HackerRank in Racket: Part 1¶

这里是使用 Racket 完成 HackerRank 函数式编程题目的第一章，包含基础函数和列表操作（0 - 10 题）。

00-Solve Me First FP¶

使用 read 读取命令行输入的两个数字，相加之后使用 display 输出结果。

solution

#lang racket

(display (+ (read) (read)))

read 读取表达式并返回对应的值。

(read [in]) → any
  in : input-port? = (current-input-port)

01-Hello World¶

solution

#lang racket

(display "Hello World")

02-Hello World N Times¶

使用 in-range 创建一个长度为输入数值的序列，并在每次迭代它时输出 Hello World\n。

solution

#lang racket

(for ([_ (in-range (read))])
  (display "Hello World\n"))

03-List Replication¶

函数签名：

replicate-elements : Integer (Listof Integer) -> (Listof Integer)

利用 append-map 和 make-list 创建元素重复的新列表。

solution

#lang racket

;; replicate-elements : Integer (Listof Integer) -> (Listof Integer)
(define (replicate-elements n lst)
  (append-map (lambda (num) (make-list n num)) lst))

(define (read-list)
  (let read-list-helper ([acc '()])
    (let ([x (read)])
      (if (eof-object? x)
          (reverse acc)
          (read-list-helper (cons x acc))))))

(let* ([input (read-list)]
       [n (car input)]
       [lst (cdr input)])
  (for-each displayln (replicate-elements n lst)))

（HackerRank 居然不支持 typed/racket）

04-Filter Array¶

函数签名：

filter-less-than : Integer (Listof Integer) -> (Listof Integer)

使用 filter 过滤出小于 delim 元素。

solution

#lang racket

;; filter-less-than : Integer (Listof Integer) -> (Listof Integer)
(define (filter-less-than delim lst)
  (filter (lambda (num) (< num delim)) lst))

(define (read-list)
  (let read-list-helper ([acc '()])
    (let ([x (read)])
      (if (eof-object? x)
          (reverse acc)
          (read-list-helper (cons x acc))))))

(let* ([input (read-list)]
       [n (car input)]
       [lst (cdr input)])
  (for-each displayln (filter-less-than n lst)))

05-Filter Positions in a List¶

函数签名：

remove-odd : (Listof Integer) -> (Listof Integer)

remove-odd-helper 函数检测当前元素位置 index 是否为奇数：

如果是奇数，则不修改累加器 acc，直接递归调用 remove-odd-helper；
如果是偶数，则将当前元素加入累加器 acc，而后递归调用 remove-odd-helper。

列表处理完毕时，返回累加器 acc 的倒序。

solution

#lang racket

;; remove-odd : (Listof Integer) -> (Listof Integer)
(define (remove-odd lst)
  (let remove-odd-helper ([lst lst] [index 0] [acc '()])
    (cond
      [(empty? lst) (reverse acc)]
      [(odd? index) (remove-odd-helper (cdr lst) (+ index 1) acc)]
      [else (remove-odd-helper (cdr lst) (+ index 1) (cons (car lst) acc))])))

(define (read-list)
  (let read-list-helper ([acc '()])
    (let ([x (read)])
      (if (eof-object? x)
          (reverse acc)
          (read-list-helper (cons x acc))))))

(let* ([input (read-list)]
       [lst (cdr input)])
  (for-each displayln (remove-odd lst)))

06-Array Of N Elements¶

identity 函数不做任何操作，只是将其参数原样返回，所以 (build-list n identity) 可以得到从 0 到 n - 1 的列表。

solution

#lang racket

(define (f n)
  (build-list n identity)）

(define n (string->number (read-line (current-input-port) 'any)))
(print (list(f n)))

我不喜欢它给我的代码模板。

07-Reverse a List¶

要求不能使用 reverse 函数。

函数签名：

reverse-list : (Listof Integer) -> (Listof Integer)

递归地从列表头部开始，每次将列表的第一个元素添加到累积结果的前面，直到列表为空，这样就实现了列表的翻转。

solution

#lang racket

;; reverse-list : (Listof Integer) -> (Listof Integer)
(define (reverse-list lst)
  (let reverse-list-helper ([lst lst] [acc '()])
    (cond
      [(empty? lst) acc]
      [else
       (reverse-list-helper
        (cdr lst)
        (cons (car lst) acc))])))

(define (read-list)
  (let read-list-helper ([acc '()])
    (let ([x (read)])
      (if (eof-object? x)
          (reverse acc)
          (read-list-helper (cons x acc))))))

(for-each displayln (reverse-list (read-list)))

相当于使用foldl + cons：

solution

(define (reverse-list lst)
  (foldl cons '() lst))

08-Sum of Odd Elements¶

函数签名：

sum-odd : (Listof Integer) -> Integer

手动递归版：

solution

#lang racket

;; sum-odd : (Listof Integer) -> Integer
(define (sum-odd lst)
  (let sum-odd-helper ([lst lst] [acc 0])
    (cond
      [(empty? lst) acc]
      [else
       (define x (car lst))
       (if (odd? x)
           (sum-odd-helper (cdr lst) (+ acc x))
           (sum-odd-helper (cdr lst) acc))])))

(define (read-list)
  (let read-list-helper ([acc '()])
    (let ([x (read)])
      (if (eof-object? x)
          (reverse acc)
          (read-list-helper (cons x acc))))))

(displayln (sum-odd (read-list)))

使用apply + filter：

solution

(define (sum-odd lst)
  (apply + (filter odd? lst)))

09-List Length¶

函数签名：

list-length (Listof Integer) -> Integer

solution

#lang racket

;; list-length (Listof Integer) -> Integer
(define (list-length lst)
  (let list-length-helper ([lst lst] [acc 0])
    (if (empty? lst)
        acc
        (list-length-helper (cdr lst) (+ acc 1)))))

(define (read-list)
  (let read-list-helper ([acc '()])
    (let ([x (read)])
      (if (eof-object? x)
          (reverse acc)
          (read-list-helper (cons x acc))))))

(displayln (list-length (read-list)))

10-Update List¶

函数签名：

update-list : (Listof Integer) -> (Listof Integer)

map + abs。

solution

#lang racket

;; update-list : (Listof Integer) -> (Listof Integer)
(define (update-list lst)
  (map abs lst))

(define (read-list)
  (let read-list-helper ([acc '()])
    (let ([x (read)])
      (if (eof-object? x)
          (reverse acc)
          (read-list-helper (cons x acc))))))

(for-each displayln (update-list (read-list)))