2.1. Functions

We've seen functions for addition, subtraction, multiplication, and division. We're far from limited to those, though! Not only does Clojure have hundreds of functions built-in, we can define our own functions to use!

To define a function, we use defn. The form for this looks like:

(defn function-name
    [input]
    (do-something-with input))

Remember that this is a list, where the first item is defn. In this case:

• the second item is the name we want to give to the function we're defining
• the third item specifies what inputs (in square brackets) we want this function to work with
• the fourth item is an expression for what we're going to do with our input

For example:

(defn add-ten
  [x]
  (+ 10 x))

(add-ten 5)

Let's take a step back and break down exactly what we just did:

• we are defining a function named add-ten
• this function takes one input, which we call x
• we add 10 to whatever the value of x is

In the following code block, try writing completing this function that multiplies a value by five:

(defn times-five
  []
  ())

(times-five 4)