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Chapter 2
Python Lists
Learn to store, access, and manipulate data in lists: the first step toward efficiently working with huge amounts of data.
nb_of_exercises title
4
Python Lists
nb_of_exercises title
4
Subsetting Lists
nb_of_exercises title
5
Manipulating Lists

Python Lists

type: VideoExercise
key: a5886d213f
xp: 50

@projector_key a0530c4542f10988847b2dbb91f717c3

Create a list

type: NormalExercise
key: e6c527bf41
lang: python
xp: 100
skills:
  - 2

A list is a compound data type; you can group values together, like this:

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 in the exercise.

@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.

@hint

  • You can use the variables that have already been created to build the list: areas = [hall, kit, ...].
  • Make sure to use square brackets [] rather than parentheses ().
  • You don't need to use quotation marks when storing variables within a list.
  • Type print(areas) to print out the list when submitting.

@pre_exercise_code

@sample_code

hall = 11.25
kit = 18.0
liv = 20.0
bed = 10.75
bath = 9.50

# Create list areas


# Print areas

@solution

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)

@sct

predef_msg = "Don't remove or edit the predefined variables!"
areas_msg = "Define `areas` as the list containing all the area variables, in the correct order: `[hall, kit, liv, bed, bath]`. Watch out for typos. The list shouldn't contain anything else!"

Ex().check_correct(
    has_printout(0, not_printed_msg = "__JINJA__:Have you used `{{sol_call}}` to print out the `areas` list at the end of your script?"),
    check_correct(
        check_object("areas").has_equal_value(incorrect_msg = areas_msg),
        multi(
            check_object('hall', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
            check_object('kit', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
            check_object('liv', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
            check_object('bed', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
            check_object('bath', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg)
        )
    )
)

success_msg("Nice! A list is way better here, isn't it?")

Create lists with different types

type: NormalExercise
key: 1702a8bcdc
lang: python
xp: 100
skills:
  - 2

Although it's not really common, a list can also contain a mix of Python types including strings, floats, and booleans.

You're now going to add the room names to your list, so you can easily see both the room name and size together.

Some of the code has been provided for you to get you started. 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?

@hint

  • The first four elements of the list areas are coded as ["hallway", hall, "kitchen", kit, ....
  • A string will need to be in quotation marks "".

@pre_exercise_code

@sample_code

hall = 11.25
kit = 18.0
liv = 20.0
bed = 10.75
bath = 9.50

# Adapt list areas
areas = [____, hall, ____, kit, "living room", liv, ____, bed, "bathroom", bath]

# Print areas
____

@solution

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)

@sct

objs = ["hall", "kit", "liv", "bed", "bath"]
predef_msg = "Don't remove or edit the predefined variables!"
areas_msg = "You didn't assign the correct value to `areas`. Have another look at the instructions. Make sure to place the room name before the variable containing the area each time. The order matters here! Watch out for typos."

Ex().check_correct(
  check_object("areas").has_equal_value(incorrect_msg = areas_msg),
  multi([ check_object(obj, missing_msg = predef_msg).has_equal_value(incorrect_msg = predef_msg) for obj in objs])
)

Ex().has_printout(0, not_printed_msg = "__JINJA__:Have you used `{{sol_call}}` to print out the `areas` list at the end of your script?")

success_msg("Nice! This list contains both strings and floats, but that's not a problem for Python!")

List of lists

type: NormalExercise
key: 9158c577b0
lang: python
xp: 100
skills:
  - 2

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 list containing strings and floats, representing the names and areas of the rooms in your house, you can create a list of lists.

Remember: "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?

@hint

  • Add sublists to the house list by adding ["bedroom", bed] and ["bathroom", bath] inside the square brackets.
  • Remember to include a comma , after each sublist.
  • To print a variable x, write print(x) on a new line.

@pre_exercise_code

@sample_code

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],
        ____,
        ____]

# Print out house
____

@solution

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)

@sct

predef_msg = "Don't remove or edit the predefined variables!"
house_msg = "You didn't assign the correct value to `house`. Have another look at the instructions. Extend the list of lists so it incorporates a list for each pair of room name and room area. Mind the order and typos!"

Ex().check_correct(
    check_object("house").has_equal_value(incorrect_msg = house_msg),
    multi(
        check_object('hall', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
        check_object('kit', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
        check_object('liv', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
        check_object('bed', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg),
        check_object('bath', missing_msg=predef_msg).has_equal_value(incorrect_msg=predef_msg)
    )
)

Ex().has_printout(0, not_printed_msg = "__JINJA__:Have you used `{{sol_call}}` to print out the contents of `house`?")

success_msg("Great! Get ready to learn about list subsetting!")

Subsetting Lists

type: VideoExercise
key: c076b5a69c
xp: 50

@projector_key fc15ba5cb9485456df8589130b519ea3

Subset and conquer

type: NormalExercise
key: c3ce582e32
lang: python
xp: 100
skills:
  - 2

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.

