Sorting a hash table in Racket

问题

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

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"))

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))

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.

回答1:

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

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))

来源：https://stackoverflow.com/questions/46595165/sorting-a-hash-table-in-racket

标签

sorting

hashtable

racket