Getting started with Functions¶

What in the world is a function. You already know. It is that sqrt thing you just worked out. It is that sin button on your calculator (no, not that kind of sin, the other kind!). Basically, this is what a function is:

A function is a box with some code in it. We do not want to worry about the code, we just want the box to do something handy for us. Ideally, we do not even need to write the code in the box, someone else did that for us.

We will use a lot of functions other people wrote in our Python programs. For now, we just need to see how they are set up and how to use them.

Layout of a function¶

All functions have a basic structure. There is a function name we use to refer to the function. There may be function parameters, a fancy term for pieces of data we need to give to the function before it can do its job. The actual body of the function is where the real code lives. That code does the work. Finally, there may be a return set of values handed back to us when the function finishes its work. Not all functions return anything. More on that later.

In Python, a function looks like this;

def mysqrt(number):
guess = number / 2
for guess_number in range(0,10):
difference = number - guess * guess
guess = (guess + number/guess)/2
return guess


This is not a good function, but it does wrap up the logic we went over last time in a nice box.

But what does it all mean?

Function name¶

The def key-word marks the start of a function definition. The name of the function follows immediately.

Function parameters¶

After the name of the function, we must include an open parenthesis character. Any parameters we want to give to the function will be listed after this open parenthesis mark, and all will be separated by commas. When we are done with the parameters, we include a close parenthesis character follows by a colon.

Function body¶

The code for the function will be indented after the first line. All statements so indented make up the body of our function. We can refer to the parameter as we did with number in the above example. We know when we have hit the end of the function because following code will not be indented.

Function return¶

Not all functions return anything, This one does, and we see what is returned by that return statement.

All of this looks confusing, so I will use an analogy, created for game programmers, to explain what is going on.

Trolls in a box¶

Pretend you have a really cool calculator but it is locked up in a box. The box has slots on top, and another slot on the side. You have a bunch of cards on top of the box. On the box is painted a name, something like sqrt.

You can pick up a card from the top of the box and write down some value next to the name. When you are done, stick it in the top of the box

Now for the fun part¶

Kick the box to wake up the troll.

(sound of furious activity followed by a new card sliding out of the box through the slot on the side)