RE: Operation Precedence questions

[Ken Sims <mr2087@xxxxxxxxx>]

Hi Dan -

> Um, I think you just agreed with me, Scott.  I said the parantheses 
> guaranteed that it would be done first, whereas Doug, the author of that 
> snippet, said otherwise.

The thing that you and everyone else seems to be overlooking is that the 
precedence order in the source and the code generated by the compiler to 
apply that precedence are two different things.

The precedence guarantees that things with a higher precedence, whether due 
to parenthesis or the precendence of operations list, will be processed 
before anything that relies on them is processed.

If there is an eval or if with only one thing in parenthesis, that does NOT 
mean that that one thing will be processed first.

The compiler looks at the expression and can determine what parts depend on 
other parts and builds the code so as to avoid doing any unnecessary work.

>       I<=%ELEM(ARRAY) AND I>0 AND ARRAY(I)>10

The compiler can determine that if the first condition is false, it doesn't 
need to check the second and third, or if the first is true and the second 
is false, it doesn't need to check the third.  That's why this code will 
prevent an array index error if I is out of range of the array.

The precedence of operators means that the compiler treats it as:

((I <= %ELEM(ARRAY)) AND (I > 0)) AND (ARRAY(I) > 10)

rather than:

((I <= (%ELEM(ARRAY) AND I)) > (0 AND ARRAY(I))) > 10

which is what would happen if AND and OR were moved higher than compares.

Ken
http://www.ke9nr.net/
Opinions expressed are my own and do not necessarily represent the views of 
my employer or anyone in their right mind.