@hint

  • Use x[1] to select the second element of a list x.
  • Use x[-1] to select the last element of a list x.
  • Make sure to wrap your subsetting operations in a print() call.
  • The number representing the area of the living room is the 6th element in the list, so you'll need [5] here. area[4] would show the string!

@pre_exercise_code

@sample_code

# 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[____])

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

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

@solution

# 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])

@sct

msg = "Don't remove or edit the predefined `areas` list."
Ex().check_object("areas", missing_msg = msg).has_equal_value(incorrect_msg = msg)
Ex().has_printout(0, not_printed_msg = "Have another look at your code to print out the second element in `areas`, which is at index `1`.")
Ex().has_printout(1, not_printed_msg = "Have another look at your code to print out the last element in `areas`, which is at index `-1`.")
Ex().has_printout(2, not_printed_msg = "Have another look at your code to print out the area of the living room. It's at index `5`.")
success_msg("Good job!")

Slicing and dicing

type: NormalExercise
key: 7f08642d18
lang: python
xp: 100
skills:
  - 2

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. However, it's also possible not to specify these indexes. If you don't specify the start index, Python figures out that you want to start your slice at the beginning of your list.

@instructions

  • Use slicing to create a list, downstairs, that contains the first 6 elements of areas.
  • Create upstairs, as the last 4 elements of areas. This time, simplify the slicing by omitting the end index.
  • Print both downstairs and upstairs using print().

@hint

  • Use the brackets [0:6] to get the first six elements of a list.
  • To get everything except the first 5 elements of a list, l, you would use l[5:].
  • Add two print() calls to print out downstairs and upstairs.

@pre_exercise_code

@sample_code

# 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[____]

# Use slicing to create upstairs
upstairs = areas[____]

# Print out downstairs and upstairs
____
____

@solution

# 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[6:]

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

@sct

msg = "Don't remove or edit the predefined `areas` list."
Ex().check_object("areas", missing_msg = msg).has_equal_value(incorrect_msg = msg)

patt = "`%s` is incorrect. Use `areas[%s]` and slicing to select the elements you want, or something equivalent."
Ex().check_object("downstairs").has_equal_value(incorrect_msg = patt % ('downstairs', '0:6'))
Ex().check_object("upstairs").has_equal_value(incorrect_msg = patt % ("upstairs",":6"))

Ex().has_printout(0, not_printed_msg="Have you printed out `downstairs` after calculating it?")
Ex().has_printout(1, not_printed_msg="Have you printed out `upstairs` after calculating it?")

success_msg("Great!")

Subsetting lists of lists

type: NormalExercise
key: dbbbd306cf
xp: 100

A Python list can also contain other lists.

To subset lists of lists, you can use the same technique as before: square brackets. This would look something like this for a list, house:

house[2][0]

@instructions

  • Subset the house list to get the float 9.5.

@hint

  • Break this down step by step. First you want to get to the last element of the list, ["bathroom", 9.50]. Recall the index of the last element is -1.
  • Next you want to get the second element of ["bathroom", 9.50], which is at index 1.

@pre_exercise_code

@sample_code

house = [["hallway", 11.25],
         ["kitchen", 18.0],
         ["living room", 20.0],
         ["bedroom", 10.75],
         ["bathroom", 9.50]]

# Subset the house list
house___

@solution

house = [["hallway", 11.25],
         ["kitchen", 18.0],
         ["living room", 20.0],
         ["bedroom", 10.75],
         ["bathroom", 9.50]]

# Subset the house list
house[-1][1]

@sct

Ex().check_or(
  has_code("house[-1][1]", pattern=False),
  has_code("house[4][1]", pattern=False)
)

success_msg("Correctomundo! The last piece of the list puzzle is manipulation.")

Manipulating Lists

type: VideoExercise
key: d7fe818b3a
xp: 50

@projector_key 355ed52d2fb0d67508c6a311b7cbc6d3

Replace list elements

type: NormalExercise
key: 4e1bba1b55
lang: python
xp: 100
skills:
  - 2

To replace list elements, you subset the list and assign new values to the subset. You can select single elements or you can change entire list slices at once.

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 to be 10.50 square meters instead of 9.50 using negative indexing.
  • Make the areas list more trendy! Change "living room" to "chill zone". Don't use negative indexing this time.

@hint

  • To update the bathroom area, identify the subset of the bathroom area (it's the last item of the list!).
  • Then, replace the value with the new bathroom area by assigning it to this subset.
  • Do the same to update the "living room" name, which is at index 4.

@pre_exercise_code

@sample_code

# 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


# Change "living room" to "chill zone"

@solution

# 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"

@sct

bathroom_msg = 'You can use `areas[-1] = 10.50` to update the bathroom area.'
chillzone_msg = 'You can use `areas[4] = "chill zone"` to update the living room name.'
Ex().check_correct(
  check_object('areas').has_equal_value(incorrect_msg = 'Your changes to `areas` did not result in the correct list. Are you sure you used the correct subset operations? When in doubt, you can use a hint!'),
  multi(
    has_equal_value(expr_code='areas[-1]', override=10.50, incorrect_msg = bathroom_msg),
    has_equal_value(expr_code='areas[4]', override='chill zone', incorrect_msg = chillzone_msg),
  )
)
success_msg('Sweet! As the code sample showed, you can also slice a list and replace it with another list to update multiple elements in a single command.')

