# RFP - Part2: R Lists

## Master R

Previously, we discussed R vectors. We now turn to R lists. Like a vector, a R list can also grow or shrink. But unlike a vector, a R list can contain any R objects. For example, vectors, lists, functions, or environments can all be the elements of a list, and it’s perfectly okay to mix them in the same list.

## The empty list

The empty list, with syntax list(), has 0 elements. Indeed, if we call length(list()), we get 0.

## Non-empty lists

We can make a list with list(e1, ..., en) where each expression1 e is evaluated to a R object. It’s more common to make a list with c(e1, e2), called called “e1 combined with e2,” where e1 evaluates to a “list” and e2 evaluates to another “list.” The result is a new list that starts with the elements in e1 followed by the elements in e2. In the simpler case when e1 evaluates to a single value like 5, 1.2, “hello”, TRUE or NA, c(e1, e2) can also be called “e1 consed onto e2.” The result is then a new list that starts with the value of e1 followed by the elements in e2. Here’re some examples of lists:

## How to use lists

Once again, powered by recursion, we only need three simple operations when working with lists:

1. Check if a list is empty.
2. Get the first element of a list, raising an exception if the list is empty.
3. Get the tail of a list without its first element, raising an exception if the list is empty.

Like in the case of vectors, R doesn’t provide these operations perfectly out of the box. So we need to write them ourselves:

Having defined is_empty(), hd() and tl(), we can use them inside of recursive functions to perform complex operations on lists. For example, given a list of integer pairs, we can write a function to sum them up.

Or we can write functions to put the first or second element of each pair into separate lists.

It should make us cringe that firsts() and seconds() look almost identical yet we wrote them as two different functions. We’ll learn how to fix it later. Notice that is_empty(), hd() and tl() defined above are very similarly to the ones defined in part1 of this series when we discussed R vectors. This is because lists are just a special kind of vectors in R. To see this, realize that another way of declaring an empty list is vector("list").

1. The word ‘expression’ here is being used in the sense of expressions in any programming language or mathematical expressions. Do NOT confuse it with the R expression object.