A relation is a set of inputs and outputs, often written as ordered pairs
(input, output). We can also represent a relation as a mapping diagram or a
graph. For example, the relation can
be represented as:
Mapping Diagram of Relation
Lines connect the inputs with their outputs. The relation can also be represented as:

A function is a relation in which each input has only one output.

In the relation , y is a function of x, because for each input x (1, 2, 3, or 0), there is only one output y. x is not a function of y, because the input y = 3 has multiple outputs: x = 1 and x = 2.

Examples:

\: y is a function of x, x is a function of y.
: y is not a function of x (x = 3 has multiple outputs), x is a function of y.
: y is a function of x, x is not a function of y (y = 9 has multiple outputs).
: y is not a function of x (x = 1 has multiple outputs), x is not a function of y (y = 2 has multiple outputs).

To check if a relation is a function, given a mapping diagram of the relation, use the following criterion: If each input has only one line connected to it, then the outputs are a function of the inputs.

Example: In the following mapping diagram, y is a function of x, but x is not a function of y: