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