quick-reference.md

Clojure quick guide

Simple clojure stuff

; what version of clojure are we using? (clojure-version)

; understanding what something does (doc doc) (source doc)

Prefix Notation

; Basic prefix notaition examples (* 2 2) (+ 1 2 3) (\ 24 4 3 2) (\ 2 4) (\ 2.0 4) (+ (* 4 5) 22) (+ 4 (* 3 2) 7) (+ 3 (* 2 (- 7 2) 4) (/ 16 4))

(+ 3 3) (- 3 3) (- 2 3) (+ 1 2 3 4 5 6)

; showing the advantage of prefix notaition for helping you ; follow the values of the expression

(+ 1 2 (+ 1 2) (* 2 2) (/ 25 5) (- (- 12 18)))

;; Prefix makes understanding the evaluation order much simpler and consistent than infix notation used in languages such as Java, C++ and C#.

;; Prefix notation is one example where clojure can minimise the amount of cerimony (typing) involved in coding.

Ratio

; Using the division function (/ ) shows another interesting characteristic of Clojure, the fact that it is lazy. This is not lazy in a bad way, but lazy evaluation of data structures. This actually helps to make clojure more efficient at dealing with data, especially very large data sets.

(/ 10 3) 10/3

(/ 10 3.0) 3.3333333333333335

(/ 22 7) 22/7

(/ 22 7.0) 3.142857142857143

(/ 2 4) (/ 2.0 4) (/ 1 3) (/ 1.0 3) (class (/ 1 3)

;; Using a Ratio means that the mathmatical division is not evaluated when using whole numbers (Integers) that would produce a decimal number. If you do return a decimal number then what precision of decimal are you expecting. By specifying one or more of the numbers as a decimal value you are giving Clojure a precision to infer and can therefore provide a specific decimal result.

;; example of Java integration - PI is a static double from the java.lang.Math object http://docs.oracle.com/javase/6/docs/api/java/lang/Math.html

(. Math PI) 3.141592653589793

; An interesting aside is that 22 divided by 7 is not the same value as pie, but a close approximation to it.

What class is that...

(class (str "Jr0cket"))

(class (defn hello-world [name] (str "Hello cruel world")))

The type function will return the metadata or class of something.

defining functions

(def square (fn [x] (* x x)))

(fn [x] (* x x)) ;; annonymous function

Using functions with functions

(map inc [1 2 3 4])

(2 3 4 5)

(defn hello-world [name] (println(str "Hello " name)))

(hello-world "jr0cket")

Data Structures

; There are four data structure types in Clojure:

• List - an ordered linked list
• Vector - an array
• Map - a key value pair
• Set - an ordered set

The most commonly used are the vector and map.

(doc list)

(doc vector)

(doc map)

(doc set)

(set [2 3 4 5])

Coin toss

(defn coin-toss [] (= 1 (rand-int 2)))

(defn toss-score [toss] (if toss 1 -1))

user=> (toss-score true) 1 user=> (toss-score false) -1 user=> (toss-score (coin-toss)) -1 user=> (toss-score (coin-toss)) 1

(repeatedly 5 coin-toss) (true true false false true)

Whats my salary

;; really bad example of Clojure code

(whats-my-monthly-takehome 60000) (whats-my-yearly-takehome 60000)

Interesting stuff

(defonce name value) ; only define this name once, even when recompiled

Caves of clojure

Shows a terminal in either text or swing

http://stevelosh.com/blog/2012/07/caves-of-clojure-01/

Quil

Coloured-balls - lein run - displays coloured balls CljBoids - lein run - follows the mouse around - uses atoms, quite involved code

Recursion

(defn recursive-counter [value] (print value) (if (< value 1000) (recur (+ value 4))))

(recursive-counter 100)

(defn recursive-counter [value] (println value) (if (< value 1000) (recur (+ value 100))))

(recursive-counter 100)

To Sort

(whats-my-monthly-takehome 60000) (whats-my-yearly-takehome 60000)

(type rand-int)

(doc rand-int)

(class "jr0cket")

(class (defn hello-world [name] (str "Hello cruel world")))

;;(javax.swing.JOptionPane/showMessageDialog nil "Hello Java Developers")

(Math/cos 3)

(println "Clojure for java developers")

(list 1 2 3 4)

(def me {:name "john" :twitter "@jr0cket"} )

(println me)

(type(/ 10 3)) (/ 22.7 7.0) (. Math PI)

(type 7.0) (type 7)

(type (list 1 2 3 4))

(type (defn square [x] (* x x)))

Code to show off LightTable

(+ 1 2)

(def person {:name "John" :twitter "@jr0cket"})

(println person)

(use 'whats-my-salary.core)

(yearly-takehome? 35000)

(source yearly-takehome?)

(class yearly-takehome?)

(def random-data [1 2 3 (rand-int 5)])

(type (rand-int 5))

(conj (list 1 2 3)(list 4 5 6))

(conj [1 3] [2 4])

(doc cons)