Hash tables

Although we've mainly focussed on lists, racket supports many other data structures. One very useful data structure is the hash table These support fast key based retrieval (and updating) of information.

• The function hash creates an immutable hash table. Values can be retrieved with hash-ref

    #lang racket

    (define ht (hash "apple" 'red "banana" 'yellow))

    (module+ test
      (require rackunit)
      (check-equal? (hash-ref ht "apple") 'red))

• Immutable hash tables can be "updated" in the same way as lists, by returning a new, updated data structure. These updates are "constant time", which means they are pretty fast. In the following, hash-set is acting like cons, adding new data to ht and returning the result. Note that ht is not modified here, as the tests show.

    (define ht2 (hash-set ht "coconut" 'brown))

    (module+ test
      (check-equal? (hash-ref ht2 "coconut") 'brown)
      (check-exn exn:fail? (lambda () (hash-ref ht "coconut"))))

• In the following example, a hash set is used to count the times a given element occurs in a list. Fill in update to the accumulator in the following code so that the tests pass.

    (define (census . lst)
      (define (helper lst acc-hash)
        (cond
          [(empty? lst) (hash->list acc-hash)]
          [else
           (let* ([key (first lst)]
                  [current (hash-ref acc-hash key 0)])
             (helper (rest lst)                         ))]))
      (helper lst (hash)))

    (module+ test
      (check-equal?
       (sort (census 1 2 3 1 2) #:key car <) '((1 . 2) (2 . 2) (3 . 1)))
      (check-equal?
       (sort (census "aardvark" "dingo" "buffalo" "buffalo" "bear") #:key car string<?)
       '(("aardvark" . 1) ("bear" . 1) ("buffalo" . 2) ("dingo" . 1))))

• We are using pairs, which are related to lists, but not exactly the same. car is like first, but works for both pairs and lists.

Explorer

• Run the following program in DrRacket. You can navigate the data structure with the mouse, and evaluate expressions involving this (e.g. (first this), (hash-ref "apple" this)) in the bottom REPL window.

    #lang racket
    (require explorer)

    (define a-list
      (list 1 (list 'foo "hello")
            (hash 'apple "red" 'sky "blue" 'violets "purple")))
    (explore a-list)