Functions

Functions let us reuse and organize code. 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! Who wants to redo the same work?!? What if we wanted to calculate the area of thousands of circles??? That's where functions help.

Instead, let's 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

• It takes one input (aka "parameter" or "argument") called r
• After the :, the indented lines form the function body, that is, the code that runs each time we use (aka "call") the function
• It returns (outputs) a single value. In this case, 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, storing 78.5 in the result variable in the function body
3. The function returns the result variable, meaning 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: