October 18th, 2020
Written by Brahm Capoor, Juliette Woodrow, Peter Maldonado, Kara Eng, Tori Qiu and Parth Sarin
s = 'PythonTime'
How would you slice into this string to obtain the following results?
Remember, strings in Python are 0-indexed. In addition, the slice
s[1:8] is inclusive of the first index, and exculsive of
the second (that is, it will get the string beginning at index 1 and up
to, but not including, index 8, i.e.
Implement the following functions:
only_one_first_char(s): removes all occurrences of the first character of
sexcept the first character itself. For example,
'recuence'. You may assume
shas at least one character.
make_gerund(s): which adds
'ing'to the end of the given string
sand returns this new word. If
salready ends with
'ing', add an
'ly'to the end of
sinstead. You may assume that
sis at least 3 characters long.
put_in_middle(outer, inner): which returns a string where
innerhas been inserted into the middle of the string
outer. To find the middle of a string, take the length of the string and divide it by 2 using integer division. The first half of the string should be all characters leading up to, but not including, the character at this index. The second half should start with the character at this index and include the rest of the characters in the string.
In these problems, we'll investigate properties of words in the English language. In each problem, we'll define a special rule and write a function to determine whether a word obeys that rule or violates that rule. For this problem, you can assume that word will be a string containing uppercase alphabetic characters only.
We say that a word is a palindrome if it reads the same forwards as backwards. For example, "Abba" is a palindrome because it is the same word read forwards and backwards. Here are some more examples:
Trueif a word is a palindrome and
We say that a word is a tridrome if the first three letters of
the word are the same as the last three letters of the word (and appear
in the same order). All tridromes must be at least 6 letters long. For
example, ENTERTAINMENT, MURMUR, and UNDERGROUND are tridromes. Write a
is_tridrome(word) that returns
True if a word is a tridrome and
We say that a word is peaceful if its letters are in alphabetical order.
For example, ALMOST, CHIPS, DIRTY, FIRST, and HOST are all
peaceful words. Write a function
is_peaceful(word) that returns
True if a word
is peaceful and
False otherwise. You may assume you have
access to a constant
ALPHABET which is a string of the
uppercase letters in the alphabet, in sequential order, i.e.,
ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.
We say that a word is a stacatto word if all of the letters in even
positions are vowels (i.e., the second, fourth, sixth, etc. letters are
vowels). For this problem, the vowels are A, E, I, O, U, and Y. For
example, AUTOMATIC, CAFETERIA, HESITATE, LEGITIMATE, and POPULATE are
stacatto words. Write a function
True if a word is a stacatto word and
Suppose you're given a file that contains all the words in the English language, where each one is on a different line. Write the following functions, using the functions you wrote in the previous problem:
count_tridromes(filename)which returns the number of English words that are tridromes.
count_peaceful(filename)which returns the number of English words that are peaceful.
count_stacatto(filename)which returns the number of English words that are stacatto words.
A few things to note:
\n. You can remove this character from a string using the
You can actually run this program! We've provided a file of all the
words in the English language called
words.txt in the
Implement the following functions:
exclaim(s): Given a string
s, look for the first exclamation mark. If there is a substring of 1 or more alphabetic characters immediately to the left of the exclamation mark, return this substring including the exclamation mark. Otherwise, return the empty string. For example,
exclaim('xx Hello! yy')returns
vowels(s): Given a string
s, look for the first colon. If there is a substring of 1 or more vowels immediately to the right of the colon, return this substring without the colon. Otherwise, return the empty string. For example,
Try out your solutions here.
Implement the following function:
That takes as a parameter a
representing a file with a single integer on each line,
and returns the smallest unique positive integer in the file.
An integer is positive if is greater than 0, and unique if it
occurs exactly once in the file. For example, suppose
filename.txt looks like this:
42 1 13 12 1 -8 20
You may assume that each line of the file contains exactly one integer, although it may not be positive and that there is at least one positive integer in the file.
Download the PyCharm project for this section here.
Now, we're going to turn our attention to a parsing task we'd be more likely to see in the real world: parsing email addresses. For the purposes of this problem, we'll be using a simplified format of an email address as follows:
hostname is a string with at least 4 characters.
It consists of alphabetic characters and at least one period. In
addition, the username can be any length, including 0 characters.
Some examples are:
email@example.com # valid email address firstname.lastname@example.org # valid email address jillian@website # invalid email address, needs at least one period. email@example.com # invalid, since 1 isn't a letter or period @gmail.com # valid email address firstname.lastname@example.org # invalid, less than 4 characters long.
Suppose you have a file called
emails.txt that looks
Please forward this email to email@example.com for me. Thanks! Can someone tell me who owns the firstname.lastname@example.org email address? The email email@example.com keeps sending me spam mail. Please forward this email to firstname.lastname@example.org for me. Thanks! Omg @ye is my favorite! Do you think email@example.com is spam? This one isn't spam: firstname.lastname@example.org Hello, world! Why am I getting emails from email@example.com?
which has at most one email address per line of the file. Your job is to write the following function:
which takes in a string representing a file's name and returns a
list of all the unique hostnames in the file.
