R in a Nutshell, 2nd Edition

Variables

Like most other languages, R lets you assign values to variables and refer to them by name. In R, the assignment operator is <-. Usually, this is pronounced as “gets.” For example, the statement:

x <- 1

is usually read as “x gets 1.” (If you’ve ever done any work with theoretical computer science, you’ll probably like this notation: it looks just like algorithm pseudocode.)

After you assign a value to a variable, the R interpreter will substitute that value in place of the variable name when it evaluates an expression. Here’s a simple example:

> x <- 1
> y <- 2
> z <- c(x,y)
> # evaluate z to see what's stored as z
> z
[1] 1 2

Notice that the substitution is done at the time that the value is assigned to z, not at the time that z is evaluated. Suppose that you were to type in the preceding three expressions and then change the value of y. The value of z would not change:

> y <- 4
> z
[1] 1 2

I’ll talk more about the subtleties of variables and how they’re evaluated in Chapter 8.

R provides several different ways to refer to a member (or set of members) of a vector. You can refer to elements by location in a vector:

> b <- c(1,2,3,4,5,6,7,8,9,10,11,12)
> b
 [1]  1  2  3  4  5  6  7  8  9 10 11 12
> # let's fetch the 7th item in vector b
> b[7]
[1] 7
> # fetch items 1 through 6
> b[1:6]
[1] 1 2 3 4 5 6
> # fetch only members of b that are congruent to zero (mod 3)
> # (in non-math speak, members that are multiples of 3)
> b[b %% 3 == 0]
[1]  3  6  9 12

You can fetch multiple items in a vector ...