Section #3: Strings & File Reading

January 31st, 2021

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Written by Juliette Woodrow, Anna Mistele, John Dalloul, and Parth Sarin

String Slicing

          
                s = 'PythonTime'
    [0123456789]
          
        

How would you slice the string to receive the following results?

• 'ython'
• 'Py'
• 'Tim'
• 'Time'
• 'T'
• 'PythonTime'
• 'PYTHONTIME'
• 'pythontime'

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. 'ythonTi').

String Searching

Implement the following functions:

• defront(s): If the string parameter has more than 1 character, return it without its first two characters. Otherwise, return the original string. What would we expect this function to return if the string is exactly 2 characters long?
• x_end(s): If the string parameter contains the 'x' character, return the string, from the first 'x' to the end of the string. Otherwise, return an empty string. For example, calling x_end('excited') would return the string 'xcited'.
• is_valid_password(s): Returns True if the string s meets the following criteria, and False otherwise. Password Criteria:
  • Contains the special characters ! and &, in that order (but characters may occur before, in between, and after them)
  • Contains explicitly more characters before the ! than after the &
  Yes, these criteria are as arbitrary and annoying as actual password requirements
• at_words(s): If the string parameter contains 2 or more '@' characters, return the substring between the first two such characters. Otherwise, return the empty string. For example, calling at_word('xx@hello@xx') returns the string 'hello'.

  You might find the 2-parameter version of the s.find(target, start) function useful. This function returns the index in s of the first instance of the string target, searching the range s[start: len(s)].

String Construction

Implement the following functions:

• make_gerund(s): which adds 'ing' to the end of the given string s and returns this new word. If s already ends with 'ing', add an 'ly' to the end of s instead. You may assume that s is at least 3 characters long.
• put_in_middle(outer, inner): which returns a string where inner has 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.
• character_counts(s, ch): that takes in two parameters, an input string s and target character ch, and replaces all occurrences of that character with the number representing the order in which it appears in the word (starting from the beginning). For example, given the input string s = 'hello' and character ch = 'l', the function should return 'he12o'. If the target character doesn't appear in the string, the string should be returned unaltered.

Word Puzzles

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.

Peaceful Words

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'.

Stacatto Words

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 is_stacatto(word) that returns True if a word is a stacatto word and False otherwise.

File Reading

No More Counting Dollars, We'll Be Counting Words

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_peaceful(filename): which returns the number of words that are peaceful in the file.
• count_stacatto(filename): which returns the number of words that are stacatto words in the file.

A few things to note:

• You're given the name of a file that contains English words, so you have to open the file and process the words.
• The words are not all uppercase. Convert them to uppercase before you call the function that you wrote in the last problem.
• Lines will end with a newline character, \n. You can remove this character from a string using the strip function, i.e. s.strip().

GameStop Trades

Write a function, parse_trades(filename) that takes in the name of a file containing trades for the NYSE stock Gamestop (GME) and determines the total number of shares bought and sold. Each line of the file is a trade, consisting of: a trade ID, stock symbol, trade type, and number of shares transacted. The trade type and number of shares for a given trade are delimited by the '|' and '$' characters, respectively. The only two trade types are "BUY" and "SELL." It may be helpful to write a helper function that given a line of the file and delimeter, gives back the value between two of that delimeter in the line.

Consider the file: gamestop_trades.txt

        
4965 GME |BUY| $8$
2725 GME |BUY| $3$
9543 GME |BUY| $11$
8390 GME |SELL| $7$
9114 GME |BUY| $8$
      
    

If you run parse_trades('gamestop_trades.txt'), it would print:

30 shares bought.
7 shares sold.
    

Py-Libs

Write a function, create_pylib(filename) that reads a PyLib (like a Madlib but with help from you and your Python skills) from the given filename and prints out a completed story! For each line in the file replace any bracketed categories (like [noun]) with a word from that category. We've provided you with a get_word(category) function that given a category gives you back a random word of that category. There will be exactly one bracketed category per line. Print out the completed Pylib line by line.

Here is an example file to call your function on: story.txt

When covid ends the first place I am traveling to is [location].
I cannot wait to [verb] when I get there! 
I will make sure to bring [noun].
Until then, I will [adverb] await the trip.