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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
R Programming week1-Subsetting
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原文地址：http://www.cnblogs.com/jpld/p/4434320.html
