4Clojure 練習

4clojure 是一個非常棒的 Clojure 練習網站，可以在練習的過程中逐漸熟悉 clojure 的 核心命令以及常用函式。

這邊列出我最近重寫 4clojure 整理的資訊。

目前排名資訊:

Username: coldnew
Rank: 1916 out of 38587
Problems Solved: 85

001: Nothing but the Truth

URL: https://www.4clojure.com/problem/1

This is a clojure form. Enter a value which will make the form evaluate to true.
Don't over think it! If you are confused, see the getting started page. Hint:
true is equal to true.

(= __ true)

點我顯示/隱藏內容

true

這是基本題，目的就是將題目的比較結果弄成 true ，因此餵 true 進去就好了。

在 clojure 中，只有 true 才是 true ，如果你餵其他的東西，就為發現:

(= true 1)       ; => false
(= true [1 2 3]) ; => false

002: Simple Math

URL: https://www.4clojure.com/problem/2

If you are not familiar with polish notation, simple arithmetic might seem
confusing.

Note: Enter only enough to fill in the blank (in this case, a single number) -
do not retype the whole problem.

(= (- 10 (* 2 3)) __)

點我顯示/隱藏內容

4

這題就是單純去了解 Lisp 的數學運算模式，Lisp 類語言皆是採用 前置式 表達式，弄 成我們看得懂的形式的話就是:

10 - (2 * 3) = ?

003: Intro to Strings

URL: https://www.4clojure.com/problem/3

Clojure strings are Java strings. This means that you can use any of the Java
string methods on Clojure strings.

(= __ (.toUpperCase "hello world"))

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"HELLO WORLD"

在 Clojure 中，字串其實就是 java.lang.String ，因此 toUpperCase 就是將所有字串轉 換成大寫。

org9429b5c" class="outline-2">

004: Intro to Lists

URL: https://www.4clojure.com/problem/4

Lists can be constructed with either a function or a quoted form.

(= (list __) '(:a :b :c))

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:a :b :c

在 clojure 中我們可以使用 list 將參數轉換成鍊表 (List) :

(list 'a 'b 'c 'd 'e 'f 'g)
;; => (a b c d e f g)
(list 1 2 3)
;; => (1 2 3)

005: Lists: conj

URL: https://www.4clojure.com/problem/5

When operating on a list, the conj function will return a new list with one or
more items "added" to the front.

Note that there are two test cases, but you are expected to supply only one
answer, which will cause all the tests to pass.

(= __ (conj '(2 3 4) 1))

(= __ (conj '(3 4) 2 1))

點我顯示/隱藏內容

'(1 2 3 4)

conj 要注意的地方是對於 List 以及 Vector 有不同的處置方式：

;; notice that conjoining to a vector is done at the end
(conj [1 2 3] 4)
;;=> [1 2 3 4]

;; notice conjoining to a list is done at the beginning
(conj '(1 2 3) 4)
;;=> (4 1 2 3)

006: Intro to Vectors

URL: https://www.4clojure.com/problem/6

Vectors can be constructed several ways. You can compare them with lists.

Note: the brackets [] surrounding the blanks __ are part of the test case.

(= [__] (list :a :b :c) (vec '(:a :b :c)) (vector :a :b :c))

點我顯示/隱藏內容

:a :b :c

在 Clojure 中，只要是 sequential 的都可以透過 = 來進行比較，而 vector 和 list 皆屬此類。

(= [1 2] '(1 2))
;; => true

至於什麼時候要用 vector 什麼時候要用 list, 這點可以看看 Raynes 在 irc 上面詢問 Clojure 作者 Rich Hickey 的紀錄: (原始討論串)

[12:21] <Raynes>  Vectors aren't seqs, right?
[12:21] <rhickey> Raynes: no, but they are sequential
[12:21] <rhickey> ,(sequential? [1 2 3])
[12:21] <clojurebot>  true
[12:22] <Raynes>  When would you want to use a list over a vector?
[12:22] <rhickey> when generating code, when generating back-to-front
[12:23] <rhickey> not too often in Clojure

org6bb6a41" class="outline-2">

007: Vectors: conj

URL: https://www.4clojure.com/problem/7

When operating on a Vector, the conj function will return a new vector with one
or more items "added" to the end. test not run

(= __ (conj [1 2 3] 4))

(= __ (conj [1 2] 3 4))

點我顯示/隱藏內容

[1 2 3 4]

這題其實和 005: Lists: conj 是類似的，再來複習一次 conj 對於 List 以及 Vector 的 不同處置方式：

;; notice that conjoining to a vector is done at the end
(conj [1 2 3] 4)
;;=> [1 2 3 4]

;; notice conjoining to a list is done at the beginning
(conj '(1 2 3) 4)
;;=> (4 1 2 3)