What is the difference between and notations to access the elements of a list or dataframe in R

Question

As R provides two different ways to access the elements of a list or a data.frame, what is the difference between both of them?

In which situations should I use[] over [[]]?

Answer 1

R provides 3 basic indexing operators. Refer below for the syntax and example:

    a[i]
    a[i, j]
    a[[i]]
    a[[i, j]]
    a$x
    a$"x"

When it comes to vectors and matrices [[ are hardly used even though they have semantic differences from [.

While indexing multi-dimensional structures with a single index, a[[i]] or a[i] will return the ith sequential element of a.

When you use lists, [[ is used to select any single element, and [ is used to return a list of the selected elements.

[[ allows a single element to be selected using various indices like integer/character and [ indexes by vectors.