02_python_lists

CHAPTER TWO: Python Lists

=============Create a list
As opposed to int, bool etc., a list is a compound data type; you can group 
values together:

a = "is"
b = "nice"
my_list = ["my", "list", a, b]
After measuring the height of your family, you decide to collect some 
information on the house you're living in. The areas of the different parts of 
your house are stored in separate variables for now, as shown in the script.

Instructions

Create a list, areas, that contains the area of the hallway (hall), 
kitchen (kit), living room (liv), bedroom (bed) and bathroom (bath), in this 
order. Use the predefined variables.
Print areas with the print() function.

# area variables (in square meters)
hall = 11.25
kit = 18.0
liv = 20.0
bed = 10.75
bath = 9.50

# Create list areas
areas = [hall, kit, liv, bed, bath]

# Print areas
print(areas)

============Create list with different types
A list can contain any Python type. Although it's not really common, a list can 
also contain a mix of Python types including strings, floats, booleans, etc.

The printout of the previous exercise wasn't really satisfying. It's just a list 
of numbers representing the areas, but you can't tell which area corresponds to 
which part of your house.

The code in the editor is the start of a solution. For some of the areas, the 
name of the corresponding room is already placed in front. Pay attention here! 
"bathroom" is a string, while bath is a variable that represents the float 9.50 
you specified earlier.

Instructions

Finish the code that creates the areas list. Build the list so that the list 
first contains the name of each room as a string and then its area. In other 
words, add the strings "hallway", "kitchen" and "bedroom" at the appropriate 
locations.
Print areas again; is the printout more informative this time?

# area variables (in square meters)
hall = 11.25
kit = 18.0
liv = 20.0
bed = 10.75
bath = 9.50

# Adapt list areas
areas = ['hallway', hall, 'kitchen', kit, "living room", liv, 'bedroom', 
		bed, "bathroom", bath]

# Print areas
print(areas)

=============Select the valid list
A list can contain any Python type. But a list itself is also a Python type. 
That means that a list can also contain a list! Python is getting funkier by 
the minute, but fear not, just remember the list syntax:

my_list = [el1, el2, el3]
Can you tell which ones of the following lines of Python code are valid ways to 
build a list?

A. [1, 3, 4, 2] B. [[1, 2, 3], [4, 5, 7]] C. [1 + 2, "a" * 5, 3]

ANSWER: A, B, C

============List of lists
As a data scientist, you'll often be dealing with a lot of data, and it will 
make sense to group some of this data.

Instead of creating a flat list containing strings and floats, representing the 
names and areas of the rooms in your house, you can create a list of lists. The 
script in the editor can already give you an idea.

Don't get confused here: "hallway" is a string, while hall is a variable that 
represents the float 11.25 you specified earlier.

Instructions

Finish the list of lists so that it also contains the bedroom and bathroom 
data. Make sure you enter these in order!
Print out house; does this way of structuring your data make more sense?
Print out the type of house. Are you still dealing with a list?

# area variables (in square meters)
hall = 11.25
kit = 18.0
liv = 20.0
bed = 10.75
bath = 9.50

# house information as list of lists
house = [["hallway", hall],
         ["kitchen", kit],
         ["living room", liv],
         ["bedroom", bed],
         ["bathroom", bath]]

# Print out house
print(house)

# Print out the type of house
print(type(house))

=============Subset and conquer
Subsetting Python lists is a piece of cake. Take the code sample below, which 
creates a list x and then selects "b" from it. Remember that this is the second 
element, so it has index 1. You can also use negative indexing.

x = ["a", "b", "c", "d"]
x[1]
x[-3] # same result!
Remember the areas list from before, containing both strings and floats? Its 
definition is already in the script. Can you add the correct code to do some 
Python subsetting?

Instructions

Print out the second element from the areas list (it has the value 11.25).
Subset and print out the last element of areas, being 9.50. Using a negative 
index makes sense here!
Select the number representing the area of the living room (20.0) and print it 
out.

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 10.75, "bathroom", 9.50]

# Print out second element from areas
print(areas[1])

# Print out last element from areas
print(areas[-1])

# Print out the area of the living room
print(areas[5])

================Subset and calculate
After you've extracted values from a list, you can use them to perform 
additional calculations. Take this example, where the second and fourth 
element of a list x are extracted. The strings that result are pasted 
together using the + operator:

x = ["a", "b", "c", "d"]
print(x[1] + x[3])

Instructions

Using a combination of list subsetting and variable assignment, create a new 
variable, eat_sleep_area, that contains the sum of the area of the kitchen and 
the area of the bedroom.
Print the new variable eat_sleep_area.

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 10.75, "bathroom", 9.50]

# Sum of kitchen and bedroom area: eat_sleep_area
eat_sleep_area = areas[3] + areas[-3]

# Print the variable eat_sleep_area
print(eat_sleep_area)

==============Slicing and dicing
Selecting single values from a list is just one part of the story. It's also 
possible to slice your list, which means selecting multiple elements from your 
list. Use the following syntax:

my_list[start:end]
The start index will be included, while the end index is not.

The code sample below shows an example. A list with "b" and "c", corresponding 
to indexes 1 and 2, are selected from a list x:

x = ["a", "b", "c", "d"]
x[1:3]
The elements with index 1 and 2 are included, while the element with index 3 is 
not.

Instructions

Use slicing to create a list, downstairs, that contains the first 6 elements of 
areas.
Do a similar thing to create a new variable, upstairs, that contains the last 4 
elements of areas.

