In contrast to a vector, in which all elements must be of the same mode, R’s list structure can combine objects of different types. For those familiar with Python, an R list is similar to a Python dictionary or, for that matter, a Perl hash. C programmers may find it similar to a C struct. The list plays a central role in R, forming the basis for data frames, object-oriented programming, and so on.
In this chapter, we’ll cover how to create lists and how to work with them. As with vectors and matrices, one common operation with lists is indexing. List indexing is similar to vector and matrix indexing but with some major differences. And like matrices, lists have an analog for the apply() function. We’ll discuss these and other list topics, including ways to take lists apart, which often comes in handy.
Technically, a list is a vector. Ordinary vectors—those of the type we’ve been using so far in this book—are termed atomic vectors, since their components cannot be broken down into smaller components. In contrast, lists are referred to as recursive vectors.
For our first look at lists, let’s consider an employee database. For each employee, we wish to store the name, salary, and a Boolean indicating union membership. Since we have three different modes here—character, numeric, and logical—it’s a perfect place for using lists. Our entire database might then be a list of lists, or some other kind of list such as a data frame, though we won’t pursue that here.
We could create a list to represent our employee, Joe, this way:
j <- list(name="Joe", salary=55000, union=T)
We could print out j, either in full or by component:
> j $name [1] "Joe" $salary [1] 55000 $union [1] TRUE
Actually, the component names—called tags in the R literature—such as salary are optional. We could alternatively do this:
> jalt <- list("Joe", 55000, T)
> jalt
[[1]]
[1] "Joe"
[[2]]
[1] 55000
[[3]]
[1] TRUEHowever, it is generally considered clearer and less error-prone to use names instead of numeric indices.
Names of list components can be abbreviated to whatever extent is possible without causing ambiguity:
> j$sal [1] 55000
Since lists are vectors, they can be created via vector():
> z <- vector(mode="list") > z[["abc"]] <- 3 > z $abc [1] 3