ST Expressions

An expression is a construct that returns a value following its evaluation.

Expressions consist of operators and operands. In Extended Structured Text (ExST), you can also use assignments as expressions.

An operand can be a constant, variable, function call, or another expression.

Special ST expressions are also:

Access to
- Elements of array variables
- Components of a structure variable
- Variables of a function block instance or a program
Method call

Example 63. Examples

`2014`	Constant, literal
`ivar`	Variable
`fct(a,b)`	Function call
`(x*y)/z`	Expression
`real_var2 := int.var;`	In ExST: Assignment

Evaluation of expressions

An expression is evaluated by processing the operators according to specific binding rules. CODESYS first processes the operator with the strongest binding. Operators with the same binding strength are processed from left to right.

Operation	Symbol	Binding strength
Bracketing	`( <expression> )`	Strongest binding
Function call	`<function name> ( <parameter list> )` All operators with syntax: `<operator> ()`
Exponentiate	`EXPT`
Negate Complementation	`-` `NOT`
Multiplication Division Modulo	`*` `/` `MOD`
Addition Subtraction	`+` `-`
Compare	`<`,`>`,`<=`,`>=`
Equality Inequality	`=` `<>`
Boolean AND	`AND`, `AND_THEN`
Boolean XOR Boolean OR	`XOR` `OR`, `OR_ELSE`	Weakest binding