14.1 Vectors and Lists

1. Two kinds of vectors

R has two kinds of vectors: atomic vectors and lists.

An atomic vector holds one type: logical, integer, double, character (plus complex and raw, which we will not use). A list can hold mixed types, including other lists.

x <- c(1L, 10L, 2L)
typeof(x)

## [1] "integer"

length(x)

## [1] 3

c(1, 10, 2) is double. The L makes an integer. typeof() is the storage type. length() is how many elements.

A factor is an integer with labels. A Date is a double. Both still behave like atomic vectors when you subset them.

2. [ keeps structure; [[ extracts one

[ returns the same kind of object, names included. [[ pulls out one element and drops the wrapper. $ is [[ for a name.

x <- c(horse = 7, man = 1, dog = 8)
x[2:3]

## man dog 
##   1   8

x[c("man", "horse")]

##   man horse 
##     1     7

x[-3]

## horse   man 
##     7     1

x[[3]]

## [1] 8

x[3] is still a named vector of length 1. x[[3]] is the number 8.

A logical vector of the same length picks the TRUE positions:

x[c(TRUE, FALSE, TRUE)]

## horse   dog 
##     7     8

3. Recycling and names

R recycles a shorter vector to the length of the longer one:

x <- c(1, 2, 3, 4)
x + 10

## [1] 11 12 13 14

# same idea as x + c(10, 10, 10, 10)

Recycling a non-scalar (x + c(10, 20)) is legal and easy to misread. Prefer a scalar, or write a vector of the same length.

names(x) is a character vector. You can set names when you create the object:

c(Yoshi = 10L, Mario = 31L)

## Yoshi Mario 
##    10    31

4. Lists hold mixed types

my_list <- list(
  x = "a",
  y = 1,
  z = c(1L, 2L, 3L)
)

[ on a list returns a list. [[ or $ returns the element inside.

my_list[1]

## $x
## [1] "a"

my_list[[1]]

## [1] "a"

my_list$y

## [1] 1

Remove an element by assigning NULL:

my_list$x <- NULL

5. A data frame is a list of columns

A tibble is a list. Each element is a column. That is why df[[2]] is the second column and df$species is the species column.

A list can also hold a tibble as one of its elements. Subset the list to get the tibble; then use dplyr on that tibble.

6. Practice

1. Create

x <- c(
  Yoshi = 10L,
  Mario = 31L,
  Luigi = 72L,
  Peach = 11L,
  Toad  = 38L
)

Extract Yoshi and Peach four ways: positive integers, negative integers, a logical vector, and names.

1. Build a wedding-style list: a venue, a guest tibble (name, meal, age), a bride, a groom, and a date. Remove one guest row from the embedded tibble. Add a named vector that decodes the meal codes. Extract the venue two ways.