You can use the if or unless statements to conditionally execute part of your program, based on the truth value of a given expression. Here is the general form of an if statement: if (test) then-statement else else-statement If test evaluates to true, then then-statement is executed and else-statement is not. If test evaluates to false, then else-statement is executed and then-statement is not. The else clause is optional. If unless is used instead of if, then the truth value of test is flipped. Now, then-statement is executed if test is false, and so on.

You can use a series of if and/or unless statements to test for multiple conditions: if (x == 1) y = 10; else if (x == 2) y = 20; else unless (x == 4) y = 30; else y = 50;

You can also optionally define variables within the condition. This is useful if you want to compare an expression which does not have a consistent return value, such as rand. The scope of this variable lasts until the if (or unless) statement chain it was declared in is over. /* rand(3) returns a random value from 0 to 2, and we run different code depending on each of the three outcomes */ if (var r = rand(3); r == 0) y = 10; else if (r == 1) y = 20; else if (r == 2) y = 40; x = r // error