9  Subsetting R Objects

9.1 [ operator

The [ operator always returns an object of the same class as the original:

x <- c("a", "b", "c", "c", "d", "a")
# Extract the first element
x[1]

[1] "a"

# other examples
x[1:4]

[1] "a" "b" "c" "c"

x[c(1, 3, 49)]

[1] "a" "c" NA 

x <- 100:110
i <- c(1, 3 ,2)
x[i]

[1] 100 102 101

u <- x > "a" 
x[u] ## or x[x > "a"]

integer(0)

• Subsetting a matrix:

  x[1,2] ## Extract the first row, first column 
  x[1, ] ## Extract the first row 
  x[, 2] ## Extract the second column 
  x[1, 2, drop = FALSE] ## keeps the matrix format

• It can be used to select multiple elements of an object (also in lists or dataframes):

  (x <- list(foo = 1:4, bar = 0.6, baz = "hello"))

  $foo
  [1] 1 2 3 4
  
  $bar
  [1] 0.6
  
  $baz
  [1] "hello"

  x[c(1, 3)]

  $foo
  [1] 1 2 3 4
  
  $baz
  [1] "hello"

  x["foo"]

  $foo
  [1] 1 2 3 4

  class(x["foo"])

  [1] "list"

  x$foo[1]

  [1] 1

  x$baz[1]

  [1] "hello"

  ## use of negative subscript removes first element "3" 
  num10[-1]

9.2 [[ operator

The [[ operator is used to extract elements of a list or a dataframe. It can only be used to extract a single element and the class of the returned object will not necessarily be a list or data frame.

• Subsetting a Dataframe:

  # Use the notation [[ ]] to extract columns
  x[["var1"]] = x[, 1] = x$var1 # All are equivalent

• Subsetting a List:

  x[[1]] ## Extract single element from a list 
  x[["bar"]] ## Extract named index
  x$bar ## Extract named index 
  x[[c(1, 3)]] ## Get the 3rd element of the 1st element of the list 
  x[[1]][[3]] ## Get the 3rd element of the 1st element of the list

• Now if we want to get access to the individual elements of list_obj[[2]], we have to use the following command:

  (data_2variable <- data.frame(x1=c(2,3,4,5,6), x2=c(5,6,7,8,1)))

    x1 x2
  1  2  5
  2  3  6
  3  4  7
  4  5  8
  5  6  1

  (list_obj <- list(dat=data_2variable, vec.obj=c(1,2,3))) 

  $dat
    x1 x2
  1  2  5
  2  3  6
  3  4  7
  4  5  8
  5  6  1
  
  $vec.obj
  [1] 1 2 3

  list_obj[[2]][1]

  [1] 1

9.3 $ operator

The $ operator is used to extract elements of a list or data frame by literal name. Its semantics are similar to that of [[.

Example:

x[["bar"]] ## Extract named index 

[1] 0.6

x$bar ## Extract named index

[1] 0.6