Hacker's Delight

2-20. Alternating among Two or More Values

Suppose a variable x can have only two possible values a and b, and you wish to assign to x the value other than its current one, and you wish your code to be independent of the values of a and b. For example, in a compiler x might be an opcode that is known to be either branch true or branch false, and whichever it is, you want to switch it to the other. The values of the opcodes branch true and branch false are arbitrary, probably defined by a C #define or enum declaration in a header file.

The straightforward code to do the switch is

if (x == a) x = b; 
else x = a; 

or, as is often seen in C programs,

x = x == a ? b : a; 

A far better (or at least more efficient) way to code it is either

If