Functions

A toaster takes a slice of bread, does its work and hands you back toast. You do not need to know how the heating wires work, you just use it. Programs have machines like that too: they are called functions, and they make your code shorter, clearer and much easier to understand.

Input, processing, output

A function is a machine with three steps: it takes an input, processes it with a fixed rule and returns an output. The function double, for example, takes a number and returns twice as much: 3 becomes 6, 10 becomes 20. The best part: you build the machine exactly once and can then use it as often as you like.

Parameters are placeholders

So that a function can handle any input, the input gets a name, for example x. This name is called a parameter. It is a placeholder: when building the function you write the rule using x, such as x plus 10. When using it, you plug in a real number, and the rule calculates with exactly that number. A function may also have several parameters: the function sum, for example, takes two numbers a and b and returns a plus b.

Combining functions

Functions become really powerful when you chain them together: the output of one becomes the input of the next. Small, simple machines combine into a big one. That is exactly how all large programs are built, from weather apps to video games.