R Programming-week1 Subsetting

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