The following code gives a wrong result :
[EDIT]According to Ashen, this specific code works, but my code (more complicated) doesn't[/EDIT]
#define MACRO1 1
#define MACRO2 -1000
...
int tab[10];
int i=1005;
int x = tab[i+MACRO1 MACRO2];
i+MACRO1 MACRO2 should be evaluated as "i+1 -1000", so it should be equal to 1005+1-1000 = 6.
Instead, it seems to be evaluated as "i+1+1000" and it is equal to 1005+1+1000 = 2006
Or maybe I made a mistake? Maybe "-" is forbidden in macros? But then how does it come that "i+MACRO1 MACRO2" still manages to be evaluated (since it would then give "1005+1 1000")?
I'd be tempted to say that this bug is similar to the one preventing functions to be evaluated when used as the index of an array. ( example : tab[MyFunction()];
Try
int x = tab[(i+MACRO1) MACRO2];
I think it's because of right-to-left evaluation.
It seems to compile OK for me even without the brackets, and returns tab[6].
Where are they defined?
Quote from: strazer on Wed 25/07/2007 15:33:00
I think it's because of right-to-left evaluation.
You are absolutely right, thank you very much strazer!
int x = tab[(i+MACRO1) MACRO2]; works
int x = tab[(i MACRO2) + MACRO1]; also works and is more consistent with the meaning of my macros.
Just for curiosity : I wonder how the "right to left" evaluation impacts this expression, since "(i+MACRO1) MACRO2" is equivalent to "MACRO2
THEN (i+MACRO1)", which is equivalent to "MACRO2
THEN MACRO1
THEN i"... So "(i+MACRO1) MACRO2" looks like it should give exactly the same result as "i+MACRO1 MACRO2"
Edit:
Quote from: Ashen on Wed 25/07/2007 15:41:12
It seems to compile OK for me even without the brackets
The example I gave was a simple one, where I removed all unuseful info, so it may work; but there still was an evaluation problem that strazer solved. Thank you anyway for having tried.
Quotewhere are they defined?
MACRO2 is defined in the header of the module.
MACRO1 is defined in the body of the module.
Glad I could help. :)
Quote from: Monsieur OUXX on Wed 25/07/2007 16:06:17
int x = tab[(i MACRO2) + MACRO1]; also works and is more consistent with the meaning of my macros.
Right, better put the +'s in there to be on the safe side (as +- equals -).
Quote from: Monsieur OUXX on Wed 25/07/2007 16:06:17So "(i+MACRO1) MACRO2" looks like it should give exactly the same result as "i+MACRO1 MACRO2"
I'll let someone else answer this since I'd surely get it wrong... ;)
Yes, there's a "Right-to-left evaluation" checkbox that you can set on the General SEttings pane to set this.
If you're writing a module, I'd recommend you check the LRPRECEDENCE macro (or just use parenthesis to remove all ambiguity) in your code.