For example, calling the function with the parameter
would have the following result:
'emails.txt') ['d.tv', 'gmail.com', 'spam.com', 'stanford.edu', 'yahoo.com']extract_all_hostnames(
In writing this function, think about how best to decompose it into functions that are responsible for subparts of the problem. For example, consider implementing a function which extracts a hostname from a single line and how you might use it.
Download the PyCharm project here.
In the last problem, you built a program to retrieve email hostnames from a file. Unfortunately, that program was limited in several ways. For example, it could only parse a single email from each line of the file, only retrieved the hostname of each email, and finally wasn't robust to peculiar cases such as punctuation occurring immediately after the hostname.
This time, you'll leverage your skills with nested loops and string parsing to build a more sophisticated program to grab emails from a file. You'll start by writing a program that simply grabs every email address from the file by implementing functions which we specify and whose definitions we provide for you. Then, you'll make your program a little more flexible by having it to support a variety of command line arguments which alter its behaviour.
First, we're going to refine our definition of what constitutes an email address. An email address must be formatted in the following way:
Every character in both the username and the hostname must be a
letter, a digit, a period, a dash, or an underscore (the
'_' character). The username must be at least one
character long, and the hostname must be at least 4 characters long,
one of which is a period. With this in mind, implement the following
useful helper function:
that takes in a character, and returns whether that character is a valid part of an email address. This will not be a long function, but will be instrumental in the readability of the more complex functions you write later.
Your job here is to implement the following function:
Which takes as input a string representing a line of text from a file, and returns a list of all the valid email addresses in that line.
Here's some sample output for the
'xx firstname.lastname@example.org email@example.com') ['firstname.lastname@example.org', 'email@example.com'] get_all_emails('_@_ aa-bb@TV.firstname.lastname@example.org') ['aa-bb@TV.org', 'email@example.com'] get_all_emails('abc @ @ 123')  get_all_emails('') get_all_emails(
Some words of wisdom:
@character in the email, and then scan backwards and forwards to find the other characters in the email address. As a reminder, the
str.find()function accepts an optional second parameter which specifies which index to begin searching in the string from.
is_email_char()function you wrote in the previous section will be very helpful here.
Finally, implement the following function:
that takes as input a filename for your function to read through and
returns a list of all the email addresses in the file. For
example, if the file
emails.txt looks like this:
Hello firstname.lastname@example.org this is email@example.com And a.7@d_e.org and firstname.lastname@example.org email@example.com is not nick's email
then the function would behave as below:
'emails.txt') ['a.7@d_e.org', 'firstname.lastname@example.org', 'email@example.com', 'firstname.lastname@example.org', 'email@example.com']get_emails_from_file(
We've written a main function for you that puts all of these together, so you don't need to worry about modifying it for this section.
def main(): args = sys.argv[1:] if len(args) == 1: emails = parse_all_emails(args) for email in emails: print(email) # some other bookkeeping here
You can use your program as demonstrated below:
python3 emails.py emails.txt a.7@d_e.org firstname.lastname@example.org email@example.com firstname.lastname@example.org email@example.com python3 emails.py big-emails.txt --@ --@and.com --@bill --@come --@oh --@oh.com --@the.com ....lots and lots of emails.... firstname.lastname@example.org email@example.com your@walk
Now that you have a basic version of your program working, you'll now turn your attention to making it a more flexible and powerful by implementing various optional command line options for the user:
-maxcommand line option allows you to specify the maximum number of emails you'd like to grab from each line. For example, if one of the lines in the file is
firstname.lastname@example.org and email@example.com and firstname.lastname@example.org, but your program is called as below, only
email@example.com be printed to the terminal.
python3 emails.py emails.txt ... emails from other lines in the file ... firstname.lastname@example.org email@example.com firstname.lastname@example.org ... emails from other lines in the file ... python3 emails.py -max 2 emails.txt ... emails from other lines in the file ... email@example.com firstname.lastname@example.org ... emails from other lines in the file ...
-hostcommand line option allows you to specify that you would only like to grab emails with a paricular hostname. For example, calling the program as below will only print
stanford.eduemails in the shell.
python3 emails.py emails.txt email@example.com firstname.lastname@example.org email@example.com python3 emails.py -host stanford.edu emails.txt firstname.lastname@example.org
You can assume that a user will use either the
-max option, or the
-host option, but not
Elegantly supporting both these options is primarily a challenge in
decomposition and style - there is no one 'correct' way to do it.
You are free to make whatever modifications you want to the
program's functions, their parameters and return values. As a
reference, the sample solution modifies the
get_all_emails functions, although you are welcome to
pursue an alternative strategy.