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Extract or Replace parts of an Object

Syntax

x[i]
x[i,j, ...]
x[i,j, ..., drop=T]
x[[i]]
x[[i,j, ...]]
x$name

Description

These operators act on vectors, matrices, dataframes and lists. If one of these expressions is on the left hand side of an assignment then that part of x is set to the value of the right hand side of the assignment.

These operators are generic. You can write methods to handle subsetting of specific classes of data.

The [[ operator requires all relavent subscripts be supplied. With the [ operator a comma separated blank indicates that all entries in that dimension are selected.

When operating on a list the [[ operator gives the specified element of the list while the [ operator returns a list with the specified element(s) in it.

Examples

x <- 1:12; m <- matrix(1:6,nr=2); li <- list(pi=pi, e = exp(1)) x[10] # the tenth element of x m[1,] # the first row of m (which must be a matrix) li[[1]] # the first element of list li y<-list(1,2,a=4,5) y[c(3,4)] # a list containing the third and fourth elements of y y$a #the element of y named a