R Programming week1-Subsetting

Subsetting

There are 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 to select more than one

element (there is one exception)

[[ is used to extract elements of a list or a data frame; 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

$ is used to extract elements of a list or data frame by name; semantics are similar to 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 TRUE TRUE 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 literal names.

> 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 Nested Elements 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 a Matrix

Matrices can 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

Indices can also be missing.

> x[1, ]

[1] 1 3 5

> x[, 2]

[1] 3 4

By default, when a single element of a matrix is retrieved, it is returned as a vector of length 1 rather than a 1 × 1 matrix. This behavior can be turned off by 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

Partial matching 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 NA Values

A common 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

What if there are multiple things and you want to take the subset with no missing values?

> x <- c(1, 2, NA, 4, NA, 5)

> y <- c("a", "b", NA, "d", NA, "f")

> good <- complete.cases(x, y)

> good

[1] TRUE TRUE FALSE TRUE FALSE TRUE

> x[good]

[1] 1 2 4 5

> y[good]

[1] "a" "b" "d" "f"

 

> airquality[1:6, ]

 Ozone Solar.R Wind Temp Month Day

1 41 190 7.4 67 5 1

2 36 118 8.0 72 5 2

3 12 149 12.6 74 5 3

4 18 313 11.5 62 5 4

5 NA NA 14.3 56 5 5

6 28 NA 14.9 66 5 6

> good <- complete.cases(airquality)

> airquality[good, ][1:6, ]

 Ozone Solar.R Wind Temp Month Day

1 41 190 7.4 67 5 1

2 36 118 8.0 72 5 2

3 12 149 12.6 74 5 3

4 18 313 11.5 62 5 4

7 23 299 8.6 65 5 7