RE: Operation Precedence questions

You also have to consider that the evaluation STOPS when the condition
cannot be met. So if I=0 the ARRAY(I)>10 comparison is never performed.
Hence, it works.

Bob Cozzi

-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx [mailto:rpg400-l-bounces@xxxxxxxxxxxx]
On Behalf Of Dan
Sent: Thursday, March 20, 2003 11:36 AM
To: rpg400-l@xxxxxxxxxxxx
Subject: Operation Precedence questions


I have a question about Operation Precedence in RPG-IV.  FWIW, v5r2.  I have
found the chapter on Expressions in the Reference and, specifically, the
sections titled "Operation Precedence" and "Short Circuit Evaluation".  Upon
my brief foray into these topics, it appears to me that the two sections
permit a contradiction to the rules they set forth.

"Operation Precedence" says:

The precedence of operations determines the order in which operations are
performed within expressions. High precedence operations are performed
before lower precedence operations.

The following list indicates the precedence of operations from highest to
lowest:
  1. ()
  2. Built-in functions, user-defined functions
  3. unary +, unary -, NOT
  4. **
  5. *, /
  6. binary +, binary -
  7. =, <>, >, >=, <, <=
  8. AND
  9. OR

"Short Circuit Evaluation" says:

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)>10
will never result in an array indexing exception.

<my comments/questions follow>

If AND & OR are always the lowest precedence, how can the array index
example above work? 
According to the rules of precedence, as I interpret them, all of the
operands:
      I<=%ELEM(ARRAY) 
      I>0 
      ARRAY(I)>10
would be evaluated before the AND relational operators.  If 'I' is zero, I
would expect a run-time error on the ARRAY(I)>10 because the > operator has
a higher precedence than the AND operator.

BTW, I know from experience that this example works.  I am questioning
whether the precedence of operations really should be re-arranged to place
AND & OR higher in the list.

- Dan

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