Operators acting on vectors, matrices, arrays and lists to extract or replace parts.
x[i]
x[i, j, … , drop = TRUE]
x[[i, exact = TRUE]]
x[[i, j, …, exact = TRUE]]
x$name
getElement(object, name)<p></p><p>x[i] <- value
x[i, j, …] <- value
x[[i]] <- value
x$name <- value</p>| Parameter | Description |
|---|---|
x, object |
object from which to extract element(s) or in which to replace element(s). |
i, j, … |
indices specifying elements to extract or replace. Indices are numeric or character vectors or empty (missing) or NULL. Numeric values are coerced to integer as by as.integer (and hence truncated towards zero). Character vectors will be matched to the names of the object (or for matrices/arrays, the dimnames): see ‘Character indices’ below for further details. For [-indexing only: i, j, … can be logical vectors, indicating elements/slices to select. Such vectors are recycled if necessary to match the corresponding extent. i, j, … can also be negative integers, indicating elements/slices to leave out of the selection. When indexing arrays by [ a single argument i can be a matrix with as many columns as there are dimensions of x; the result is then a vector with elements corresponding to the sets of indices in each row of i. An index value of NULL is treated as if it were integer(0). |
name |
A literal character string or a name (possibly backtick quoted). For extraction, this is normally (see under ‘Environments’) partially matched to the names of the object. |
drop |
For matrices and arrays. If TRUE the result is coerced to the lowest possible dimension (see the examples). This only works for extracting elements, not for the replacement. See drop for further details. |
exact |
Controls possible partial matching of [[ when extracting by a character vector (for most objects, but see under ‘Environments’). The default is no partial matching. Value NA allows partial matching but issues a warning when it occurs. Value FALSE allows partial matching without any warning. |
value |
typically an array-like R object of a similar class as x. |
# NOT RUN {
x <- 1:12
m <- matrix(1:6, nrow = 2, dimnames = list(c("a", "b"), LETTERS[1:3]))
li <- list(pi = pi, e = exp(1))
x[10] # the tenth element of x
x <- x[-1] # delete the 1st element of x
m[1,] # the first row of matrix m
m[1, , drop = FALSE] # is a 1-row matrix
m[,c(TRUE,FALSE,TRUE)]# logical indexing
m[cbind(c(1,2,1),3:1)]# matrix numeric index
ci <- cbind(c("a", "b", "a"), c("A", "C", "B"))
m[ci] # matrix character index
m <- m[,-1] # delete the first column of m
li[[1]] # the first element of list li
y <- list(1, 2, a = 4, 5)
y[c(3, 4)] # a list containing elements 3 and 4 of y
y$a # the element of y named a
## non-integer indices are truncated:
(i <- 3.999999999) # "4" is printed
(1:5)[i] # 3
## named atomic vectors, compare "[" and "[[" :
nx <- c(Abc = 123, pi = pi)
nx[1] ; nx["pi"] # keeps names, whereas "[[" does not:
nx[[1]] ; nx[["pi"]]
# }
# NOT RUN {
## recursive indexing into lists
z <- list(a = list(b = 9, c = "hello"), d = 1:5)
unlist(z)
z[[c(1, 2)]]
z[[c(1, 2, 1)]] # both "hello"
z[[c("a", "b")]] <- "new"
unlist(z)
## check $ and [[ for environments
e1 <- new.env()
e1$a <- 10
e1[["a"]]
e1[["b"]] <- 20
e1$b
ls(e1)
## partial matching - possibly with warning :
stopifnot(identical(li$p, pi))
op <- options(warnPartialMatchDollar = TRUE)
stopifnot( identical(li$p, pi), #-- a warning
inherits(tryCatch (li$p, warning = identity), "warning"))
## revert the warning option:
if(is.null(op[[1]])) op[[1]] <- FALSE; options(op)
# }