Function (programming)

In computer programming, a function is a subroutine which can be called with a set of parameters and which is defined or declared as though it always returns a value, sometimes using a special symbol to indicate that no value is ever actually returned. For example, in the C and Java programming languages, a function that never returns a value would be defined as though the returned value was of type void. The only difference between a void function and a procedure is terminological.

If a function can in fact return a value, then the function may be referenced within an expression, in which case if a value is returned, it is used in the natural way in the evaluation of the expression.

For example, in the C programming language, in which 'int' signifies a sufficiently small integer, if f and g are declared as follows:

int f(int a);
int g(int a);

and if f(1) returns 1, and if g(2) returns 2, then the expression f(1) + f(2) would evaluate to 3.

In procedural programming languages, functions may have side-effects, that is to say, they may have outside inputs and outputs. This makes them not equivalent to mathematical functions.