6 Lists

As opposed to vectors, lists can hold components of different types, just as your to-do lists can contain different categories of tasks. This chapter will teach you how to create, name, and subset these lists.

6.1 Lists, why would you need them?

Congratulations! At this point in the course you are already familiar with:

Vectors (one dimensional array)

can hold numeric, character or logical values. The elements in a vector all have the same data type.
Matrices (two dimensional array)

can hold numeric, character or logical values. The elements in a matrix all have the same data type.
Data frames (two-dimensional objects)

can hold numeric, character or logical values. Within a column all elements have the same data type, but different columns can be of different data type. Pretty sweet for an R newbie, right? ;-)

Instructions `100 XP`

Submit the answer to start learning everything about lists!

6.2 Lists, why would you need them? (2)

A list in R is similar to your to-do list at work or school: the different items on that list most likely differ in length, characteristic, and type of activity that has to be done.

A list in R allows you to gather a variety of objects under one name (that is, the name of the list) in an ordered way. These objects can be matrices, vectors, data frames, even other lists, etc. It is not even required that these objects are related to each other in any way.

You could say that a list is some kind super data type: you can store practically any piece of information in it!

Instructions `100 XP`

Just submit the answer to start the first exercise on lists.

6.3 Creating a list

Let us create our first list! To construct a list you use the function list():

my_list <- list(comp1, comp2 ...)

The arguments to the list function are the list components. Remember, these components can be matrices, vectors, other lists, …

Instructions `100 XP`

Construct a list, named my_list, that contains the variables my_vector, my_matrix and my_df as list components.

ex_53.R

# Vector with numerics from 1 up to 10
my_vector <- 1:10 

# Matrix with numerics from 1 up to 9
my_matrix <- matrix(1:9, ncol = 3)

# First 10 elements of the built-in data frame mtcars
my_df <- mtcars[1:10,]

# Construct list with these different elements:
my_list <- list(my_vector, my_matrix, my_df)

6.4 Creating a named list

Well done, you’re on a roll!

Just like on your to-do list, you want to avoid not knowing or remembering what the components of your list stand for. That is why you should give names to them:

my_list <- list(name1 = your_comp1, name2 = your_comp2)

This creates a list with components that are named name1, name2, and so on. If you want to name your lists after you’ve created them, you can use the names() function as you did with vectors. The following commands are fully equivalent to the assignment above:

my_list <- list(your_comp1, your_comp2) names(my_list) <- c(“name1”, “name2”)

Instructions `100 XP`

Change the code of the previous exercise (see editor) by adding names to the components. Use for my_vector the name vec, for my_matrix the name mat and for my_df the name df.
Print out my_list so you can inspect the output.

ex_54.R

# Vector with numerics from 1 up to 10
my_vector <- 1:10 

# Matrix with numerics from 1 up to 9
my_matrix <- matrix(1:9, ncol = 3)

# First 10 elements of the built-in data frame mtcars
my_df <- mtcars[1:10,]

# Adapt list() call to give the components names
my_list <- list(my_vector, my_matrix, my_df)
names(my_list) <- c("vec", "mat", "df")
# Print out my_list
print(my_list)

6.5 Creating a named list (2)

Being a huge movie fan (remember your job at LucasFilms), you decide to start storing information on good movies with the help of lists.

Start by creating a list for the movie “The Shining”. We have already created the variables mov, act and rev in your R workspace. Feel free to check them out in the console.

Instructions `100 XP`

Complete the code in the editor to create shining_list; it contains three elements:

moviename: a character string with the movie title (stored in mov)
actors: a vector with the main actors’ names (stored in act)
reviews: a data frame that contains some reviews (stored in rev)

Do not forget to name the list components accordingly (names are moviename, actors and reviews).

ex_55.R

# The variables mov, act and rev are available

# Finish the code to build shining_list
shining_list <- 
    list(
        moviename = mov, 
        actors = act,
        reviews = rev
    )

6.6 Selecting elements from a list

Your list will often be built out of numerous elements and components. Therefore, getting a single element, multiple elements, or a component out of it is not always straightforward.

One way to select a component is using the numbered position of that component. For example, to “grab” the first component of shining_list you type

shining_list[[1]]

A quick way to check this out is typing it in the console. Important to remember: to select elements from vectors, you use single square brackets: [ ]. Don’t mix them up!

You can also refer to the names of the components, with [[ ]] or with the $ sign. Both will select the data frame representing the reviews:

shining_list[["reviews"]]
shining_list$reviews

Besides selecting components, you often need to select specific elements out of these components. For example, with shining_list[[2]][1] you select from the second component, actors (shining_list[[2]]), the first element ([1]). When you type this in the console, you will see the answer is Jack Nicholson.

