Expressions[LINK]

An expression is a sequence of variables and/or constants linked together by operators. An expression is always evaluated to a single value.

The rules for expressions are:

• An expression is a sequence of variables and/or constants linked by operators.

• Expressions always evaluate to a single value.

• Comparison operators evaluate to 1.0 for “true” or 0.0 for “false.”

• Compound expressions are allowed and can be organized with parentheses.

• Variables must be initialized prior to being used in an expression.

The operators shown in Table [table:operators-for-erl] are available for use in Erl programs.

Operators for Erl

Operator Symbol	Description	Evaluation Order	Example
( )	Parentheses	left-to-right	SET z = 23/(3 + 2)
+	Addition	right-to-left	SET a = 4 + 5
-	Subtraction	right-to-left	SET b = a - 3
$\ast$	Multiplication	right-to-left	SET c = a * b
/	Division	left-to-right	SET d = b/a
^	Raise to a power	left-to-right	SET e = c ^ 0.5
==	Equality comparison	left-to-right	IF a == b
< >	Inequality comparison	left-to-right	IF c < > d
>	Greater than comparison	left-to-right	IF a > e
> =	Greater than or equal to comparison	left-to-right	IF a > = 6
<	Less than comparison	left-to-right	IF b < 2
< =	Less than or equal to comparison	left-to-right	IF b < = f
& &	Logical AND	right-to-left	IF c & & d
$||$	Logical OR	right-to-left	IF c $||$ d

Because expressions can be evaluated to a single value, they can be used in SET and IF statements. That means both of the following instructions are allowed:

SET a = c < d
IF a - 1

In the case of the SET example, the value of “a” is set to 1 if “c” is less than “d”; otherwise, it is set to 0. For the IF example, the IF block of instructions are executed if a – 1 is greater than zero.

Compound expressions allow multiple operators to be sequenced or nested. For example:

a + b * 7 / 4.5
(a * 3 + 4) ^ 2
(a > b) && (c < d)

For complicated expressions, it helps to make heavy use of parentheses in your equations. By using parentheses with proper algebraic evaluation in mind to group terms, you can help the Erl parser. The language processor is simplistic compared to a full-blown programming language and sometimes has problems applying the rules of algebra. It is safer to err on the side of extra parentheses and to inspect the results of complex expressions in the EDD output.