I Like My Clojure Filtered

There are many ways we can process information both as individual values and as collections of values.

In this section we will introduce the following functions:

• filter -
• when, and, or - make a decision based on some condition
• odd?, even? - test if a value is odd or even and returns true or false (predicates)
• for - loop through values
• range - generate a range of numbers

Filtering information

Another form of decision making is to filter information to only return those parts you are interested in, based on some kind of criteria. A simple example of this is to return all the even numbers from a collection

(filter even? (range 10))

There are several functions you can use to filter values, such as odd?, even?, true?, false?, string?, integer?

Other decision making functions

When

(when (> 3 2)
      "Higher")

While

FIXME: needs a better example

(while test & body)

Repeatedly executes body while test expression is true. Presumes some side-effect will cause test to become false/nil. Returns nil

;; a var to be used for its side effects
(def a (atom 10))
;; #'user/a

(while (pos? @a) (do (println @a) (swap! a dec)))

Filtering information based on decisions or conditions

Decisions are commonly made when processing information

odd? even?

(for [x (range 10) :when (odd? x)] x)

In the above code, try changing the function odd? to the function even?.

Using a ? character at the end of a functions name is a convention used in Clojure. Functions named in this way should return a true or false answer. Functions named this way are called a predicate.