6.6.1 Instructions `100 XP`

Select from shining_list the vector representing the actors. Simply print out this vector.
Select from shining_list the second element in the vector representing the actors. Do a printout like before.

ex_56.R

# shining_list is already pre-loaded in the workspace

# Print out the vector representing the actors
print(shining_list[["actors"]])

# Print the second element of the vector representing the actors
print(shining_list[["actors"]][2])

6.7 Creating a new list for another movie

You found reviews of another, more recent, Jack Nicholson movie: The Departed!

Scores	Comments
4.6	I would watch it again
5	Amazing!
4.8	I liked it
5	One of the best movies
4.2	Fascinating plot

It would be useful to collect together all the pieces of information about the movie, like the title, actors, and reviews into a single variable. Since these pieces of data are different shapes, it is natural to combine them in a list variable.

movie_title, containing the title of the movie, and movie_actors, containing the names of some of the actors in the movie, are available in your workspace.

Instructions `100 XP`

Create two vectors, called scores and comments, that contain the information from the reviews shown in the table.
Find the average of the scores vector and save it as avg_review.
Combine the scores and comments vectors into a data frame called reviews_df.
Create a list, called departed_list, that contains the movie_title, movie_actors, reviews data frame as reviews_df, and the average review score as avg_review, and print it out.

ex_57.R

# Use the table from the exercise to define the comments and scores vectors
scores <- c(4.6, 5, 4.8, 5, 4.2)
comments <- 
    c(
        "I would watch it again",
        "Amazing!", 
        "I liked it",
        "One of the best movies", 
        "Fascinating plot"
    )

# Save the average of the scores vector as avg_review
avg_review <- mean(scores)

# Combine scores and comments into the reviews_df data frame
reviews_df <- data.frame(scores, comments)

# Create and print out a list, called departed_list
departed_list <- list(
    movie_title,
    movie_actors,
    reviews_df,
    avg_review
)
print(departed_list)

