Is it possible to use a if statement inside #define?

Question

I\'m trying to make a macro with the following formula: (a^2/(a+b))*b, and I want to make sure that the there will be no dividing by zero.

#define          


        

Answer 1

You can not use if statement, because #define is interpret by the preprocessor, and the output would be

 result=if( x == 0 || y == 0) { 0 } else { ( ( ( x * x ) / ( ( x ) + ( y ) ) ) * ( y ) )}

which is wrong syntax.

But an alternative is to use ternary operator. Change your define to

#define SUM_A( x, y )  ((x) == 0 || (y) == 0 ? 0 : ( ( ( (x) * (x) ) / ( ( x ) + ( y ) ) ) * ( y ) ))

Remember to always put your define between parentheses, to avoid syntax error when replacing.

Answer 2

if introduces a statement, not an expression. Use the "ternary" (conditional) operator:

#define SUM_A(x, y) (((x) == 0 || (y) == 0)? 0: ((((x) * (x)) / ((x) + (y))) * (y)))

Alternatively, make this an inline function:

inline float sum_a(float x, float y)
{
    if (x == 0 || y == 0)
        return 0;
    else
        return ((x * x) / (x + y)) * y;
}

This avoids the problem of multiple evaluation of x and/or y and is much more readable, but it does fix the types of x and y. You can also drop the inline and let the compiler decide whether inlining this function is worthwhile (inline is not a guarantee that it will perform inlining).