Subsetting

Question 1

[

Answer 1

Always returns an object of the same clas

Can be used to select more than one element

Question 2

[[

Answer 2

Used to extract elements of a list or a data frame

Class not necessarily the same as the initial object

Question 3

$

Answer 3

Used to extract elements by name

Semantics similar to that of [[]]

Question 4

How do I return the third element of vector x?

Answer 4

x[3]

Question 5

How do I return the 4th and 5th elements of vector x?

Answer 5

x[4:5]

Question 6

How do I return the 3rd and 7th elements of vector y?

Answer 6

y[c(3,7)]

Question 7

How do I return all elements greater than 4 in the vector t?

Answer 7

t[t > 4]

Question 8

How do I create a logical vector from x, given the condition: x == ‘c’?

Answer 8

u <- x == ‘c’

Question 9

How do I extract the first row from the matrix m?

Answer 9

m[1,]

Question 10

How do I return a matrix for element in the first row, second column, instead of returning a vector?

Answer 10

m[1, 2, drop=FALSE]

Note that the drop argument can only be passed to the [] subsetting operation, and has no effect when passed to [[]] (which always returns a vector)

Question 11

How do you extract the foo element of x by name using the $ syntax?

Answer 11

x$foo

Question 12

How do apply names to elements in a vector x with two elements?

Answer 12

names(x) <- c(“first”, “second”)

Question 13

What are two nice things about naming elements, and referencing elements by name?

Answer 13

Makes for cleaner code

Don’t have to remember where in the list/vector an element is

Question 14

What two things can you do with the [[]] syntax that you cannot do with the $ syntax?

Answer 14

Extract a sequence of elements

Allowing the use of computed indices

Question 15

If you had a named element in x called “foo”, how would you perform partial matching to return the element “foo” with the [[]] syntax?

Answer 15

x[[“f”, exact = FALSE]]

Question 16

What does the R code for removing NA’s in a vector look like?

Answer 16

x <- is.na(x)

x[!bad]

Question 17

Describe the function of complete.cases

Answer 17

a logical vector with the value TRUE for rows that are complete, and FALSE for rows that have some NA values. To remove the rows with missing data from airquality, try the following:

> x <- airquality[complete.cases(airquality),]
> str(x)

Question 18

Given x <- list(2, “a”, “b”, TRUE)

Answer 18

x returns the entire list
x[2] returns a list which contains only the second element of the list
x[[2]] returns only the second element of the list: a character vector

Question 19

na.omit()

Answer 19

Returns a new dataset without any missing (NA) values