Week 6: Basic Functions¶

Summary¶

This week we talked about functions - what they are, what’re used for, and how to write our own.

Function Basics¶

So what is a function? The short answer is, it’s a bunch of lines of code that you set aside - kind of like a special paragraph - and give a name to. Then, anywhere else in your code, you can use that same name to execute the function’s code, without having to type it all out again. Just by using the name, the computer will know what code you’re talking about.

Here’s an example: Say you wrote some code that prints a bunch of sentences in a particular order, like this:

print("First sentence\n")
print("Second sentence\n")
print("Final sentence!\n")

If you wanted to write this code as a function, it would look like this:

def three_sentences():
        print("First sentence\n")
        print("Second sentence\n")
        print("Final sentence!\n")

Things to note: - def (define) is a keyword that tells Python you’re about to write a function - The next word is the name of the function (you choose this - it can be whatever you like), followed by parentheses (these also indicate to Python that it’s a function) - The line ends with a colon, just like loops and if statements. As always, the contents of the function - its “paragraph” - are indented by 4 spaces

Defining Functions and Calling Functions¶

The code above just defines the function called three_sentences. None of the code will actually be executed; we;re just letting the computer know that in the future, if we say three_sentences(), we’re talking about this paragraph.

After you’ve defined the function like we did above, you can call it anywhere in your code. Calling a function is the same as executing a function. You can call a function simply by writing the function name, followed by parentheses. For example, look at this code block:

def three_sentences():
        print("First sentence\n")
        print("Second sentence\n")
        print("Final sentence!\n")

print("OK, let's call the function!\n")

three_sentences()

The final line of code actually calls the function. Once a function has been defined, you can call it as many times as you want! You can also define as many functions as you want in a single program.

Function Arguments¶

The function above is really simple - you just call it, and it does something. Some functions, like print(), are different - you need to put something in the parentheses, because it’s expecting something to be in the parentheses.

The thing you put in the parentheses is called an argument. That’s just another word for the input of a function.

We can write functions that take arguments too. For example, let’s say you wanted to write a function where, when somebody calls it, they need to put a name (probably a String) in the parentheses, and the function will print out a greeting for that particular person. It would probably look something like this:

def greeting(name):
        print("Hello there, " + name + "!\n")

To write a function that takes an argument in its parentheses, you simply write a variable name inside the parentheses like I did with the name variable above. Then you can use that variable in the function!

Now, when you call the greeting() function, you need to put something in the parentheses, like this:

greeting("Penny")
greeting("Emerald")
greeting("Cortana")

If you try to just call greeting(), Python will complain, because when you defined the function, you told it to expect something in the parentheses.

Python doesn’t check which type of variable an argument is, so even if you’re expecting a String, someone could still type greeting(5.127849) without crashing the program.

You can even have more than one argument in a function! Check out the example below:

def add_two_numbers(num1, num2):
        sum = num1 + num2
        print(sum)
        print("\n")

If you put that at the top of your program, now you can call it to get the sum of any two numbers! For example, try add_two_numbers(0, 5), add_two_numbers(100, -56), and add_two_numbers(.0456, .55903). s you can see, multiple arguments are just separated by commas, both when defining a function, and also calling a function.

Scope¶

We briefly discussed this in class - just a little warning to keep in mind when working with functions. In our add_two_numbers(num1, num2) function above, num1 and num2 are the arguments that the functions expects. They’re both variables that we can use within that function’s paragraph.

However, outside the paragraph, if you try to reference num1 and num2, Python will complain that it doesn’t know what variables you’re talking about. This is because num1 and num2 only exist within the function’s paragraph.

So, for example:

def add_two_numbers(num1, num2):
        sum = num1 + num2
        print(sum)
        print("\n")

print("Let's sum two numbers!")
add_two_numbers(1, 2)
print(num1)

...will crash, because of the last line. We’ll talk more about scope later on.

We finished up by experimenting with turtles and writing functions. Check the Extra Resources section after tomorrow to see some examples!

Homework¶

I have 3 exercises I’d like you to complete in PyCharm this week. They might seem simple, but it’s important to get the basics of writing and using your own functions in code early on, so it’s worth the practice.

Simply write a program that defines the add_two_numbers function from above. Call the function with 5 different pairs of numbers. Then try the following two experiments: What happens if you call it with a pair of Strings? What happens if you call it with one Int and one Boolean?
Write a program that defines the following functions.

difference_between_two_numbers(num1, num2), which subtracts one number from the other and prints the result
multiply_two_numbers(num1, num2), which multiplies the two numbers and prints the result
compare_two_numbers(num1, num2), which prints which of the two numbers is bigger (hint: you’ll need an if statement for this one!)
Then, after you’ve defined these functions, call each one once to demonstrate that it works.

Write any function using the turtle module. It can be as simple as you like (for example, draw_straight_line), as long as it 1) takes a turtle as an argument, and 2) makes that turtle do something. Be creative and challenge yourself!

Remember to email me with questions and answers as tmeo*njgifted.org. I’m always happy to help!

Good luck!

Extra Resources¶

When I finish putting it together, I’ll post some of the turtle code that we wrote using functions