Coding basics - Extra Practice

• You don’t have to do all of these. (Or any, for that matter). They are here for you to do as many as you need to get the hang of it, to practice for quizzes, etc.
• Skip around if you like.
• These questions practice the R coding basics from the Taxonomy of Data tutorial: arithmetic, assignment, object classes, functions, vectors, data frames, and knowing when R prints output.

Arithmetic

Exercise 1

What does R return?

4 + 7 * 2

Show answer

R returns 18. Multiplication happens before addition: 7 * 2 is 14, then 4 + 14 is 18.

Exercise 2

What does R return?

(4 + 7) * 2

Show answer

R returns 22. Parentheses happen first: 4 + 7 is 11, then 11 * 2 is 22.

Exercise 3

What does R return?

2 ^ 3 + 1

Show answer

R returns 9. The ^ symbol is an exponent, so 2 to the third power. Exponents happen before addition: 2 ^ 3 is 8, then 8 + 1 is 9.

Exercise 4

Write one line of R code that computes the number of minutes in 3.5 hours.

Show answer

3.5 * 60

This returns 210.

Exercise 5

Write one line of R code that computes the natural log of 100. Hint: the ln function takes the natural log (log base \(e\))

Show answer

log(100)

By default, log() computes the natural logarithm.

Exercise 6

Now compute the log base 10 of 100. There are two solutions, try to guess and figure it out.

Show answer

Solution 1:

log10(100)

This correctly returns 2. log10 is a native R function for taking logs of base 10.

Solution 2:

Alternatively, you can take logs of ANY base by passing (regular) log a second argument, e.g.

log(100, 10)

This is saying \(log_{10}{100}\)

Another example with a different base:

log(16, 2)

This computes \(log_{2}{16}\), which is 4.

Exercise 7

What does R return?

10 / 2 + 3 * 4

Show answer

R returns 17. Division and multiplication happen before addition: 10 / 2 is 5, 3 * 4 is 12, and 5 + 12 is 17.

Assignment and Changing Values

Exercise 8

After running these lines, what is stored in x?

x <- 5
x + 3

Show answer

This will print 8 to the screen, but x is still 5. The second line looks up what x is, adds 3 to it, and prints the result. x will not change unless you set it to something new with <-.

Exercise 9

After running these lines, what is stored in total?

coffee <- 20
snack <- 6
total <- coffee + snack

Show answer

total stores 26.

Exercise 10

After running these lines, what is stored in total?

coffee <- 20
snack <- 6
total <- coffee + snack
snack <- 10

Show answer

total still stores 26. Changing snack later does not automatically rerun the earlier assignment to total.

Exercise 11

After running these lines, what is stored in total?

coffee <- 20
snack <- 6
total <- coffee + snack
snack <- 10
total <- coffee + snack

Show answer

total stores 30. The final line reruns the calculation using the current values of coffee and snack.

Exercise 12

Which of these are valid object names in R?

budget
budget_2026
2026_budget
my budget
coffee2

Show answer

Valid names: budget, budget_2026, and coffee2.

Invalid names: 2026_budget starts with a number, and my budget uses a space, which makes R interpret it as two different things: my and budget

Exercise 13

Write code that stores the value 1800 in an object named rent, then stores five times this value semester_rent.

Show answer

rent <- 1800
semester_rent <- rent * 5

After this code, semester_rent stores 9000.

Exercise 14

After running these lines, what is stored in a and b?

a <- 3
b <- a + 4
a <- 10

Show answer

a stores 10 and b stores 7. The assignment to b used the old value of a, and b does not update automatically when a changes.

Object Classes and Data Types

Exercise 15

What class will R report?

class(42)

Show answer

R reports "numeric".

Exercise 16

What class will R report?

class("42")

Show answer

R reports "character". The quotation marks make "42" a blob of text, not a number.

Exercise 17

What happens if you run this code?

"42" + 8

Show answer

R gives an error because "42" is a character string (text blob). Arithmetic works on numbers, not strings. It’s like trying to to add “apple” to the number 6. It makes no sense.

The specific error it gives is: “Error in”42” + 8 : non-numeric argument to binary operator”

The + is the “binary operator.” It does something (“operates”) on two different quantities (hence, “binary”). It’s saying that it needs both things to be numbers - not one text blob and one number.

