Question

I have an expression

x += y;

and, based on a boolean, I want to be able to change it to

x -= y;

Of course I could do

if(i){x+=y;} else{x-=y;}
//or
x+=(y*sign);  //where sign is either 1 or -1

But if I have to do this iteratively, I want to avoid the extra computation. Is there a more efficient way? Is it possible to modulate the operator?

Answer 1

The most efficient way to do this iteratively is to precompute the data you need.

So, precomputation:

const YourNumberType increment  = (i? y : -y);

Then in your loop:

x += increment;

---

EDIT: re question in commentary about how to generate code, like this:

#include <stdio.h>

void display( int x ) { printf( "%d\n", x ); }

template< bool isSomething >
inline void advance( int& x, int y );

template<> inline void advance<true>( int& x, int y )   { x += y; }
template<> inline void advance<false>( int& x, int y )  { x -= y; }

template< bool isSomething >
void myFunc()
{
    int x   = 314;
    int y   = 271;

    for( ;; )
    {
        advance< isSomething >( x, y );     // The nano-optimization.
        display( x );
        if( !( -10000 < x && x < 10000 ) ) { return; }
    }
}

int main( int n, char*[] )
{
    n > 1? myFunc<true>() : myFunc<false>();
}

E.g. with Visual C++ 10.0 this generates two versions of myFunc, one with an add instruction and the other with a sub instruction.

Cheers & hth.,