The first control construct examined is the if statement. An if statement will determine whether or not a code block should be executed, and it has an optional else statement. An additional if statement can be chained to an else statement to create a cascade of options.

In its most basic form, an if statement has the following form:

if(condition)
  code block

The condition can be a logical statement constructed from the operators given in the next table, or it can be a numeric result. If the condition is numeric, then zero is considered to be FALSE, otherwise the statement is TRUE. The code block can either be a single statement or a set of statements enclosed in braces:

> # check if 1 is less than 2
> if(1<2)
+    cat("one is smaller than two.\n")
one is smaller than two.
> if(2>1) {
+    cat("But two is bigger yet.\n")
+ }
But two is bigger yet.

Note that a comment is included in the previous code. The # character is used to denote a comment. Anything after...