clojure school lesson 1 recap and exercises

lesson1.clj

;; WIP material

;; RECAP

; clojure expressions are lists
; syntactically, lists are values in parentheses delimited by white space
(1 2 3)

; clojure expects each expression is built in such a way,
; that the first element is a function name followed by its arguments
; (we will soon start calling these things correctly)

(function-name arg1 arg2 arg3)

; for example

(+ 1 2)

; => 3

; clojure expressions can be nested

(+ 1 (- 4 2))

; => 3

; you can define your own function with defn

(defn function-name [param1 param2]
  ('the 'function 'body 'is 'a 'list))

; example:

(defn my-square [a]
  (* a a))

; my-square returns the square of the provided argument
; and can be called as any built in function

(my-square 4)

; => 16

; functions can be composed of multiple expressions. the return value of the
; function is the return value of the last expression

(defn my-happy-square [a]
  (println "yay")
  (* a a))

; or to reuse what we already have

(defn my-happy-square [a]
  (println "yay")
  (my-square a))

; we can define function to support multiple arities

(defn my-increment
  ([a b] (+ a b))
  ([a] (my-increment a 1)))

(my-increment 5)

; => 6

(my-increment 9 10)

; => 19

;; data

; numbers
5   ; integers
5.2 ; floats

; characters

\c ; this represents the letter 'c'

; strings

"hello there"

; collections

(1 2 3) ; this is a list which we already know
[1 2 3] ; and this is a vector, which we'll learn more about soon.
        ; we also already know it from function definitions

{:height 182 :weight 155} ; this is a map, it associates keys with values
                          ; if this particular map describes a person, (s)he might
                          ; need to put down a little



;; some useful functions, macros and special forms

; Basic mathematical operations
; addition

(+ 1 2 3 4 5 6)

; => 21

; subtraction

(- 1 2 3 4 5 6)

; => -19

; division

(/ 2 3)

; => 2/3

; multiplication

(* 2 3 4)

; => 24

; equality

(= 1 2)

; => false

(not= 1 2)

; => true

(> 2 3)

; => false

(< 2 3)

; => true

; try also <= and >=

; applying a collection of values to a function

(apply + [1 2 3 4 5 6])

; => 21

; collection related functions
; constructiong collections programatically

(vector 1 2 3)

; => [1 2 3]

(list 1 2 3)

; => (1 2 3)

(hash-map :height 182 :weight 31)

; => {:weight 31, :height 182} - the colon only enhances readability. you
; can use it, but you do not have to

; adding to the collection - conj
(conj [1 2 3] 4)

; => [1 2 3 4]

(conj (list 1 2 3) 4)

; => (4 1 2 3)

;with maps, conj expects a vector with key-value pair
(conj {:width 5} [:legth 2])

; => {:width 5, :length 2}


; conj adds to the beginning of the list, but to the end of the vector.
; this is because of the nature of the collection types and the goal of
; conj to be fast. for each collection type, conj adds new elements in such
; a way, that the traversal of the collection is not necessary

; determining collection length

(count [1 1 1 1 1])

; => 5 - works same way with lists

; to insert multiple elements, use 'into'

(into [1 2 3] [4 5])

; => [1 2 3 4 5]

;; EXERCISES

; 0) write a function that computes average of values in a collection
; example call: (my-average [1 2 3 4 5]) should return 3
; expect the input array to have at least one element

; 1) how would you represent a 2D triangle in clojure? can you think of multiple
; representations? write a function that creates a triangle in your representation.

; 2) write a function that determines, whether your triangle is righ-angled
; you will likely want to define some helper functions too