Learning R

Chapter 5. Lists and Data Frames

The vectors, matrices, and arrays that we have seen so far contain elements that are all of the same type. Lists and data frames are two types that let us combine different types of data in a single variable.

Chapter Goals

After reading this chapter, you should:

• Be able to create lists and data frames
• Be able to use length, names, and other functions to inspect and manipulate these types
• Understand what NULL is and when to use it
• Understand the difference between recursive and atomic variables
• Know how to perform basic manipulation of lists and data frames

Lists

A list is, loosely speaking, a vector where each element can be of a different type. This section concerns how to create, index, and manipulate lists.

Creating Lists

Lists are created with the list function, and specifying the contents works much like the c function that we’ve seen already. You simply list the contents, with each argument separated by a comma. List elements can be any variable type—vectors, matrices, even functions:

(a_list <- list(
  c(1, 1, 2, 5, 14, 42),    #See http://oeis.org/A000108
  month.abb,
  matrix(c(3, -8, 1, -3), nrow = 2),
  asin
))

## [[1]]
## [1]  1  1  2  5 14 42
##
## [[2]]
##  [1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov"
## [12] "Dec"
##
## [[3]]
##      [,1] [,2]
## [1,]    3    1
## [2,]   -8   -3
##
## [[4]]
## function (x)  .Primitive("asin")

As with vectors, you can name elements during construction, or afterward using the names function:

names(a_list) <- c