Introduction to Functions

A named little machine that does one job for you

Big Idea: A function is a reusable, named block of steps. You give it some inputs, it does its steps, and it hands back an output. Once you build a function, you can use it again and again without writing the steps out every time.

By the end you will be able to:

say what a function is in simple words,
explain inputs and an output,
say why we split a big program into small named pieces.

1. What is a function?

A function is a group of steps that we put together, give a name, and use whenever we want. Instead of writing the same steps over and over, we write them once inside a function. Then we just say the function's name to run all of those steps at once.

Think of making toast. The steps are: put bread in the toaster, push the lever down, wait, take out the toast. That is four steps. If we put those steps in a function called makeToast, then next time we only need to say makeToast and all four steps happen. The name stands for the whole block of steps.

A function is a reusable, named block of steps. Reusable means you can use it many times.

2. Inputs and an output

Most functions take some inputs and give back one output.

Input = what you put in (the things the function needs to do its job).
The steps = the work the function does with the inputs.
Output = what comes back out when the function is done.

For example, a function called addTwoNumbers takes two inputs (say 3 and 5), adds them, and its output is 8. Change the inputs and you get a different output: give it 10 and 2, and the output is 12. Same function, same steps, but different results depending on the inputs.

Introduction to Functions