Prof. Dr. James Propp of the University of Massachusetts Lowell (Boston, VSA) tweeted¹ the following riddle on December 6, 2018:

Its ingenuity comes from the answer being given away by the way the riddle has been formulated: it's an e-lipogram.

A lipogram (from Ancient Greek: λειπογράμματος, leipográmmatos, "leaving out a letter") is a kind of constrained writing or word game consisting of writing paragraphs or longer works in which a particular letter or group of letters is avoided — usually a common vowel, and frequently e, the most common letter in the English language. As such, an e-lipogram is a text that does not contain the letter e and an an-lipogram is a text in which the letters a and n have been avoided. Writing a lipogram may be a trivial task when avoiding uncommon letters like z, j, q or x, but it is much more challenging to avoid common letters like e, t or a, as the author must omit many ordinary words.

Assignment

• Write a function occurrences that takes the location of a text file (str). The function must return a dictionary (dict) that maps each letter (str) in the given text file onto the number of occurrences (int) of the letter in the text file. In counting the number of occurrences of a letter, no distinction must be made between uppercase and lowercase letters. The keys of the dictionary must be represented as lowercase letters, and only the letters that occur in the given text file may occur as keys.

• Write a function missing_letters that takes the location of a text file (str). The function must return the set (set) containing all letters (str) that do not occur in the given text file. In determining the missing letters, no distinction must be made between uppercase and lowercase letters. The missing letters must be represented as lowercase letters in the set returned by the function.

• Write a function make_lipogram that takes two arguments: i) a collection (str, list, tuple or set) of letters (str) and ii) the location of a text file (str). The function must print the contents of the given text file, with each letter from the given collection discarded. In discarding the letters from the given collection, no distinction must be made between uppercase and lowercase letters. The function also has a third optional parameter that may take the location of a text file (str). If a value is explicitly passed to the third parameter, the function must not print the file contents with the discarded letters, but write it to this text file. If the text file did not yet exist, it must be created. If the text file already existed, it must be overwritten.

Example

In the following interactive session we assume the text file lipo.txt² to be located in the current directory.

>>> occurrences('lipo.txt3')
{'i': 21, 'm': 5, 't': 22, 'h': 8, 'n': 16, 'k': 3, 'g': 4, 'o': 12, 'f': 2, 'a': 19, 'r': 7, 'l': 6, 'q': 1, 'u': 9, 'y': 3, 'p': 1, 'c': 4, 's': 8, 'd': 6, 'w': 4, 'b': 2}

>>> missing_letters('lipo.txt4')
{'e', 'j', 'v', 'x', 'z'}

>>> make_lipogram('aeiou', 'lipo.txt5')
'm thnkng f n rrtnl
qntty mprtnt n clcls
(t's hrd t dscss  ntrl
lgrthms wtht t).
Wht cnstnt m  thnkng f,
nd why m  tlkng bt t
n ths dd rndbt wy?
>>> make_lipogram({'a', 'e', 'i', 'o', 'u'}, 'lipo.txt6', 'copy.txt7')

The last statement creates a new text file copy.txt⁸, with the same contents as printed by the previous statement.