Exercise 18

What class will R report?

class(c("Adelie", "Gentoo", "Chinstrap"))

Show answer

R reports "character". Even though this is a vector containing many strings, the class function just tells you what kind of “stuff” is in the vector. It doesn’t tell you that it’s a vector.

Exercise 19

What will the resulting dataframe look like? Make a little drawing on paper, or describe it verbally before checking your answer.

df <- data.frame(name = c("Ava", "Ben"), score = c(9, 7))

Show answer

name	score
Ava	9
Ben	7

Functions and Their Responses

Exercise 20

Identify the function(s), argument(s), and return value (if any).

sqrt(49)

Show answer

The function is sqrt(), which is passed is one argument: 49. It will return 7.

Exercise 21

Identify the function(s), argument(s), and return value (if any).

mean(c(10, 20, 30))

Show answer

The first function that you see is mean. It expects one argument: a vector of numbers.

That vector is created right on the spot, using the c function, which is given three arguments: 10, 20, and 30, and it returns a vector (container with [10, 20, 30] in it)

Finally mean can compute the average and returns 20.

Exercise 22

Identify the function(s), argument(s), and return value (if any).

length(c("red", "blue", "green", "blue"))

Show answer

R returns 4, the number of elements in the vector.

There are two functions being called here:

• c which is given four arguments, each of which is a string, and returns a vector with these strings.
• This vector becomes the input for length, which just counts and returns how many things are in the vector (4 here).

Exercise 23

Write one line of code that simultaneously calculates the square root of each of these numbers: 16, 25, and 36.

Show answer

sqrt(c(16, 25, 36))

R returns c(4, 5, 6).

This one is different because of how sqrt works. If you give it just a number (e.g., sqrt(4)) it will give you one number back (2). But if you give it a vector of numbers, it will return a vector with the square roots of all the numbers.

Exercise 24

What does R return?

c(1, 2, 3) * 10

Show answer

R returns c(10, 20, 30). Similar to sqrt, when multiplication * is used on a vector, R multiplies once for each value in the vector.

Exercise 25

What does R return?

c(1, 2, 3) + c(10, 20, 30)

Show answer

R returns c(11, 22, 33). When things like + or * are given two vectors, it returns a vector of the result for each corresponding pair.

Exercise 26

What class will R report?

mixed <- c(1, "two", 3)
class(mixed)

Show answer

R reports "character". A vector must store one basic type, so R converts the numbers to character strings.

Essentially, when you ran mixed <- c(1, "two", 3), R thought, “Well those aren’t all the same type of thing. Can I convert them all to the same type? I guess they can all be text (character strings), so lets do that.”

Run this cell to see the conversion:

Data Frames

Exercise 27

Create a data frame called pets that has information about three different pets: a 4-year-old cat named Milo, a 2-year-old dog named Luna, and a 7-year-old cat named Nori. We’ve written some of it for you, just fill in the blanks. It may help think about what the final dataframe will look like.

Show answer

pets <- data.frame(
  name = c("Milo", "Luna", "Nori"),
  age = c(4, 2, 7),
  species = c("cat", "dog", "cat")
)

You could also do it this way:

name <- c("Milo", "Luna", "Nori")
age <- c(4, 2, 7)
species <- c("cat", "dog", "cat")
pets <- data.frame(name, age, species)

The resulting dataframe will look like:

name	age	species
Milo	5	cat
Luna	2	dog
Nori	7	cat

Exercise 28

In the pets data frame from the previous question, what is the unit of observation?

Show answer

The unit of observation is one pet. Each row stores information about one pet. This may seem stupid/trivial here, but for every dataset you’re ever given, ask yourself: what does one row represent?

Exercise 29

In this modified pets dataframe, what is a unit of observation?

name	date	age	weight_pounds
Milo	1	12	cat
Milo	2	13	cat
Milo	3	13	cat
Luna	2	40	dog
Luna	4	44	dog
Luna	5	45	dog
Nori	7	18	cat

Show answer

It may be that, for example, we have data on many pets through many check-ins with the veterinarian. In which case the dataset may look like:

Exercise 30

Write code to create this data frame.

city	temperature	weather
Berkeley	64	cloudy
Oakland	67	sunny
Richmond	63	foggy

Make city and weather character vectors and temperature numeric.