--- format: html: theme: darkly fig-width: 8 fig-height: 4 code-fold: true code-link: true highlight-style: dracula code-line-numbers: true code-block-border-left: true code-tools: source: true toggle: false caption: none --- # Lists As opposed to vectors, lists can hold components of different types, just as your to-do lists can contain different categories of tasks. This chapter will teach you how to create, name, and subset these lists. ## Lists, why would you need them? Congratulations! At this point in the course you are already familiar with: - Vectors (one dimensional array) : can hold numeric, character or logical values. The elements in a vector all have the same data type. - Matrices (two dimensional array) : can hold numeric, character or logical values. The elements in a matrix all have the same data type. - Data frames (two-dimensional objects) : can hold numeric, character or logical values. Within a column all elements have the same data type, but different columns can be of different data type. Pretty sweet for an R newbie, right? ;-) ### Instructions `100 XP` {.unnumbered} Submit the answer to start learning everything about lists! ## Lists, why would you need them? (2) A list in R is similar to your to-do list at work or school: the different items on that list most likely differ in length, characteristic, and type of activity that has to be done. A list in R allows you to gather a variety of objects under one name (that is, the name of the list) in an ordered way. These objects can be matrices, vectors, data frames, even other lists, etc. It is not even required that these objects are related to each other in any way. You could say that a list is some kind super data type: you can store practically any piece of information in it! ### Instructions `100 XP` {.unnumbered} Just submit the answer to start the first exercise on lists. ## Creating a list Let us create our first list! To construct a list you use the function `list()`: ```{.r code-line-numbers="false"} my_list <- list(comp1, comp2 ...) ``` The arguments to the list function are the list components. Remember, these components can be matrices, vectors, other lists, … ### Instructions `100 XP` {.unnumbered} Construct a list, named `my_list`, that contains the variables `my_vector`, `my_matrix` and `my_df` as list components. ```{.r filename="ex_53.R"} # Vector with numerics from 1 up to 10 my_vector <- 1:10 # Matrix with numerics from 1 up to 9 my_matrix <- matrix(1:9, ncol = 3) # First 10 elements of the built-in data frame mtcars my_df <- mtcars[1:10,] # Construct list with these different elements: my_list <- list(my_vector, my_matrix, my_df) ``` ## Creating a named list Well done, you're on a roll! Just like on your to-do list, you want to avoid not knowing or remembering what the components of your list stand for. That is why you should give names to them: my_list <- list(name1 = your_comp1, name2 = your_comp2) This creates a list with components that are named name1, name2, and so on. If you want to name your lists after you've created them, you can use the `names()` function as you did with vectors. The following commands are fully equivalent to the assignment above: my_list <- list(your_comp1, your_comp2) names(my_list) <- c("name1", "name2") ### Instructions `100 XP` {.unnumbered} - Change the code of the previous exercise (see editor) by adding names to the components. Use for my_vector the name vec, for my_matrix the name mat and for `my_df` the name df. - Print out `my_list` so you can inspect the output. ```{.r filename="ex_54.R"} # Vector with numerics from 1 up to 10 my_vector <- 1:10 # Matrix with numerics from 1 up to 9 my_matrix <- matrix(1:9, ncol = 3) # First 10 elements of the built-in data frame mtcars my_df <- mtcars[1:10,] # Adapt list() call to give the components names my_list <- list(my_vector, my_matrix, my_df) names(my_list) <- c("vec", "mat", "df") # Print out my_list print(my_list) ``` ## Creating a named list (2) Being a huge movie fan (remember your job at LucasFilms), you decide to start storing information on good movies with the help of lists. Start by creating a list for the movie "The Shining". We have already created the variables `mov`, `act` and `rev` in your R workspace. Feel free to check them out in the console. ### Instructions `100 XP` {.unnumbered} Complete the code in the editor to create `shining_list`; it contains three elements: - `moviename`: a character string with the movie title (stored in mov) - `actors`: a vector with the main actors' names (stored in act) - `reviews`: a data frame that contains some reviews (stored in rev) Do not forget to name the list components accordingly (names are moviename, actors and reviews). ```{.r filename="ex_55.R"} # The variables mov, act and rev are available # Finish the code to build shining_list shining_list <- list( moviename = mov, actors = act, reviews = rev ) ``` ## Selecting elements from a list Your list will often be built out of numerous elements and components. Therefore, getting a single element, multiple elements, or a component out of it is not always straightforward. One way to select a component is using the numbered position of that component. For example, to "grab" the first component of `shining_list` you type ```{.r code-line-numbers="false"} shining_list[[1]] ``` A quick way to check this out is typing it in the console. Important to remember: to select elements from vectors, you use single square brackets: `[ ]`. Don't mix them up! You can also refer to the names of the components, with `[[ ]]` or with the `$` sign. Both will select the data frame representing the reviews: ```{.r code-line-numbers="false"} shining_list[["reviews"]] shining_list$reviews ``` Besides selecting components, you often need to select specific elements out of these components. For example, with `shining_list[[2]][1]` you select from the second component, actors (`shining_list[[2]]`), the first element (`[1]`). When you type this in the console, you will see the answer is Jack Nicholson. ### Instructions `100 XP` - Select from shining_list the vector representing the actors. Simply print out this vector. - Select from shining_list the second element in the vector representing the actors. Do a printout like before. ```{.r filename="ex_56.R"} # shining_list is already pre-loaded in the workspace # Print out the vector representing the actors print(shining_list[["actors"]]) # Print the second element of the vector representing the actors print(shining_list[["actors"]][2]) ``` ## Creating a new list for another movie You found reviews of another, more recent, Jack Nicholson movie: The Departed! |Scores | Comments |- |:--- | 4.6 | I would watch it again | 5 | Amazing! | 4.8 | I liked it | 5 | One of the best movies | 4.2 | Fascinating plot It would be useful to collect together all the pieces of information about the movie, like the title, actors, and reviews into a single variable. Since these pieces of data are different shapes, it is natural to combine them in a list variable. `movie_title`, containing the title of the movie, and `movie_actors`, containing the names of some of the actors in the movie, are available in your workspace. ### Instructions `100 XP` {.unnumbered} - Create two vectors, called `scores` and `comments`, that contain the information from the reviews shown in the table. - Find the average of the `scores` vector and save it as `avg_review`. - Combine the `scores` and `comments` vectors into a data frame called `reviews_df`. - Create a list, called `departed_list`, that contains the ` movie_title`, `movie_actors`, `reviews` data frame as `reviews_df`, and the average review score as `avg_review`, and print it out. ```{.r filename="ex_57.R"} # Use the table from the exercise to define the comments and scores vectors scores <- c(4.6, 5, 4.8, 5, 4.2) comments <- c( "I would watch it again", "Amazing!", "I liked it", "One of the best movies", "Fascinating plot" ) # Save the average of the scores vector as avg_review avg_review <- mean(scores) # Combine scores and comments into the reviews_df data frame reviews_df <- data.frame(scores, comments) # Create and print out a list, called departed_list departed_list <- list( movie_title, movie_actors, reviews_df, avg_review ) print(departed_list) ```

6.1 Lists, why would you need them?

Instructions 100 XP

6.2 Lists, why would you need them? (2)

Instructions 100 XP

6.3 Creating a list

Instructions 100 XP

6.4 Creating a named list

Instructions 100 XP

6.5 Creating a named list (2)

Instructions 100 XP

6.6 Selecting elements from a list

6.6.1 Instructions 100 XP

6.7 Creating a new list for another movie

Instructions 100 XP

Instructions `100 XP`

Instructions `100 XP`

Instructions `100 XP`

Instructions `100 XP`

Instructions `100 XP`

6.6.1 Instructions `100 XP`

Instructions `100 XP`