Extend a list

type: NormalExercise
key: ff0fe8d967
lang: python
xp: 100
skills:
  - 2

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.

@hint

  • Follow the code sample in the assignment. x is areas here, and ["e", "f"] is ["poolhouse", 24.5].
  • To add more elements to areas_1, use areas_1 + ["element", 123].

@pre_exercise_code

@sample_code

# 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 = ____

# Add garage data to areas_1, new list is areas_2
areas_2 = ____

@solution

# Create the areas list (updated version)
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", 15.45]

@sct

msg = "Don't remove or edit the predefined `areas` list."
Ex().check_object("areas", missing_msg = msg).has_equal_value(incorrect_msg = msg)
Ex().check_object("areas_1").has_equal_value(incorrect_msg = "Use `areas + [\"poolhouse\", 24.5]` to create `areas_1`. Watch out for typos!")
Ex().check_object("areas_2").has_equal_value(incorrect_msg = "Use `areas_1 + [\"garage\", 15.45]` to create `areas_2`. Watch out for typos!")
success_msg("Cool! The list is shaping up nicely!")

Delete list elements

type: NormalExercise
key: 85f792356e
xp: 100

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!

Unfortunately, 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'll need to remove it from the list. You decide to remove the corresponding string and float from the areas list.

@instructions

  • Delete the string and float for the "poolhouse" from your areas list.
  • Print the updated areas list.

@hint

  • You'll need to use del twice to delete two elements. Be careful about changing indexes though!

@pre_exercise_code

@sample_code

areas = ["hallway", 11.25, "kitchen", 18.0,
        "chill zone", 20.0, "bedroom", 10.75,
         "bathroom", 10.50, "poolhouse", 24.5,
         "garage", 15.45]

# Delete the poolhouse items from the list


# Print the updated list

@solution

areas = ["hallway", 11.25, "kitchen", 18.0,
        "chill zone", 20.0, "bedroom", 10.75,
         "bathroom", 10.50, "poolhouse", 24.5,
         "garage", 15.45]

# Delete the poolhouse items from the list
del areas[10]
del areas[10]

# Print the updated list
print(areas)

@sct

Ex().check_or(
  multi(
    has_code("del areas[10]", pattern=False),
    has_code("del areas[10]", pattern=False)
  ),
  has_code("del areas[-4:-2]", pattern=False),
  has_code("del(areas[-4:-2])", pattern=False),
  multi(
    has_code("del(areas[10])", pattern=False),
    has_code("del(areas[10])", pattern=False)
  ),
  has_code("del areas[10:12]", pattern=False),
  has_code("del(areas[10:12])", pattern=False),
  multi(
    has_code("del areas[-4]", pattern=False),
    has_code("del areas[-3]", pattern=False)
  ),
  multi(
    has_code("del(areas[-4])", pattern=False),
    has_code("del(areas[-3])", pattern=False)
  )
)

Ex().has_printout(0, not_printed_msg="Have you printed out `areas` after removing the poolhouse string and float?")
success_msg("Correct! You'll learn about easier ways to remove specific elements from Python lists later on.")

Inner workings of lists

type: NormalExercise
key: af72db9915
lang: python
xp: 100
skills:
  - 2

Some code has been provided for you in this exercise: a list with the name areas and a copy named areas_copy.

Currently, the first element in the areas_copy list is changed and the areas list is printed out. If you hit the run code button 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 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.

@hint

  • Change the areas_copy = areas call. Instead of assigning areas, you can assign list(areas) or areas[:].

@pre_exercise_code

@sample_code

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

# Change this command
areas_copy = areas

# Change areas_copy
areas_copy[0] = 5.0

# Print areas
print(areas)

@solution

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

# Change this command
areas_copy = list(areas)

# Change areas_copy
areas_copy[0] = 5.0

# Print areas
print(areas)

@sct

Ex().check_correct(
  check_object("areas_copy").has_equal_value(incorrect_msg = "It seems that `areas_copy` has not been updated correctly."),
  check_function("list", missing_msg = "Make sure to use `list(areas)` to create an `areas_copy`.")
)

mmsg = "Don't remove the predefined `areas` list."
imsg = "Be sure to edit ONLY the copy, not the original `areas` list. Have another look at the exercise description if you're unsure how to create a copy."
Ex().check_correct(
  check_object("areas", missing_msg = mmsg).has_equal_value(incorrect_msg = imsg),
  check_function("list", missing_msg = "Make sure to use `list(areas)` to create an `areas_copy`.")
)

success_msg("Nice! The difference between explicit and reference-based copies is subtle, but can be really important. Try to keep in mind how a list is stored in the computer's memory.")