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 
		10.75, "bathroom", 9.50]

# Use slicing to create downstairs
downstairs = areas[0:6]

# Use slicing to create upstairs
upstairs = areas[-4:]

# Print out downstairs and upstairs
print(downstairs)
print(upstairs)

==============Slicing and dicing (2)
In the video, Hugo first discussed the syntax where you specify both where to 
begin and end the slice of your list:

my_list[begin:end]
However, it's also possible not to specify these indexes. If you don't specify 
the begin index, Python figures out that you want to start your slice at the 
beginning of your list. If you don't specify the end index, the slice will go 
all the way to the last element of your list. To experiment with this, try the 
following commands in the IPython Shell:

x = ["a", "b", "c", "d"]
x[:2]
x[2:]
x[:]
Print both downstairs and upstairs using print().

Instructions

Create downstairs again, as the first 6 elements of areas. This time, simplify 
the slicing by omitting the begin index.
Create upstairs again, as the last 4 elements of areas. This time, simplify the 
slicing by omitting the end index.

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 10.75, "bathroom", 9.50]

# Alternative slicing to create downstairs
downstairs = areas[:6]

# Alternative slicing to create upstairs
upstairs = areas[-4:]

===============Subsetting lists of lists
You saw before that a Python list can contain practically anything; even other 
lists! To subset lists of lists, you can use the same technique as before: 
square brackets. Try out the commands in the following code sample in the 
IPython Shell:

x = [["a", "b", "c"],
     ["d", "e", "f"],
     ["g", "h", "i"]]
x[2][0]
x[2][:2]
x[2] results in a list, that you can subset again by adding additional square 
brackets.

What will house[-1][1] return? house, the list of lists that you created 
before, is already defined for you in the workspace. You can experiment with 
it in the IPython Shell.

Instructions

ANSWER: A float: the bathroom area

=============Replace list elements
Replacing list elements is pretty easy. Simply subset the list and assign new 
values to the subset. You can select single elements or you can change entire 
list slices at once.

Use the IPython Shell to experiment with the commands below. Can you tell 
what's happening and why?

x = ["a", "b", "c", "d"]
x[1] = "r"
x[2:] = ["s", "t"]
For this and the following exercises, you'll continue working on the areas list 
that contains the names and areas of different rooms in a house.

Instructions

Update the area of the bathroom area to be 10.50 square meters instead of 9.50.
Make the areas list more trendy! Change "living room" to "chill zone".

# Create the areas list
areas = ["hallway", 11.25, "kitchen", 18.0, "living room", 20.0, "bedroom", 
		10.75, "bathroom", 9.50]

# Correct the bathroom area
areas[-1] = 10.50

# Change "living room" to "chill zone"
areas[4] = "chill zone"

===========Extend a list
If you can change elements in a list, you sure want to be able to add elements 
to it, right? You can use the + operator:

x = ["a", "b", "c", "d"]
y = x + ["e", "f"]

You just won the lottery, awesome! You decide to build a poolhouse and a 
garage. Can you add the information to the areas list?

Instructions

Use the + operator to paste the list ["poolhouse", 24.5] to the end of the 
areas list. Store the resulting list as areas_1.
Further extend areas_1 by adding data on your garage. Add the string "garage" 
and float 15.45. Name the resulting list areas_2.

# Create the areas list and make some changes
areas = ["hallway", 11.25, "kitchen", 18.0, "chill zone", 20.0,
         "bedroom", 10.75, "bathroom", 10.50]

# Add poolhouse data to areas, new list is areas_1
areas_1 = areas + ["poolhouse", 24.5]

# Add garage data to areas_1, new list is areas_2
areas_2 = areas_1 + ["garage", float(15.45)]

===========Delete list elements
Finally, you can also remove elements from your list. You can do this with the 
del statement:

x = ["a", "b", "c", "d"]
del(x[1])
Pay attention here: as soon as you remove an element from a list, the indexes 
of the elements that come after the deleted element all change!

The updated and extended version of areas that you've built in the previous 
exercises is coded below. You can copy and paste this into the IPython Shell 
to play around with the result.

areas = ["hallway", 11.25, "kitchen", 18.0,
        "chill zone", 20.0, "bedroom", 10.75,
         "bathroom", 10.50, "poolhouse", 24.5,
         "garage", 15.45]
There was a mistake! The amount you won with the lottery is not that big after 
all and it looks like the poolhouse isn't going to happen. You decide to remove 
the corresponding string and float from the areas list.

The ; sign is used to place commands on the same line. The following two code 
chunks are equivalent:

# Same line
command1; command2

# Separate lines
command1
command2

Which of the code chunks will do the job for us?

Instructions

ANSWER: del(areas[-4:-2])

============Inner workings of lists
At the end of the video, Hugo explained how Python lists work behind the 
scenes. In this exercise you'll get some hands-on experience with this.

The Python code in the script already creates a list with the name areas and a 
copy named areas_copy. Next, the first element in the areas_copy list is 
changed and the areas list is printed out. If you hit Run Code you'll see that, 
although you've changed areas_copy, the change also takes effect in the areas 
list. That's because areas and areas_copy point to the same list.

If you want to prevent changes in areas_copy from also taking effect in areas, 
you'll have to do a more explicit copy of the areas list. You can do this with 
list() or by using [:].

Instructions

Change the second command, that creates the variable areas_copy, such that 
areas_copy is an explicit copy of areas. After your edit, changes made to 
areas_copy shouldn't affect areas. Submit the answer to check this.

# Create list areas
areas = [11.25, 18.0, 20.0, 10.75, 9.50]

# Create areas_copy
areas_copy = list(areas[:])

# Change areas_copy
areas_copy[0] = 5.0

# Print areas
print(areas)

==============