Preview
Video Player
00:00
00:00
Multidimensional Lists
6:20You can put lists inside of lists. This allows for you to create dimensions like rows and columns. Come explore!
I know that you're really
starting to love lists.
0:00
And it's only a matter of time
before you attempt to put one list
0:03
inside of another list.
0:06
And of course, you can do that,
a list allows you to store any data type.
0:07
And a list is just another data type.
0:11
Now at first, this might not seem
like it would be very beneficial.
0:15
Until you realized that you can use this
list inside of a list concept to represent
0:19
rows and columns.
0:24
The syntax may seem a little
confusing at first.
0:25
So I figured,
0:28
you should get a tiny taste cuz you might
see it out in a wild in code samples.
0:29
Let's take a quick look.
0:33
So let's imagine it's sometime
in your near future and
0:34
you're freelancing with
your Python skills.
0:38
You've been tracking your time and
0:40
your expenses that you incur
traveling to your client's site.
0:42
You've been keeping your work
log here in a spreadsheet.
0:45
So here's an hours tab, these are how many
hours you worked each of those days, and
0:47
here's your travel,
here's how much money it costs you.
0:50
Sometimes you use public transit,
sometimes you work from home, big savings.
0:52
Do you see how it has rows of weeks and
then each one of these columns here,
0:56
these are the days of the week, right?
1:01
So here is Tuesday and here is week two.
1:03
Now let's take a look here
at the values of week one.
1:07
So here we go, here we go,
week one, right?
1:11
You can build a list like
this pretty easily, right?
1:15
I mean,
basically just put some hard brackets and
1:17
some commas between these values, right?
1:19
So picture that list like a value
as part of a bigger list, right?
1:21
So there is item one,
here is item two, here is item three,
1:26
just three lists inside of the list.
1:32
Let's do this.
1:36
Let's go over to our work space and
let's create a new file.
1:37
We're gonna call that expenses.py.
1:42
So go ahead and
check out the teacher's notes.
1:47
I went ahead and converted that worksheet
so that we could use it in Python.
1:49
Go ahead and copy and
paste that, here we go.
1:53
Travel expenses, excellent.
1:57
So we've got for week 1 of Monday,
we got 5, right?
2:01
Let's just make sure, 5,
2.75, 22, 2.75, 22, awesome.
2:05
So let's explore this list
of lists using the shell.
2:11
So I'll say python -i expenses.
2:16
So let's see what happens if we get the
length of that, length of travel expenses?
2:21
3, okay, that makes sense.
2:28
It's a list of lists and
the length of the list,
2:31
the outer list is how many
top level elements there are.
2:34
So there's 1, 2,
2:38
3, there's three lists inside of
this list, just like a normal list.
2:39
I said list a lot didn't I?
2:43
So then I suppose I can access
the elements the way that we always do.
2:46
So I should be able to get back
that first week by just saying
2:50
travel_expenses[0], and
that should get the first week.
2:53
Okay, it did, right,
it pulled that list out.
2:59
There we go.
3:03
Now since that returns a list,
I should be able to chain it.
3:04
Like if I wanted to get this Tuesday,
I wanted to get this 2.75, I could use
3:07
travel_expenses(0) and then chain on
the second element, which is 1, right?
3:12
Starts at 0, so
the first week of 1 should be here, right?
3:17
So let's see what happens.
3:22
Awesome, now this is the syntax
that I was wanting you to see.
3:24
Note how you first select the week,
and then you select the day.
3:28
Now this works, of course,
with any row/column combination.
3:32
These are dimensions, our data is
multi dimensional, sounds outerspacey.
3:35
Currently it's just two dimensions, right?
3:41
So we have the first one,
which is weeks and the second one,
3:43
which is day of the week.
3:47
So, I just got asked to report on how much
travel expenses I am incurring weekly.
3:49
So why don't we loop through the weeks and
then calculate the total?
3:56
So, let's do that up here.
4:00
So we'll say print("Travel Expenses:").
4:01
So let's see,
let's keep track of a counter for
4:08
each week that we go through and
we'll increment that in our loop.
4:10
So we'll say the week_number,
we'll start at week number one, okay?
4:14
So, let's see, each row in that
first dimension represented a week.
4:19
So, let's name our variable that,
that makes the most sense, right?
4:25
So, for a week in travel_expenses and
then each time through,
4:28
it's going to pull that row.
4:33
So, let's print out that out,
4:35
we'll say print week, oops,
there's a little bullet point there.
4:37
And we'll do week #, and then we'll
4:42
do that was that much money,
and we will format that,
4:47
and pass in the week_number.
4:53
And there's a handy function called
sum that takes an iterable and
4:58
guess what our week is here, it's a list,
so therefore, it's iterable.
5:03
So let's just use it.
5:06
We'll say sum, and
you just pass it in iterable, and
5:07
it will add it all up for us.
5:09
So we will close the format
statement there and
5:11
we'll close the print statement, swesome.
5:13
And now we gotta remember to
increment this week number
5:16
as we go through the loop each time.
5:19
So we'll say week number +=1, save that.
5:21
Let's go ahead, let's drop out here,
let's give this thing a run.
5:25
Awesome, pretty cool, right?
5:33
Wanna know what's even cooler than that?
5:36
You can actually export spreadsheets
into a format that is known as CSV
5:38
which stands for comma separated values.
5:43
And there's a Python module
that lets you read and
5:45
process CSVs and it works much like this,
rows and columns.
5:48
Now, that module is out of
the scope of this course, but
5:52
check the teacher's notes for more.
5:55
Now remember, these lists are still
mutable, so make sure to be explicit
5:57
when you change them and be careful about
changing them while you're looping.
6:02
I'm so glad that you had
a chance to witness the power of
6:06
multi-dimensional lists.
6:09
Hopefully that syntax will not seem merely
as intimidating when you see it next.
6:11
All right, let's get to building our
application right after this quick break.
6:16