me delete these lines right here I run this again and then I get back 30.

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So this is a really good use case for what you would want to use a dictionary for.

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You have a key value pairing and what you're going to be interested in is looking up these keys and

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then getting back their corresponding values something like this.

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When it really makes sense for a list because then that would require you to memorize every single pair

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location and it would really make sense because then you'd be mixing in names and numbers.

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So instead of using a dictionary like this allows you to easily look things up based off the key.

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In this case the key is quite intuitive.

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It's just that person's first name and then the value here is their salaries.

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Now let's imagine we just hired a new employee and we want to add them to the salaries dictionary that

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we do that is you call salaries and then you reference the key as if it already existed there.

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So let's say we just hired Cindy and then you set it equal to whatever the corresponding value was so

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let's say he's the CEO.

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So she's going to get paid $100 an hour and then just to check that it's working.

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Instead of asking for salaries or Sally's salary I'm going to ask for Cynthy's save that run.

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And then I get back 100.

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So here what I'm doing and just corresponding the Sindhi key with the value of 100.

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So that's how you can add values and key value pairs to an existing dictionary if you ever want to adjust

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a value pair.

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Let's say John got a raise then you need to do is say salaries call the existing He says John and let's

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give them a raise.

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So there's two ways of doing this.

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One is I could just set equal to a new value saying let's say John's new salary is 30 or I can actually

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reference the old value I could say.

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OK.

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Grab John's old salary value add that add let's say 42 it so got a big raise there.

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Let's check if that worked.

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So I'm going to create that dictionary of salaries and then I'm going to print John's salary right after

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we make it so we should see 20.

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And then we're going to leave this in the line here that I'm going to give him a raise.

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And then it prints his salary again.

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So we should see 20 and then 60 when I run this.

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And there it is 20 and then 60.

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So it starts off with $20.

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But then I adjust it to his existing salary plus 40 something else I could just set it straight to 40.

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And then if I run if I were to run this again that I would see 20 and 40.

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OK so that's how you can add new key value pairs as well as just old key value pairs.

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Something else you can do which is not super common but it is available to you is passen things like

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dictionaries or lists as values.

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So for example let's say we want this to be defined as people instead.

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So we have people let's say we have John and Sally let's just delete samea off of this and they can

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have multiple aspects linked to them by passing and a list as that value pair.

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So say John and we can give him some numbers like 1 2 3 and then we can say OK salary or excuse me Sally

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is going to have some other metrics like 40 10 and 30.

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Right now these numbers are just made out they don't really represent anything.

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I just want to show you they can actually link a list to this value or this keep pair so that I could

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say OK people John and let's print that out.

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Print people John print that and now and I run this I get back the list.

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One two three which means I can then do something like this.

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I could say Sally and then let's grab that index with zero from this list so I can actually make a stack

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call here.

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So keep in mind just Pozieres video and think what you should see as the output.

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If I ask for people Sally 0.

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OK hopefully understood it's going to be 40 because when I run this what's happening is we first grab

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this list which is people Salli and then we're going to grab in the first item of this list.

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Zero.

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Again these numbers don't really represent anything.

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They're just here for demonstration purposes.

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And if you really want you can also have a nested dictionary.

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So this can be a further dictionary.

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So you could say something like salary 10 and then something like age 30 and let's go ahead and delete

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this.

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So this is now a nested dictionary for John.

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So then I could say people John and then go again and ask for age and then print that out.

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And this is starting to look almost like Jaison if you're familiar with that so I can print this and

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I get back 30.

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So this is an example of a nested dictionary.

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Again calling John and then getting back that dictionary.

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So you can always break this down by steps.

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Let's cut out age there.

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So if I call people John I get back to this dictionary and then off of that dictionary I can then call

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off more keys so I could say something like salary save that run this again and then I get back $10

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per hour as his salary.

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Last thing I want to mention is that there's a few useful pairs or useful method calls you can call

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for.

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So let me show you that and create a very simple dictionary right now.

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We'll say K-1 V-1.

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So say K-1 10 K-2 has a value of 20 and we'll say K3 has a value of 30.

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And if I print out the dictionary I'll see everything.

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So I see this whole dictionary.

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But I can also have methods for this.

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So if I just want the keys of a dictionary I can call keys open close print sees and or turnback the

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dictionary keys if I want just the values of a dictionary.

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I can call it the values run that and then I get back the values and if I want the actual key value

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pairs I can call items and I'll return back this list object of these tuple key value pairs and we're

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going to talk about tuples in the very next lecture.

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So keep that in mind.

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OK so those are keys values and items which are kind of convenient when you want to iterate through

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a dictionary and we'll talk about that when we talk about control flow and for loops.

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But right now an idea should be understanding the syntax how to add new items or key value pairs to

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a dictionary and then those key methods.

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All right if you have any questions feel free to post the Q&A forums.

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We'll see at the next lecture.