Lab 02: Lists & Trees

The main purpose of this lab is to practice elementary recursive manipulation with lists. Lists can be decomposed by functions car and cdr. On the other hand, lists can be built by functions cons, list or append. Also, higher-order functions filter and map can be used as they were introduced in the second lecture (map only applied to a single list).

Exercise 1

Write the function (my-reverse lst) taking a list lst and returning a list consisting of elements from lst in the reverse order. E.g. (my-reverse '(a b c)) => (c b a). The function should use the tail recursion.

TIP

The idea is to use an accumulator acc storing the intermediate result. We start with an empty accumulator and recursively deconstruct the list lst element by element by means of car,cdr and join them to the accumulator by cons. The computation for lst = '(a b c) and acc = '() go as follows:

(cdr lst)	(cons (car lst) acc)
'(b c)	'(a)
'(c)	'(b a)
'()	'(c b a)

Solution

(define (my-reverse lst [acc '()])
  (if (null? lst)
      acc
      (my-reverse (cdr lst) (cons (car lst) acc))))

Note that using accumulators is a general concept.

Exercise 2

Write a function (letter-frequencies str) which takes a string str and returns a histogram of letters occurring in str so that the most frequent characters come first. The histogram is just a list of pairs (char . num) where char is a character and num is the number of its occurrences in str. E.g. (letter-frequencies "good") => ((#\o . 2) (#\d . 1) (#\g . 1)). The string str should be first converted into lowercase characters, so that #\A and #\a represent the same character. Non-alphabetic characters should be removed.

Idea: The function letter-frequencies is just a composition of several functions.

string-downcase -> string->list -> filter-alphabetic -> sort
  -> group-same -> join-lengths -> sort

• The function string-downcase translates all characters into lowercase letters, and it is implemented in Racket.
• The function string->list is implemented as well, and it decomposes a given string into a list of its characters.
• Then non-alphabetic characters can be filter out by filter function using the predicate char-alphabetic?.
• To compute the number of occurrences of characters, we apply the sort function, which groups together the same characters, e.g., (sort '(#\c #\z #\c) char<?) => (#\c #\c #\z). The function sort takes a boolean function as its second argument, taking two arguments and comparing them.
• The function group-same scans the input list and returns a list consisting of lists of the same consecutive characters, e.g., (group-same '(#\c #\c #\z)) => ((#\c #\c) (#\z)).
• The function join-lengths creates for each group of the same character a pair of the for (char . num) where the number of occurrences num is computed by function length.
• Finally, the output is sorted by the number of occurrences.

The function group-same is the only recursive function in our program. It has to keep a partially built group of the same character as an intermediate result. If the new character (car l) coming from the list is the same as the current character in the group, the partial group is extended by this character. Once the new character (car l) differs from the current character in the group, the partial group is closed, joined to the output, and a new group is created.

Solution

(define (group-same lst)
  (define (iter l gr)
    (cond
      [(null? l) (list gr)]
      [(eqv? (car gr) (car l)) (iter (cdr l) (cons (car gr) gr))]
      [else (cons gr (iter (cdr l) (list (car l))))]))
  (if (null? lst)
      '()
      (iter (cdr lst) (list (car lst)))))

(define (join-lengths grs)
  (map (lambda (g) (cons (car g) (length g))) grs))


(define (letter-frequencies str)
  (sort
   (join-lengths
    (group-same
     (sort
      (filter char-alphabetic? (string->list (string-downcase str)))
      char<?)))
   >
   #:key cdr))
   
; Might be more readable:
#|
(define (letter-frequencies-2 str)
  (let* [(lowercase (string-downcase str))
         (listified (string->list lowercase))
         (alphabetic (filter char-alphabetic? listified))
         (sorted-chars (sort alphabetic char<?))
         (grouped (group-same sorted-chars))
         (joined (join-lengths grouped))
         (sorted-occurs (sort joined > #:key cdr))]
    sorted-occurs))
|#

If you wish, you can use function file->string to check letter frequencies in any file, for instance, in Shakespeare's Sonnets by calling (letter-frequencies (file->string "sonnets.txt")) and comparing the result with the letter frequencies in English alphabet Wikipedia.

Task 1

Write a function (average-list lst) taking a list of numbers lst and returning their arithmetical average. E.g. (average-lst '(1 2 3)) => 2. The function should be tail-recursive.

Hint: As the function should be tail-recursive, it has to use an accumulator storing a partial sum of elements from the list. Finally, the resulting sum is divided by the number of all elements in the list. For the number of elements in lst, you can use the function length. Depending on your implementation function can return precise rational numbers like (average-list '(0 1)) => 1/2. If you want to have the usual floating-point representation, use the function exact->inexact, transforming the result into the imprecise floating-point representation.

Solution

(define (average-list lst)
  (define (iter l acc)
    (if (null? l)
        acc
        (iter (cdr l) (+ acc (car l)))))
  (exact->inexact (/ (iter lst 0) (length lst))))

Task 2

Taking an inspiration from the group-same function, write a function (split-list n lst) which takes a natural number n and a list lst and returns a list of lists consisting of n-tuples of consecutive elements from lst, e.g.:

(split-list 2 '(a b 1 2 3 4)) => ((a b) (1 2) (3 4))

In case the number of elements is not divisible by n, make the last list in the output shorter, e.g.:

(split-list 3 '(a b 1 2)) => ((a b 1) (2))

Using functions split-list and average-list from the previous task, write a function (n-block-average n lst) which splits a given list of numbers lst into n-tuples of consecutive numbers and returns a list of averages of these n-tuples. E.g. (n-block-average 2 '(1 3 1 5)) => (2 3).

Hint: The function split-list needs two accumulators. The first accumulator keeps a partially built segment of consecutive elements. The second tracks how many elements we must read from the list to complete the n-tuple of consecutive elements.

Solution

(define (split-list n lst)
  (define (iter l k segment)
    (cond
      [(null? l) (list segment)]
      [(zero? k) (cons segment (iter l n '()))]
      [else (iter (cdr l) (- k 1) (append segment (list (car l))))]))
  (iter lst n '()))
  
(define (n-block-average n lst)
  (map average-list (split-list n lst)))