Glasgow Clojurians - 6 March 2014

collections_as_functions.clj

; We had an interesting chat about using collections
; as functions.  In summary...

; get returns an item from a collection
(get [:a :b :c] 1) 
:b
(get {:a 1, :b 2} :b)
2
(get #{:a :b :c} :c)
:c
(get #{:a :b :c} :foo)
nil

; Calling the collection as a function has the same effect
([:a :b :c] 1) 
:b
({:a 1, :b 2} :b)
2
(#{:a :b :c} :c)
:c
(#{:a :b :c} :foo)
nil

; which is awfully useful when the collection is passed to 
; a higher order function
(map [:a :b :c] [1 2])
(:b :c) 
(map '(:a :b :c) [1 2]) ; sadly, it doesn't work for lists
(map {:a 1, :b 2} [:b :foo])
(2 nil)
(filter #{:b :c} [:a :b :c :d])
(:b :c)

vending_machine.clj

(ns clj-vending.core)

(def nickel  5)
(def dime    10)
(def quarter 25)
(def dollar  100)

(defn calc-change [item-value amount-tendered]
  (let [change (- amount-tendered item-value)] 
    {:amount change :nickel (/ change nickel)}))

(calc-change 60 100)

(defn find-highest-coin [amount]
  (cond 
   (>= amount dollar)  [dollar (- amount dollar)]
   (>= amount quarter) [quarter (- amount quarter)]  
   (>= amount dime)    [dime (- amount dime)]
   (>= amount nickel)  [nickel (- amount nickel)]))

(find-highest-coin 45)