Appendix A.3 - Additional Vector Topics

Selecting Vector Elements

On ocassion, we will want to select one or more elements from a vector. In this section, we will discuss two methods for doing as such: Indexing and Slicing. Later in this notebook, we will discuss a more advanced method called boolean masking.

Indexing

If we want to select a specific entry from a vector, we can do so by following the name of the vector with [n], where n indicates the position of the element we wish to select. This position is called the element’s index.

Note: Unlike most languages, R uses 1 as the index of the first element in a vector (as opposed to 0).

my_vec <- c(10, 20, 30, 40, 50, 60, 70, 80, 90)

my_vec[1]

[1] 10

my_vec[3]

[1] 30

Putting a negative sign in from of an index returns a copy of the vector with the element at that position removed.

my_vec[-3]

[1] 10 20 40 50 60 70 80 90

Slicing

We can select multiple elements from a vector by replacing the index with a vector of indices.

my_vec[c(3, 4, 8)]

[1] 30 40 80

my_vec[2:6]

[1] 20 30 40 50 60

We can also use negative indexing to drop several from a vector.

my_vec[-(2:6)]

[1] 10 70 80 90

Vector Comparisons

We can use the operators <, >, <=, =>, and == to compare vectors and scalars. As with other vector operations, this comparison is performed elementwise. The comparison will return a boolean (TRUE/FALSE) vector of the same size as the original vector.

v <- c(5, 6, 2, 3, 9, 4, 8, 1)

v < 6

[1]  TRUE FALSE  TRUE  TRUE FALSE  TRUE FALSE  TRUE

Conditional Counting

R, like most programming languages, assigns a numerical value of 0 to the boolean value FALSE, and assigns a value of 1 to the boolean value TRUE. This fact allows us to use the sum() function to count the number of elements in a vector that satisfy a certain critera.

sum(v < 6)

[1] 5

Boolean Subsetting

We can select elements of a vector by passing it a seperate boolean vector of the same size. For example, assume that we have a vector v and a boolean vector sel of the same size as v. If we pass sel to v in place of an index, then we will get back only the elements of v that are associated with TRUE values in sel.

w <- c(11, 22, 33, 44, 55)
sel <- c(TRUE, FALSE, TRUE, TRUE, FALSE)
w[sel]

[1] 11 33 44

Boolean Masking

Using a boolean vector might not seem like a particularly convenient way of selecting elements from a vector. However, when combined with a vector comparison, this is an extremely useful tool for selecting elements of a vector that satisfy a certain criteria. This method is called boolean masking.

v <- c(5, 6, 2, 3, 9, 4, 8, 1)
sel <- v < 6
v[sel]

[1] 5 2 3 4 1

The last two lines in the previous cell can be combined into a single line.

v[v < 6]

[1] 5 2 3 4 1