球拍中的哈希表 [英] Hash table in Racket

问题描述

我是Racket的新手，我试图定义一个将对哈希表进行排序的函数sort-mail.

I'm new to Racket and I'm trying to define a function sort-mail that is gonna sort a hash table.

我有一些已定义的列表:

I've some defined lists:

(define test-dates
    '("Sun, 10 Sep 2017 09:48:44 +0200"
      "Wed, 13 Sep 2017 17:51:05 +0000"
      "Sun, 10 Sep 2017 13:16:19 +0200"
      "Tue, 17 Nov 2009 18:21:38 -0500"
      "Wed, 13 Sep 2017 10:40:47 -0700"
      "Thu, 14 Sep 2017 12:03:35 -0700"
      "Wed, 18 Nov 2009 02:22:12 -0800"
      "Sat, 09 Sep 2017 13:40:18 -0700"
      "Tue, 26 Oct 2010 15:11:06 +0200"
      "Tue, 17 Nov 2009 18:04:31 -0800"
      "Mon, 17 Oct 2011 04:15:12 +0000"
      "Sun, 16 Oct 2011 23:12:02 -0500"
      "Mon, 11 Sep 2017 14:41:12 +0100"))

   (define sorted-dates
    '("Tue, 17 Nov 2009 18:04:31 -0800"
      "Tue, 17 Nov 2009 18:21:38 -0500"
      "Wed, 18 Nov 2009 02:22:12 -0800"
      "Tue, 26 Oct 2010 15:11:06 +0200"
      "Sun, 16 Oct 2011 23:12:02 -0500"
      "Mon, 17 Oct 2011 04:15:12 +0000"
      "Sat, 09 Sep 2017 13:40:18 -0700"
      "Sun, 10 Sep 2017 09:48:44 +0200"
      "Sun, 10 Sep 2017 13:16:19 +0200"
      "Mon, 11 Sep 2017 14:41:12 +0100"
      "Wed, 13 Sep 2017 10:40:47 -0700"
      "Wed, 13 Sep 2017 17:51:05 +0000"
      "Thu, 14 Sep 2017 12:03:35 -0700"))

该功能应该通过此测试.

The function is supposed to pass this test.

(module+ test       
  (define test-hashes (map (lambda (x) (hasheq 'Date x)) test-dates))       
  (define sorted-hashes (map (lambda (x) (hasheq 'Date x)) sorted-dates))     
  (check-equal? (sort-mail test-hashes) sorted-hashes)) 

那么，我该如何开始?我发现Racket中的哈希表非常困难.我考虑过使用sort函数，但是它猜测它并没有使用哈希表作为参数.

So, how do I even start? I find hash tables in Racket very difficult. I thought of using the sort function, but it guess it doesn't take a hash table as an argument.

推荐答案

哈希表本质上是排序的.通过设计，它们通过将唯一键映射到索引而允许(理论上)即时查找时间.因此，由于没有必要，因此没有排序机制可对哈希图起作用.如果您试图将键值对汇总到一个列表中，然后进行排序，那肯定是可能的.

Hash tables are inherently sorted. By design, they allow for (theoretically) instant lookup time by mapping a unique key to an index. So, there is no sorting mechanism to act on a hash-map as there is no need. If you are trying to aggregate the key value pairs into a list, and then sort, that is certainly possible.

hash-keys将返回表中的键列表. 哈希值将返回表中的值列表.

hash-keys will return a list of keys in the table. hash-values will return a list of values in the table.

这些列表可以排序.您还可以将每个列表的每个元素配对在一起(因此是键值对列表).请尝试以下操作:

These lists can be sorted. You can also pair each element of each list together, (so a list of key-value pairs). Try the following:

(define h (make-immutable-hash
   (list (cons 1 2)
         (cons 3 4)
         (cons 5 6)
         (cons 7 8))))


(define (pair-up key value)
   (list key value))

(map pair-up (hash-keys h) (hash-values h))  

; Alternative to above, where pair-up is essentially defined inside.  
(map (lambda (key value) (list key value)) (hash-keys h) (hash-values h))

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