Relational operations AND and OR are evaluated from left to right. However, as soon as the value is known, evaluation of the expression stops and the value is returned. As a result, not all operands of the expression need to be evaluated.
For operation AND, if the first operand is false, then the second operand is not evaluated. Likewise, for operation OR, if the first operand is true, the second operand is not evaluated.
There are two implications of this behaviour. First, an array index can be both tested and used within the same expression. The expression
I<=%ELEM(ARRAY) AND I>0 AND ARRAY(I)>10will never result in an array indexing exception.
The second implication is that if the second operand is a call to a user-defined function, the function will not be called. This is important if the function changes the value of a parameter or a global variable.