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 ...
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