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Addition
7:51What good are lists if you can't add to them? Let's learn how to append, extend and concatenate.
So now that we've seen how to create a
list, let's take a look at a powerful and
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common feature of lists, adding items.
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So, let's go ahead and pop open a REPL.
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So I'm gonna open up
the Python Shell with Python.
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And one thing I should note is that lists
can store any object, any data type.
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So, let's say I wanted to store
my temperature over time.
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As I went to the doctor,
they can just add to my records, right?
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So, let's create an empty
list to store these readings.
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So we'll say temperatures =,
and I'm gonna do an empty list,
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and again that's [ ], empty list, awesome.
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So now, let's add my temperature to that.
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So what we wanna do is,
we want to append a value to our list.
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So it'll say temperatures and there's
a method that allows you to do this.
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So I'm going to put a dot,
because I wanna get to the methods.
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And the method name that allows
you to add a value to the end of
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the list is called append.
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So I'm gonna say append.
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And I will pass in the item and
the first time I was running (98.6).
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Now, don't worry I'm not super sick,
those are in Fahrenheit and
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that's the standard temperature.
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And then the next trip to my
doctor they took my temperature,
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I was running a little bit hot.
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So I'm gonna append another value.
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We'll do (99.4), and
let's take a look here at our list.
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Here we go, we got two lists in there,
just keep on appending and
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wait until the end of the list, right?
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You'll note that when I'm appending these
values, I'm not being returned a new list.
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I'm actually modifying
the existing list in place.
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That's because lists are mutable,
you're able to change them.
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So I end up catching a flu.
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And it was so
bad that I ended up the emergency room.
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And they stored their
temperatures there for me.
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And this is what it look like in there,
it was pretty bad.
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So these are my er_temps.
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And I'm going to create a new list.
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I was at [102.2, that's really high
if you don't speak in Fahrenheit.
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And then I was 101.1 and
started going back down to 99.9].
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So, when I came back to my doctor,
I wanted to combine the two lists, right?
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It's the same data that
we're talking about,
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it's just from two different sources.
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So, if you wanna add all items from
one list onto an existing list,
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you can use a method known as Extend.
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So I'm gonna use my original list,
which was temperatures.
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I'm going to extend that with my
other value, which is (er_temps).
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Now, if I take a look at temperatures,
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you'll see that I have both of
those lists together, right?
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It just appending these all at the end,
it extended the list.
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Now sometimes, extending a list
isn't exactly what you want.
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You might wanna have a combination
of the two lists, but
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not modify either one of them.
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You want a brand new list and
leave the previous lists alone.
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This is where concatenation
comes into play.
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Concatenation works with the plus sign and
it's just like string concatenation.
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So here, let's do this.
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We'll say that I have some
primary_care_doctors.
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You might of heard of these doctors.
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Go to a doctor called ["Dr Scholls"], and
my other go to doctor is "Dr Pepper"].
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And when I was at the I didn't catch their
names, all I caught was their first names.
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And so, it was ["Doug", and
"Susan"], I was super out of it,
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they were really super attractive though.
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But I don't wanna change my
primary care doctors, right?
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Suppose I wanted to show a list of
everybody who helped me through this
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flu season.
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What I can do is, I can concatenate
those two lists together.
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So if I wanted to say all_doctors.
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And I could say, I wanted to
use my primary_care_doctors and
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I wanted to use my er_doctors.
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That created a brand new list and
stuck it in all_doctors.
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So all_doctors is now referring to a list
that has both 'Doug' and 'Susan' and
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'Dr Scholls' and 'Dr Pepper' together, but
it didn't affect my primary_care_doctors.
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One more thing that I'd like to make sure
we remember is that these lists can store
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any data type, right?
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So, if I wanted to store in my
temperatures, which before we were just
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putting floats in there, right,
cuz they had that numbers.
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If I can append, I can totally append
to the (99), it's an integer, but
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it would go in there.
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And I can even actually, nothing's
stopping me from putting in a string,
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all right, of like ("Burning up").
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And if I take a look at temperatures,
it's totally gonna be fine.
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There's an integer and a string in this
list, the data type doesn't matter.
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Okay, so, back to our meeting example.
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What we were trying to do here is, we
were trying to get Ashley to come along.
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She had pinged me and
I wanted to add her to these list.
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Now, I could just put her here.
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Let's practice our skills,
let's give them a little stretch.
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So let's add her to adhere to this list.
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So what do we do to add her?
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We want her at the end of the list, so
we are just going to append("Ashley").
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There we go.
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And as meetings work, Ashley suggested
that we should also have Guil and
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James come as well.
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So, let's extend our list
with Ashley's suggestions.
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So she suggested that we add ["James",
"Guil"], so that's a list in it's own.
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And we want to just put them
at the end there of our list.
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And since this is a list literal,
we've created a new list, and
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we want to say attendees.extend(["James",
"Guil"]).
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Now, if we accidentally said append,
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it will would actually
append a list to our item.
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We'll get to that later, but
now the list should be extended and
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James and
Guil should be appended to the end.
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Well, you know what, we should probably
add some optional invitees, too.
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These meetings tend to grow, don't they?
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So we'll do optional_invitees.
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And these two are super busy, so
I'm gonna not suggest that they come,
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but they can come if they want to.
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So we got Ben Jacobin and David Farland,
so they're optional invitees here.
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So, you know what now though,
our account's gonna be off here, right?
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Cuz this is only looking at the attendees
but we might need a bigger room.
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So, let's make sure that we have
a large enough meeting space.
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Let's concatenate these two lists.
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So we'll say potential_attendees.
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And we don't want to modify either
the optional or the original attendees,
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we just wanna make a new list.
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So we'll say attendees +
optional_attendees, and this should be
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everybody that could potentially be there
and that we will have a large enough room.
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So let's make sure that this says there
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are a length of potential_attendees,
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say there are potential here.
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We don't need this space
because it's built-in for us.
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There we go.
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And let's give that a run.
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I'm gonna drop out of here, and
I will type python meeting.py.
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And I spelled something wrong,
it looks like, right?
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So NameError: optional_attendees.
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I said invitees.
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So this is optional_invitees.
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There are 8 potential attendees currently,
awesome.
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Now, the order of this list that we're
working with here doesn't really matter.
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But as you can imagine, there are some
list where the order matters.
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And you might not want just
to add the end of your list,
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you might want to insert
at a specific location.
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Let's take a look at that problem
right after this quick break.
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