Logical indexing in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is logical indexing in R?

Logical indexing is a way to select elements from a vector or other data structure using a logical vector of TRUE and FALSE values. Only elements corresponding to TRUE are selected.

beginner

How does logical indexing differ from numeric indexing in R?

Numeric indexing uses numbers to select elements by position, while logical indexing uses TRUE/FALSE values to select elements based on a condition.

beginner

Example: What does x[c(TRUE, FALSE, TRUE)] return if x = c(10, 20, 30)?

It returns c(10, 30) because the first and third positions are TRUE, so those elements are selected.

intermediate

How can logical indexing be used with conditions?

You can create a logical vector by testing a condition on elements, like x > 5, and use it to select only elements that meet the condition.

intermediate

What happens if the logical vector is shorter than the vector being indexed?

The logical vector is recycled (repeated) to match the length of the vector being indexed.

What does logical indexing select in R?

A Elements at numeric positions

B Elements where the logical vector is FALSE

C Elements where the logical vector is TRUE

D All elements

Logical indexing selects elements corresponding to TRUE values in the logical vector.

If x = c(5, 10, 15), what does x[x > 7] return?

A c(10, 15)

B c(TRUE, TRUE, TRUE)

C c(5)

D c(5, 10, 15)

x > 7 creates a logical vector c(FALSE, TRUE, TRUE), so x[x > 7] returns elements 10 and 15.

What happens if the logical vector is shorter than the vector being indexed?

A The logical vector is recycled to match the length

B An error occurs

C Only the first elements are selected

D No elements are selected

R recycles the shorter logical vector to match the length of the vector being indexed.

Which of these is a valid logical index for x = c(1, 2, 3, 4)?

A c(1, 3)

B c(TRUE, FALSE, TRUE, FALSE)

C c('a', 'b')

D c(0, 1, 0, 1)

Logical indexing requires a logical vector of TRUE/FALSE values.

How can you select all elements of x that are equal to 10 using logical indexing?

A x[c(10)]

B x[10]

C x == 10

D x[x == 10]

x == 10 creates a logical vector; using it inside x[...] selects elements equal to 10.

Explain how logical indexing works in R and give a simple example.

Think about how TRUE and FALSE values pick elements from a vector.

Describe what happens when the logical vector used for indexing is shorter than the vector being indexed.

Consider how R handles vectors of different lengths.