Functions

Functions allow us to reuse and organize code. For example, say we have some code that calculates the area of a circle:

radius = 5
area = 3.14 * radius * radius

That works! The problem is when we want to calculate the area of other circles, each with its own radius. We could just copy the code and change the variable names like this:

radius = 5
area1 = 3.14 * radius * radius

radius2 = 7
area2 = 3.14 * radius2 * radius2

But we want to reuse our code! Why would we want to redo our work? What if we wanted to calculate the area of thousands of circles??? That's where functions help.

Instead, we can define a new function called area_of_circle using the def keyword.

def area_of_circle(r):
    pi = 3.14
    result = pi * r * r
    return result

Let's break this area_of_circle function down:

• It takes one input (aka "parameter" or "argument") called r
• After the :, the indented lines form the function body – this is the code block that will run each time we use (aka "call") the function
• It returns a single value (the output of the function). In this case, it's the value stored in the result variable

To "call" this function (fancy programmer speak for "use this function") we can pass in any number as the argument (in this case, 5) for the parameter r, and capture the output into a new variable:

area = area_of_circle(5)
print(area)
# 78.5

1. 5 goes in as the input r
2. The body of the function runs, which stores 78.5 in the result variable within the function body
3. The function returns the result variable, which means the area_of_circle(5) expression evaluates to 78.5
4. 78.5 is stored in the area variable and then printed

Because we've already defined the function, now we can use it as many times as we want with different inputs!

area = area_of_circle(6)
print(area)
# 113.04

area = area_of_circle(7)
print(area)
# 153.86

Try calling the function yourself and watch the body run: