Accessing elements ([], [[]], $) in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the single bracket [ ] operator do when accessing elements in an R list?

The single bracket [ ] returns a sublist containing the selected elements. It preserves the list structure.

beginner

How does the double bracket [[ ]] operator differ from the single bracket [ ] in R?

The double bracket [[ ]] extracts the actual element inside the list, not a sublist. It returns the element itself.

beginner

What is the purpose of the $ operator in R when working with lists or data frames?

The $ operator accesses elements by name directly, returning the element itself, similar to [[ ]] but using the element's name.

intermediate

Given list x <- list(a = 1:3, b = "hello"), what is the result of x["a"]?

It returns a sublist containing the element named 'a', so the output is a list with one element: a = 1:3.

intermediate

Given list x <- list(a = 1:3, b = "hello"), what is the result of x[["a"]]?

It returns the actual element named 'a', which is the vector 1, 2, 3 (not a list).

Which operator returns a sublist when accessing elements in an R list?

A [[ ]]

B [ ]

C $

D None of the above

The single bracket [ ] returns a sublist, preserving the list structure.

Which operator extracts the actual element from a list in R?

A Both B and C

B [[ ]]

C $

D [ ]

Both [[ ]] and $ extract the actual element, not a sublist.

What does x$element return in R?

A The actual element named 'element'

B NULL

C A sublist containing 'element'

D An error

The $ operator returns the actual element named 'element' from the list or data frame.

If x is a list, what is the type of x["name"]?

A The element itself

B NULL

C A vector

D A sublist containing the element

Using single brackets with a name returns a sublist containing that element.

Which operator can you use to access a list element by name without quotes?

A [ ]

B [[ ]]

C $

D None of the above

The $ operator accesses elements by name without needing quotes.

Explain the difference between [ ], [[ ]], and $ when accessing elements in an R list.

Think about whether you want a list or the element itself.

Describe a situation where using [ ] instead of [[ ]] would cause unexpected results in R.

Consider what happens if you try to use a vector function on a sublist.