During game development you often need a variable that switches between two values with each call/loop/frame. Here is a short example:

const int a = 38;
const int b = 21;
int x = 0;
// [ ... ]
if (x == a) {
    x = b;
} else {
    x = a;
}

In C/C++ (and also other languages like Java, PHP, ActionScript…), you also often see the following shorter syntax, that does the exact same thing:

const int a = 38;
const int b = 21;
int x = 0;
// [ ... ]
x = (x == a) ? b : a;

But a better and more efficient way to code this is:

const int a = 38;
const int b = 21;
int x = b; // make sure x is initialised with one of the constants
// [ ... ]
x = a + b - x;

This is faster because the compiled code requires no instructions for the if statement.

If you want to switch between true and false, you can use:

bool x = true;
// [ ... ]
x = !x;

If you want to switch between a negative and positive value, you can use:

int x = 10;
// [ ... ]
x *= -1;