Basics
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
[ [ - used to extract elements of a list or data frame. 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
$ - 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"
Lists
> x <- list(foo = 1:4, bar = 0.6)
First element is foo. Second element is bar.
> x[1] # returns list with sequence
$foo
[1] 1 2 3 4
> x[[1]] # returns sequence from list
[1] 1 2 3 4
If you can't remember the position of "bar" in the list, you can access it using its name rather than its index.
> x$bar # returns element associated with "bar"
[1] 0.6
> x[["bar"]] # equivalent to above
[1] 0.6
> x["bar"] # returns list with element
$bar
[1] 0.6
To extract multiple elements from a list, use the [] operator.
> x <- list(foo = 1:4, bar = 0.6, baz = "hello")
> x[c(1, 3)]
$foo
[1] 1 2 3 4
$baz
[1] "hello"
You can't use the [[]] or $ operators to extract multiple elements from a list.
The [[]] operator can be used with indices; $ can only be used with literal names.
> x <- list(foo = 1:4, bar = 0.6, baz = "hello")
> name <- "foo"
> x[[name]]
[1] 1 2 3 4
> x$name
NULL
> x$foo
[1] 1 2 3 4
[[]] 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
Matrices
> x <- matrix(1:6, 2, 3)
> x
\t[ ,1]\t[ ,2]\t[ ,3]
[1, ]\t1\t3\t5
[2, ]\t2\t4\t6
> 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 from a matrix is retrieved, it is returned as a vector of length 1 rather than a 1 x 1 matrix. This behavior can be turned off by setting drop = FALSE.
> x[1, 2, drop = FALSE]
\t[ ,1]
[1, ]\t3
> x[1, , drop = FALSE]
\t[ ,1]\t[ ,2]\t[ ,3]
[1, ]\t1\t3\t5
Partial Matching
Partial matching of names is allowed with [[]] and $.
$ looks for a name in the list that matches the letter "a"
> x <- list(aardvark = 1:5)
> x$a
[1] 1 2 3 4 5
[[]] looks for a name that's an exact match.
> x[["a"]]
NULL
The exact = FALSE argument drops the exactness requirement.
> x[["a", exact = FALSE]]
[1] 1 2 3 4 5
Removing Missing (NA) Values
> x <- c(1, 2, NA, 4, NA, 5)
> bad <- is.na(x)
> x[!bad]
[1] 1 2 4 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]
"a" "b" "d" "f"
You can also use complete.cases to remove missing values from data frames. To get the rows of a data frame where all the values are not missing:
> good <- complete.cases(dataframename)
> dataframename[good, ]