Due: Thursday 23 September.
The reading for this assignment is Chapters 7-8 of
How to think....
- Download filters.py by typing:
Read the file and get an idea of what it does. You should be able
to get a sense of how it works even though it uses some features we
haven't worked with yet.
The function is_palindrome is an example of a filter. It
checks words and indicates whether or not they are palindromes.
Or, at least, it is supposed to. At the moment it indicates whether
or not a word is equal to the string 'palindrome', which is
probably not what we had in mind.
If you run the program as is, it will run through a list of words (in
/usr/share/dict/words) and print palindrome. Your job
is to change is_palindrome so that it does what it is supposed
to do. A palindrome, since you asked, is a word that reads the same
forward and backward, like `reviver' and `noon.'
How many palindromes are there in our little dictionary?
Hint: use a pipe.
- Write a filter function named is_word that always
returns True. Now you can run filters.py again with the new
python filters.py is_word | wc
So now we know how many words there are in the dictionary.
- Read the handout regarding lipograms (attached), and then
write a filter called has_no_e that returns
True if the given word doesn't have the letter `e' in it.
What percentage of the words in the dictionary have no `e'?
- Write a function named avoids
that takes a word and a string of forbidden letters, and
that returns true if the word doesn't use any of the forbidden
letters. You can call your function from the command line like this:
python filters.py avoids xyz | wc
Can you find a combination of 5 forbidden letters that
excludes the smallest fraction of the dictionary?
- Write a function that takes a word and a list of required
letters, and that returns True if the word uses all the required letters
at least once.
How many words are there that use
all the vowels aeiou? How about aeiouy?
Hint: You can convert a string to a list using the
built-in function list.
- Write a function that takes a word and a list of letters,
and that returns True if the word contains only letters in
the list. How many words can be formed using only the letters
acefhlo? Other than `hoe alfalfa'.
- Write a function called is_abecedarian that returns
True if the letters in a word appear in alphabetical order.
How many abecedarian words are there?
- Download Palindrome.py from the class web page.
Read through the program and
then run it. If the version of is_palindrome in your
filters.py is correct, then Palindrome.py should
provide a working graphical palindrome-checker.
- In filters.py, write a function called rotate_word
that takes a string and an integer as parameters, and that returns
a new string that contains the letters from the original string
`rotated' by the given amount. To rotate a letter means to
shift it through the alphabet, wrapping around to the beginning
if necessary. For example, Y shifted by 1 is Z, and Z shifted
by 1 is A. A shifted by 5 is F.
For you hard-drinking electrical engineers, what do you get if
you rotate 'ohms' by 1? For you entrepreneurial bakers, what do
you get if you rotate 'hawk' by 8? And for you environmentalists,
what's 'terra' rotated by 13?
Hint: check out the ord and chr functions documented
A good solution to this problem should be appropriately general
and encapsulated. It should work for upper- and lower-case letters,
and for positive and negative shifts.
- As a JFFE, how many rotate-pairs can you find in the
- As a second JFFE, go back to Palindrome.py and make a copy
of check_palindrome called rotate_entry. Instead
of checking for palindromes, it should take the contents of the
text entry, rotate it by 1, and display the result in the label.
- As a third JFFE, add a second text entry to the GUI,
and modify rotate_entry so that it gets the amount of the
shift from the new entry.
WHAT TO TURN IN: You should hand in a copy of filters.py
(printed with a2ps), along with a page of plain text that
presents your findings regarding words with various properties,
rotate-pairs, and anything else you find that you think is