Do you think this macro is a bad programming practice?

[Adrian20XX]

Hi,

I want to define a macro in C like this, but I wonder if these is too obscure and I'm starting to get too nasty.

This is the macro I want to use:

#define CompareXYAndDoActions(X, Y, greaterAction, equalAction, lowerAction)            \
{                                                                                    \
    if (X>Y) {                                                                        \
        greaterAction;                                                                \
    }                                                                                \
    if (X==Y) {                                                                        \
        equalAction;                                                                \
    }                                                                                \
    if (X<Y) {                                                                        \
        lowerAction;                                                                \
    }                                                                                \
}

Later I plan to call it like this:

CompareXYAndDoActions(X, Y,
        {
            Action1;
        },
        {
            Action2;
        },
        {
            Action3;
        }
        );

So, do you think the call later is readable? Or does it make you want to puke?
Any idea will be greatly appreciated.
TIA & Regards ...

[JosAH]

Hi,

I want to define a macro in C like this, but I wonder if these is too obscure and I'm starting to get too nasty.

This is the macro I want to use:

#define CompareXYAndDoActions(X, Y, greaterAction, equalAction, lowerAction)            \
{                                                                                    \
    if (X>Y) {                                                                        \
        greaterAction;                                                                \
    }                                                                                \
    if (X==Y) {                                                                        \
        equalAction;                                                                \
    }                                                                                \
    if (X<Y) {                                                                        \
        lowerAction;                                                                \
    }                                                                                \
}

Later I plan to call it like this:

CompareXYAndDoActions(X, Y,
        {
            Action1;
        },
        {
            Action2;
        },
        {
            Action3;
        }
        );

So, do you think the call later is readable? Or does it make you want to puke?
Any idea will be greatly appreciated.
TIA & Regards ...

That macro stinks, both syntactically and semantically; have a look:

if (<some_other_condition>)
   CompareXYAndDoActions(X, Y,
        {
            Action1;
        },
        {
            Action2;
        },
        {
            Action3;
        }
        ); // <--- note this semi colon
else
   <something_else>
 

Semantically it doesn't work either; better put parentheses around the X and Y
macro parameters. But, even better, if you want to use Fortran you know where
to find it.

kind regards,

Jos

[Adrian20XX]

That macro stinks, both syntactically and semantically.

Semantically it doesn't work either; better put parentheses around the X and Y macro parameters. But, even better, if you want to use Fortran you know where to find it.

kind regards,

Jos

Thanks Jos,

I thought I was getting too nasty in here, but to get that from the Chief Editor is a clear indication I've crossed the line by too much :-(

The problem is that I had already optimized the algorithm at the higher levels, so every extra performance gain I can get in here is very important. And I wanted also to be able to decide later how to do the comparisons, I think I've read once that some Intel Processor (I guess Pentium) tests at the same time a number for greater than zero, equal than zero or less than zero, so wanted to be able to change later the conditions for (X-Y>0), (X-Y==0) and (X-Y<0) to do one comparison instead of 3.

No other idea about how to do this? :-(

TIA & Regards ...

[JosAH]

Thanks Jos,

I thought I was getting too nasty in here, but to get that from the Chief Editor is a clear indication I've crossed the line by too much :-(

Yep, implementing macros that seem to add a bit of different statement syntax
is almost never a good idea. Simply write out your statements and leave the
optimization to the compiler (esp. the code generator): it is much better at it.

The problem is that I had already optimized the algorithm at the higher levels, so every extra performance gain I can get in here is very important. And I wanted also to be able to decide later how to do the comparisons, I think I've read once that some Intel Processor (I guess Pentium) tests at the same time a number for greater than zero, equal than zero or less than zero, so wanted to be able to change later the conditions for (X-Y>0), (X-Y==0) and (X-Y<0) to do one comparison instead of 3.

No other idea about how to do this? :-(

TIA & Regards ...

Those 'micro optimizations' should only be applied (if ever) at the end of the
development phase: first make the entire thing work using an efficient algorithm.
As I wrote above: a compiler is a good code optimizer. If you want to use those
very low level processor trickery-dickeries it's even better to write a (small?)
assembly function that uses it and can be called from the rest of your code.

Your particular example can be implemented in a 'sign' macro as follows:

#define sign(x, y) (((y) > (x)) - ((x) > (y)))
 

This little monster has result -1, 0 or 1 according to x > y, x == y or x < y, you
can even use that number as an index in a function pointer array if you are
absolutely possitively definitely sure that all three situations occur frequently
in your code; otherwise simply use a simple if statement for the comparison.

An optimizing compiler can even break the code for that thing down to one single
comparison and a bit of status flags (bits) fiddling.

But as a rule of thumb: if you feel the urge to optimize: optimize late.

kind regards,

Jos