Show answer

city <- c("Berkeley", "Oakland", "Richmond")
temperature <- c(64, 67, 63)
weather <- c("cloudy", "sunny", "foggy")

weather_df <- data.frame(city, temperature, weather)

Exercise 31

What happens if you try to create this data frame?

data.frame(
  name = c("A", "B", "C"),
  score = c(10, 12)
)

Show answer

R gives an error because the columns have different lengths. There are three names, but only two scores, so R doesn’t have a full table of data. All rows have to have the same number of columns.

Exercise 32

Write code to create this data frame with an ordered factor for year.

name	height	year
Leia	160	sophomore
Luke	170	freshman
Han	182	senior
Lando	178	junior

Show answer

students <- data.frame(
  name = c("Leia", "Luke", "Han", "Lando"),
  height = c(160, 170, 182, 178),
  year = factor(
    c("sophomore", "freshman", "senior", "junior"),
    levels = c("freshman", "sophomore", "junior", "senior"),
    ordered = TRUE
  )
)

What Prints?

Exercise 33

Which lines print output to the console?

x <- c(5, 2)
x
c(5, 2)

Show answer

The second and third lines print output. The assignment line stores a value but does not print it.

Exercise 34

Which lines print output to the console?

scores <- c(10, 20, 30)
mean(scores)
avg <- mean(scores)
avg

Show answer

The second and fourth lines print output. The first and third lines are assignments, so they do not print.

Exercise 35

What appears in the console?

answer <- 2 ^ 4

Show answer

Nothing prints to the console. The value 16 is stored in answer.

Exercise 36

What appears in the console?

answer <- 2 ^ 4
answer

Show answer

R prints 16 on the second line because typing an object’s name asks R to display the object.

Exercise 37

Which lines print output to the console?

species <- factor(c("Adelie", "Gentoo"))
class(species)
levels(species)
penguins <- data.frame(species)

Show answer

The second and third lines print output. The first and fourth lines are assignments, so they do not print.

Exercise 38

Write two lines of code that store c(3, 6, 9) in values and then print the mean to the console.

Show answer

values <- c(3, 6, 9)
mean(values)

The first line stores the vector. The second line prints 6.

Exercise 39

Write two lines of code that store c(3, 6, 9) in values and store the mean in values_mean without printing the mean.

Show answer

values <- c(3, 6, 9)
values_mean <- mean(values)

Both lines are assignments, so neither line prints the mean.

Exercise 40

What class will R report?

resident <- c(TRUE, FALSE, TRUE)
class(resident)

Show answer

R reports "logical". TRUE and FALSE are logical values, not character strings.

Exercise 41

What class will R report?

resident <- c("TRUE", "FALSE", "TRUE")
class(resident)

Show answer

R reports "character". The quotation marks make these strings, not logical values.

BONUS CHALLENGE: Write your own function

Programmers constantly write their own functions to perform repetitive tasks and generally make their work more readable.

The examples below won’t seem very useful yet, but we wanted to expose it to the idea. You might find great use for them later.

Here’s an example (trivial) function that adds two numbers. Read the cell, then run it, then we’ll explain.

Let’s break down the pieces.

addup is the name we are giving our new function. We could have called it anything we wanted - add or banana or whatever - it’s just like a variable.

<- is the assignment operator, saying whatever comes after this is going to be saved under the label addup

function is a special keyword announcing, “I AM NOW GOING TO DEFINE A NEW FUNCTION”

(a, b) says “this function has two arguments - I’m going to refer to them as a and b in my code.” You can pick any names you want for the arguments, whatever is readable.

{ says “now the function starts. whenenver someone calls addup, run this code”

total <- a + b defines a new variable as the sum of a and b

return(total) says “this is my return value - send that back to whoever called me”

} indicates the function is over.

Below are a few other example function definitions:

Here we made the function a little more condensed. We put the math right inside the return statement. But the idea is the same.

This is a function that takes zero arguments! But we still have to write function() (empty parentheses) both in defining the function and in calling it later (sayhi())

Exercise 42

Write a function called spread that takes three arguments (all assumed to be numbers) and returns the difference between the largest and smallest number. The native R functions max and min may be helpful….

To clarify, calling spread(3, 5, 7) should return 4

Show answer

spread <- function(x, y, z) {
  low = min(x, y, z)
  high = max(x, y, z)
  return(high - low)
}