Negative indexing for exclusion in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does negative indexing do in R when used with vectors?

Negative indexing in R excludes elements at the specified positions from a vector. For example, x[-2] returns all elements except the second one.

beginner

How do you exclude multiple elements from a vector using negative indexing?

You provide a vector of negative indices. For example, x[-c(1,3)] excludes the first and third elements from x.

intermediate

What happens if you mix positive and negative indices in R when subsetting a vector?

R will give an error because you cannot mix positive and negative indices in the same subset operation.

beginner

Example: Given x <- c(10, 20, 30, 40), what is the result of x[-2]?

The result is c(10, 30, 40) because the second element (20) is excluded.

beginner

Why is negative indexing useful in data manipulation?

It allows you to quickly remove unwanted elements without creating a new vector or using complex functions.

What does x[-1] do in R if x is a vector?

A Causes an error

B Returns only the first element

C Returns the vector unchanged

D Returns all elements except the first one

Negative indexing excludes the element at that position, so x[-1] excludes the first element.

Which of these is a valid way to exclude the 2nd and 4th elements from a vector x?

A x[-c(2,4)]

B x[c(-2,-4)]

C x[c(2,4)]

D x[-2,4]

Only C: x[-c(2,4)] is a valid way to exclude the 2nd and 4th elements. A selects them instead. B is invalid because mixing negative indices inside c() is not allowed. D is invalid syntax for vector indexing.

What happens if you try x[c(1, -2)] in R?

A Returns elements 1 and excludes 2

B Gives an error

C Returns all elements

D Returns elements 1 and 2

Mixing positive and negative indices in the same subset causes an error.

If x <- c(5, 10, 15, 20), what is x[-3]?

A c(5, 10, 20)

B c(5, 10, 15)

C c(15, 20)

D c(10, 15, 20)

x[-3] excludes the third element (15), so the result is c(5, 10, 20).

Why might you use negative indexing instead of positive indexing?

A To reverse the vector

B To select only the first element

C To exclude unwanted elements easily

D To sort the vector

Negative indexing is a quick way to exclude elements without specifying all the ones you want.

Explain how negative indexing works in R and give an example.

Think about removing elements by position.

What error occurs if you mix positive and negative indices in R? Why?

Consider how R expects indexing to be consistent.