Warning

## Lab 2: Lists

The main purpose 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.

Hint: 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

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

1. The function string-downcase translates all characters into lowercase letters and it is implemented in racket.
2. The function string->list is implemented as well and it decomposes a given string into a list of its characters.
3. Then non-alphabetic characters can be filter out by filter function using the predicate char-alphabetic?.
4. To compute the number of occurrences of characters, we apply sort function which groups together the same characters, e.g. (sort '(#\c #\z #\c) char<?) ⇒ (#\c #\c #\z). The function sort takes as its second argument a boolean function taking two arguments and comparing them.
5. 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)).
6. 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.
7. Finally, the output is sorted by numbers of occurrences.

The function group-same is the only recursive function in our program. It has to keep as an intermediate result a partially built group of the same character. 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

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 imprecise floting-point representation.

Solution

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 and the second tracks how many elements we have to read from the list to complete the n-tuple of consecutive elements.

Solution