Lecture 2

Question 1

What is the syntax for a data.table operation in R?

Answer 1

A: DT[i, j, by] where:

i: Row conditions

j: Column operations

by: Grouping

Question 2

How do you create a data.table in R?

Answer 2

data.table(x = c(), y = c())

Question 3

What function converts a data.frame to a data.table?

Answer 3

as.data.table()

Question 4

Which function is used to load large data files efficiently into a data.table?

Answer 4

fread()

Question 5

How can you access the 2nd row of a data.table?

Answer 5

DT[2]

Question 6

How can you subset rows using multiple conditions?

Answer 6

Use & for AND and | for OR, e.g., DT[AIRLINE == “AA” & DEPARTURE_TIME > 600]

Question 7

How can you select rows where a column value is in a set of values?

Answer 7

Use %in%, e.g., DT[DESTINATION_AIRPORT %in% c(“JFK”, “LGA”)]

Question 8

How can you ensure changes in a data.table do not affect the original data?

Answer 8

Use copy(), e.g., new_DT <- copy(DT)

Question 9

How do you access a specific column by name?

Answer 9

DT[, COLUMN_NAME]

Question 10

How do you access multiple columns as a data.table?

Answer 10

DT[, .(col1, col2)]

Question 11

How do you add a new column in a data.table?

Answer 11

Use :=, e.g., DT[, NEW_COLUMN := OLD_COLUMN * 2]

Question 12

How do you remove a column in a data.table?

Answer 12

DT[, COLUMN_NAME := NULL]

Question 13

What does .N represent in data.table?

Answer 13

.N is a built-in variable that counts the number of rows in the table or group.

Question 13

What is the purpose of the by argument in data.table?

Answer 13

It is used for grouped operations, e.g., DT[, .(mean_col = mean(COLUMN)), by = GROUP_COLUMN]

Question 14

How do you calculate the mean of a column in a data.table?

Answer 14

DT[, mean(COLUMN_NAME, na.rm = TRUE)]

Question 15

What is the output of rep(6:9, 2) ?
a. 6 7 8 9 6 7 8 9
b. 6 6 7 7 8 8 9 9
c. “6”, “7”, “8”, “9”, “6”, “7”, “8”, “9”
d. “6”, “6”, “7”, “7”, “8”, “8”, “9”, “9”

Answer 15

The correct answer is A

rep(6:9, 2)
[1] 6 7 8 9 6 7 8 9

answer B is wrong because rep(6:9, 2) repeats the sequence “6 7 8 9” twice, not each number of the sequence individually.
answer C and D are wrong because their elements are characters (and not integers)

Question 16

What is the output of the following code: c(3 != sqrt(9), TRUE == (3 > 8))?
a. FALSE TRUE
b. TRUE
c. FALSE FALSE
d. FALSE

Answer 16

The correct answer is C

c(3 != sqrt(9), TRUE == (3 > 8))
[1] FALSE FALSE

NOTE: in order to understand its logic better, the expression could be simplified to:
c(3 != 3, TRUE == FALSE)

Question 17

Let x <- c(1, 6, 3, 2). What is the output of sort(x)?
a. 1 2 3 6
b. 6 3 2 1
c. 1 4 3 2
d. 2 3 4 1