Subsetting
Thereare a number of operators that can be used to extract subsets of R objects.
[always returns an object of the same class as the original; can be used toselect more than one
element(there is one exception)
[[ isused to extract elements of a list or a data frame; it can only be used toextract a single
elementand the class of the returned object will not necessarily be a list or dataframe
$ isused to extract elements of a list or data frame by name; semantics are similarto that of [[.
> x <-c("a", "b", "c", "c", "d","a")
> x[1]
[1] "a"
> x[2]
[1] "b"
> x[1:4]
[1] "a""b" "c" "c"
> x[x >"a"]
[1] "b""c" "c" "d"
> u <- x> "a"
> u
[1] FALSE TRUETRUE TRUE TRUE FALSE
> x[u]
[1] "b""c" "c" "d"
Subsetting Lists
> x <-list(foo = 1:4, bar = 0.6)
> x[1]
$foo
[1] 1 2 3 4
> x[[1]]
[1] 1 2 3 4
> x$bar
[1] 0.6
>x[["bar"]]
[1] 0.6
>x["bar"]
$bar
[1] 0.6
> x <-list(foo = 1:4, bar = 0.6, baz = "hello")
> x[c(1, 3)]
$foo
[1] 1 2 3 4
$baz
[1]"hello"
The[[ operator can be used with computed indices; $ can only be used with literalnames.
> x <-list(foo = 1:4, bar = 0.6, baz = "hello")
> name <-"foo"
> x[[name]] ##computed index for ‘foo’
[1] 1 2 3 4
> x$name ##element ‘name’ doesn’t exist!
NULL
> x$foo
[1] 1 2 3 4 ##element ‘foo’ does exist
Subsetting NestedElements of a List
The[[ can take an integer sequence
> x <-list(a = list(10, 12, 14), b = c(3.14, 2.81))
> x[[c(1, 3)]]
[1] 14
> x[[1]][[3]]
[1] 14
> x[[c(2, 1)]]
[1] 3.14
Subsetting aMatrix
Matricescan be subsetted in the usual way with (i,j) type indices.
> x <-matrix(1:6, 2, 3)
> x[1, 2]
[1] 3
> x[2, 1]
[1] 2
Indicescan also be missing.
> x[1, ]
[1] 1 3 5
> x[, 2]
[1] 3 4
Bydefault, when a single element of a matrix is retrieved, it is returned as avector of length 1 rather than a 1 × 1 matrix. This behavior can be turned offby setting drop = FALSE.
> x <-matrix(1:6, 2, 3)
> x[1, 2]
[1] 3
> x[1, 2, drop= FALSE]
[,1]
[1,] 3
Similarly,subsetting a single column or a single row will give you a vector, not a matrix(by default).
> x <-matrix(1:6, 2, 3)
> x[1, ]
[1] 1 3 5
> x[1, , drop= FALSE]
[,1] [,2][,3]
[1,] 1 3 5
Partial Matching
Partialmatching of names is allowed with [[ and $.
> x <-list(aardvark = 1:5)
> x$a
[1] 1 2 3 4 5
>x[["a"]]
NULL
>x[["a", exact = FALSE]]
[1] 1 2 3 4 5
Removing NAValues
Acommon task is to remove missing values (NAs).
> x <- c(1,2, NA, 4, NA, 5)
> bad <-is.na(x)
> x[!bad]
[1] 1 2 4 5
Whatif there are multiple things and you want to take the subset with no missingvalues?
> x <- c(1,2, NA, 4, NA, 5)
> y <-c("a", "b", NA, "d", NA, "f")
> good <-complete.cases(x, y)
> good
[1] TRUE TRUEFALSE TRUE FALSE TRUE
> x[good]
[1] 1 2 4 5
> y[good]
[1] "a""b" "d" "f"
>airquality[1:6, ]
OzoneSolar.R Wind Temp Month Day
1 41 190 7.4 67 51
2 36 118 8.0 72 52
3 12 149 12.6 745 3
4 18 313 11.5 625 4
5 NA NA 14.3 56 55
6 28 NA 14.9 66 56
> good <-complete.cases(airquality)
> airquality[good,][1:6, ]
OzoneSolar.R Wind Temp Month Day
1 41 190 7.4 67 51
2 36 118 8.0 72 52
3 12 149 12.6 745 3
4 18 313 11.5 625 4
7 23 299 8.6 65 57
