### First clojure examples and Lab 1

``````

(ns examples)

; numbers and simple expessions - just like Scheme
(def a 6)

(println a)

(println (+ 2 3))

(println [(+ 2 3)
(< 3 5)
(/ 10 5)
(/ 5 2)
(+ 1 2 3 4 5)
(. Math sqrt 4) ; calling a java method
(. Math pow 2 200)
(Math/sqrt 4) ; a shorter way of calling a java method
(Math/pow 2 200)])

; lists are familiar from Scheme
(def fruit '("apple" "banana" "strawberry"))

(println (nth fruit 2))

(println (cons "kiwi" fruit))

(println (first fruit))

(println (rest fruit))

; vectors are more common than lists:
(def colors ["red" "green" "blue"])

(println colors)

; cant's do
;("apple" "banana" "strawberry")
; without '

; define a function
(defn average [x y] (/ (+ x y) 2))

(println (average 3 5))

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

; map maps a fucntion over a sequence, returns the result as
; a sequence (that looks like a list)
(println [(map square [1 2 3 4])
(map square '(1 2 3 4))
(map average [1 2 3 ] [-1 -2 -3])
(map average [1 2 3 ] '(-1 -2 -3))
])

(defn starts-a
[string]
(= (.charAt string 0) \a))

(println [(filter even? '(1 2 3 4 5 6 7 8 9 0))
(filter starts-a fruit)])

(println [(reduce + [1 2 3 4])
(reduce * [1 2 3 4])
(range 1 20 2)
(reduce + (range 1 20 2))
(min 2 3)
(reduce min [1 2 3 4 5]) ; use "reduce" to find the minimum element in a sequence
])

; anonymous functions
(println [(map (fn [x] (* x x)) [1 2 3 4])
(map #(* %1 %1) [1 2 3 4])
])

(println
(if (< 2 3) 5 7))

; recursion (a direct definition, not tail-recursive)
(defn fact [n]
(if (<= n 0) 1
(* n (fact (- n 1)))))

(println [ "testing factorial:" (fact 4) (fact 0) (fact 7)] )

; checking multiple conditions:
(println [
"Checking conditions with cond: \n"

(cond false 1 true 2)

(cond (= 1 2) 5 (< 3 2) 6 true 7)

(cond (= 1 2) 5 (< 3 2) 6 false 7) ; returns nil
])

;; Lab 1 problem 1:

; you are given a list of yearly incomes. Incomes are taxed based on the
; following brackets:
; 0 - 10,000 is taxed at 10%,
; 10,000 - 30,000 is taxed at 15%
; 30,000 - 80,000 is taxed at 25%
; Note that the *entire* amount is taxed according to the bracket, so
; the income is 50,000 then the tax is 7,500.
; use map-reduce method to compute the *total* tax paid on the list of incomes.

;; Lab 1 problem 2:
;; write a reverse_list function: a function that takes a list (or a vector) and
;; returns a list with all the same elements, but in the opposite order.

(println [
(cons 5 [6 7])
(first [5 6 7])
(rest [5 6 7])
(empty? [5 6 7])
(empty? [])
(empty? '())
(concat [6 7] [9 8])
(list 5)

])

;;; CAUTION: don't use "list" as a variable name since it's a pre-defined function!!!!

;; Lab 1 problem 3:
;; write a reverse_list2 function that takes a list and returns that list
;; in reverse order *using reduce*.

(defn factorial [num]
(loop [n num accum 1]
(if (<= n 0)
accum
(recur (- n 1) (* accum n)))))

(println ["testing loop/recur factorial:"
(factorial 4)
(factorial 0)
(factorial 7)
])

(defn every-other [param_list]
(loop [the_list param_list keep true result '()]
(cond
(empty? the_list) result
keep (recur (rest the_list) false (concat result (list (first the_list))))
(not keep) (recur (rest the_list) true result))))

(println ["testing every-other:"
(every-other [1 2 3 4 5])
(every-other [])
(every-other [1])
])

;; another (more efficient) version that builds a vector
;; Thanks to Brian Goslinga for pointing out the efficiency differences

(defn every-other2 [param_list]
(loop [the_list param_list keep true result []]
(cond
(empty? the_list) result
keep (recur (rest the_list) false (conj result (first the_list)))
(not keep) (recur (rest the_list) true result))))

(println ["testing every-other:"
(every-other2 [1 2 3 4 5])
(every-other2 [])
(every-other2 [1])
])

;; Lab 2 problem 1:
;; write a function that is similar to every-other but
;; keeps even-numbered elements instead of odd-numbered ones, i.e.
;; produces [2, 4, 9] given the list [1 2 3 4 5 9].
``````

UMM CSci 4409

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