To recap briefly, a formula is divided into multiple sections, separated by labels. Some sections (for example init and loop) can contain statements. Most statements are just expressions, and this topic explains what expressions really are.
Almost everything is an expression. Variables, parameters and constants are all expressions. By using operators or functions, expressions can be composed of one or two subexpressions. Examples:
a 3 3 + 2 sin(a) (3 + sin(a)) / 2
These are all valid expressions (note that “a” is a variable). The important property of an expression is that you can always calculate its value, no matter how complicated the expression is. Moreover, you can do something with this value, for example assigning it to a variable by using the = (assignment) operator:
b = 3 + 2 b = (3 + sin(a)) / 2
These expressions are called assignments. It is important to realize that assignments are expressions themselves, so this is also a valid expression:
c = b = 3 + 2
This gives both c and b the value 5.
Now, remember that statements can be expressions, so that means any expression is a valid statement. Of course, it does not make sense to use expressions like “3″ or “(3 + sin(a)) / 2″ as statements, since nothing is done with the value of the expression, so the statement is ignored by the compiler (and results in a warning message). Only assignments actually use the value of the expression, therefore statements usually are assignments.
Note: do not confuse the = (assignment) operator with the == (equality) operator. The == operator is used to test if two expressions are equal, and returns a boolean value (true or false).