Subsets of data vectors

Often it will be useful or necessary to work with a subset of a dataset. There are several ways to do this in S.

1. Explicit subscripting, using a vector of positive integers. For example x[1] will extract the first element of x. x[1:5] refers to the first five values of x. x[c(1,3,5)] selects x[1], x[3], and x[5].
2. A logical expression of the same length as x. The elements of x for which the logical expression is true will be extracted. For example, x[x>0] selects the elements of x that are greater than zero. If y is the same length as x, y[x>0] selects the cases from y corresponding to the positive values of x.
3. A vector of negative integers. Subscripting by a negative integer tells S to omit the cases corresponding to the negative subscripts. For example, x[-1] selects all but the first element of x. x[c(-1,-5,-7)] refers to all but the first, fifth, and seventh elements of x.

Examples:

> x
[1] 1 2 3 4 5

> x > 3
[1] F F F T T

> x[x>3]
[1] 4 5

> y<-x*x
> y
[1]  1  4  9 16 25

> y[x>3]
[1] 16 25

> y[2]
[1] 4

> y[-2]
[1]  1  9 16 25