Using the % operator with preprocessor constants

[zahy[dot]bnaya[At]gmail[dot]com]

hello,
I have a constant declared as

#define TABLESIZE 16*15*14*13*12* 11*10*9/3 /// 172972800

I tried the following code:

if (TABLESIZE == 172972800)
{
printf("%d\n",5 18763960%172972 800);
printf("%d\n",5 18763960%TABLES IZE);
printf("%d\n",T ABLESIZE);
}



My output is:

172818360
86486400
172972800



How can it be?
Thanks

[Victor Bazarov]

Re: Using the % operator with preprocessor constants

zahy[dot]bnaya[At]gmail[dot]com wrote:[color=blue]
> hello,
> I have a constant declared as
>
> #define TABLESIZE 16*15*14*13*12* 11*10*9/3 /// 172972800[/color]

It's not a constant. It's a piece of text. If you need a constant, use

const long TABLESIZE = ....
[color=blue]
> I tried the following code:
>
> if (TABLESIZE == 172972800)
> {
> printf("%d\n",5 18763960%172972 800);
> printf("%d\n",5 18763960%TABLES IZE);[/color]

Substitute the contents of your 'TABLESIZE' macro here. What do you get?
Write it down. Look what the '%' applies to.
[color=blue]
> printf("%d\n",T ABLESIZE);
> }
>
>
>
> My output is:
>
> 172818360
> 86486400
> 172972800
>
>
>
> How can it be?[/color]

Do NOT use macros when you need a constant.

V
--
Please remove capital As from my address when replying by mail

[Ben Pope]

Re: Using the % operator with preprocessor constants

zahy[dot]bnaya[At]gmail[dot]com wrote:[color=blue]
> hello,
> I have a constant declared as
>
> #define TABLESIZE 16*15*14*13*12* 11*10*9/3 /// 172972800
>
> I tried the following code:
>
> if (TABLESIZE == 172972800)
> {
> printf("%d\n",5 18763960%172972 800);
> printf("%d\n",5 18763960%TABLES IZE);
> printf("%d\n",T ABLESIZE);
> }
>
>
>
> My output is:
>
> 172818360
> 86486400
> 172972800
>
>
>
> How can it be?[/color]

Macros are evil.

Rule 1* is to always put it in brackets:
#define TABLESIZE (16*15*14*13*12 *11*10*9/3)

Macros are text replacement so your second printf expands to:

printf("%d\n",5 18763960%16*15* 14*13*12*11*10* 9/3);

Rule one comes after at least 3 other rules, which essentially all say:
Don't use macros!

Ben Pope
--
I'm not just a number. To many, I'm known as a string...