Null function

In computer science, a null function (or null operator) is subroutine that returns no data values and leaves the program state unchanged. When it is part of the instruction set of a processor, it is called a NOP or NOOP (No OPeration).

Mathematically, a (computer) function f is null if and only if its execution leaves the program state s unchanged. That is, a null function is an identity function whose domain and codomain are both the state space S of the program, and for which: