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Addition

7:51 with Craig Dennis

What good are lists if you can't add to them? Let's learn how to append, extend and concatenate.

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Remember Strings are immutable, they cannot be changed. However, Lists are mutable, they can be changed. String methods return a brand new string entirely, because you can't change them. List methods do not return a new list, they modify the existing list, because lists are mutable.

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So now that we've seen how to create a list, let's take a look at a powerful and 0:00

common feature of lists, adding items. 0:04

So, let's go ahead and pop open a REPL. 0:06

So I'm gonna open up the Python Shell with Python. 0:08

And one thing I should note is that lists can store any object, any data type. 0:12

So, let's say I wanted to store my temperature over time. 0:17

As I went to the doctor, they can just add to my records, right? 0:21

So, let's create an empty list to store these readings. 0:26

So we'll say temperatures =, and I'm gonna do an empty list, 0:29

and again that's [ ], empty list, awesome. 0:35

So now, let's add my temperature to that. 0:41

So what we wanna do is, we want to append a value to our list. 0:44

So it'll say temperatures and there's a method that allows you to do this. 0:47

So I'm going to put a dot, because I wanna get to the methods. 0:52

And the method name that allows you to add a value to the end of 0:57

the list is called append. 1:00

So I'm gonna say append. 1:01

And I will pass in the item and the first time I was running (98.6). 1:03

Now, don't worry I'm not super sick, those are in Fahrenheit and 1:09

that's the standard temperature. 1:13

And then the next trip to my doctor they took my temperature, 1:15

I was running a little bit hot. 1:18

So I'm gonna append another value. 1:20

We'll do (99.4), and let's take a look here at our list. 1:21

Here we go, we got two lists in there, just keep on appending and 1:29

wait until the end of the list, right? 1:31

You'll note that when I'm appending these values, I'm not being returned a new list. 1:34

I'm actually modifying the existing list in place. 1:38

That's because lists are mutable, you're able to change them. 1:42

So I end up catching a flu. 1:46

And it was so bad that I ended up the emergency room. 1:48

And they stored their temperatures there for me. 1:51

And this is what it look like in there, it was pretty bad. 1:55

So these are my er_temps. 1:56

And I'm going to create a new list. 1:58

I was at [102.2, that's really high if you don't speak in Fahrenheit. 2:00

And then I was 101.1 and started going back down to 99.9]. 2:06

So, when I came back to my doctor, I wanted to combine the two lists, right? 2:09

It's the same data that we're talking about, 2:15

it's just from two different sources. 2:16

So, if you wanna add all items from one list onto an existing list, 2:18

you can use a method known as Extend. 2:23

So I'm gonna use my original list, which was temperatures. 2:27

I'm going to extend that with my other value, which is (er_temps). 2:30

Now, if I take a look at temperatures, 2:39

you'll see that I have both of those lists together, right? 2:41

It just appending these all at the end, it extended the list. 2:44

Now sometimes, extending a list isn't exactly what you want. 2:49

You might wanna have a combination of the two lists, but 2:53

not modify either one of them. 2:56

You want a brand new list and leave the previous lists alone. 2:57

This is where concatenation comes into play. 3:02

Concatenation works with the plus sign and it's just like string concatenation. 3:04

So here, let's do this. 3:09

We'll say that I have some primary_care_doctors. 3:11

You might of heard of these doctors. 3:13

Go to a doctor called ["Dr Scholls"], and my other go to doctor is "Dr Pepper"]. 3:16

And when I was at the I didn't catch their names, all I caught was their first names. 3:23

And so, it was ["Doug", and "Susan"], I was super out of it, 3:29

they were really super attractive though. 3:32

But I don't wanna change my primary care doctors, right? 3:37

Suppose I wanted to show a list of everybody who helped me through this 3:41

flu season. 3:45

What I can do is, I can concatenate those two lists together. 3:45

So if I wanted to say all_doctors. 3:49

And I could say, I wanted to use my primary_care_doctors and 3:52

I wanted to use my er_doctors. 3:57

That created a brand new list and stuck it in all_doctors. 4:00

So all_doctors is now referring to a list that has both 'Doug' and 'Susan' and 4:03

'Dr Scholls' and 'Dr Pepper' together, but it didn't affect my primary_care_doctors. 4:07

One more thing that I'd like to make sure we remember is that these lists can store 4:13

any data type, right? 4:18

So, if I wanted to store in my temperatures, which before we were just 4:20

putting floats in there, right, cuz they had that numbers. 4:23

If I can append, I can totally append to the (99), it's an integer, but 4:26

it would go in there. 4:29

And I can even actually, nothing's stopping me from putting in a string, 4:30

all right, of like ("Burning up"). 4:33

And if I take a look at temperatures, it's totally gonna be fine. 4:35

There's an integer and a string in this list, the data type doesn't matter. 4:40

Okay, so, back to our meeting example. 4:44

What we were trying to do here is, we were trying to get Ashley to come along. 4:48

She had pinged me and I wanted to add her to these list. 4:52

Now, I could just put her here. 4:55

Let's practice our skills, let's give them a little stretch. 4:56

So let's add her to adhere to this list. 4:59

So what do we do to add her? 5:01

We want her at the end of the list, so we are just going to append("Ashley"). 5:03

There we go. 5:09

And as meetings work, Ashley suggested that we should also have Guil and 5:10

James come as well. 5:14

So, let's extend our list with Ashley's suggestions. 5:15

So she suggested that we add ["James", "Guil"], so that's a list in it's own. 5:19

And we want to just put them at the end there of our list. 5:25

And since this is a list literal, we've created a new list, and 5:31

we want to say attendees.extend(["James", "Guil"]). 5:35

Now, if we accidentally said append, 5:40

it will would actually append a list to our item. 5:42

We'll get to that later, but now the list should be extended and 5:45

James and Guil should be appended to the end. 5:49

Well, you know what, we should probably add some optional invitees, too. 5:51

These meetings tend to grow, don't they? 5:56

So we'll do optional_invitees. 5:58

And these two are super busy, so I'm gonna not suggest that they come, 6:01

but they can come if they want to. 6:05

So we got Ben Jacobin and David Farland, so they're optional invitees here. 6:08

So, you know what now though, our account's gonna be off here, right? 6:13

Cuz this is only looking at the attendees but we might need a bigger room. 6:17

So, let's make sure that we have a large enough meeting space. 6:22

Let's concatenate these two lists. 6:25

So we'll say potential_attendees. 6:28

And we don't want to modify either the optional or the original attendees, 6:31

we just wanna make a new list. 6:36

So we'll say attendees + optional_attendees, and this should be 6:38

everybody that could potentially be there and that we will have a large enough room. 6:45

So let's make sure that this says there 6:50

are a length of potential_attendees, 6:54

say there are potential here. 6:59

We don't need this space because it's built-in for us. 7:03

There we go. 7:06

And let's give that a run. 7:07

I'm gonna drop out of here, and I will type python meeting.py. 7:10

And I spelled something wrong, it looks like, right? 7:16

So NameError: optional_attendees. 7:19

I said invitees. 7:21

So this is optional_invitees. 7:22

There are 8 potential attendees currently, awesome. 7:30

Now, the order of this list that we're working with here doesn't really matter. 7:34

But as you can imagine, there are some list where the order matters. 7:38

And you might not want just to add the end of your list, 7:42

you might want to insert at a specific location. 7:45

Let's take a look at that problem right after this quick break. 7:47

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