Section #4: Strings & File Reading

July 12th, 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'
• 'e'
• 'ime'

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'). You can use negative indices to get letters from the back of the string. For example, the slice s[-2:] starts at the second to last character of the string and goes to the end of string. It would be 'me'.

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.

Word Puzzle

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. For this problem, you can assume that word will be a string containing uppercase alphabetic characters only.

File Reading

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 for a given trade comes after '||' and is followed by a single '|'. The price for a given trade starts with '$$' and is followed by a single '$'. 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, a string that comes before a value, and the string that comes after a value, gives back the value between two of those strings 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. 
    

The main function

Your job is to write a program that emulates the 3 calculator functions shown below:

          
$ python3 calculator.py -square 42
1764 # prints the square of the number passed in

$ python3 calculator.py -exp 2 10
1024 # prints the first number raised to the power of the second number

$ python3 calculator.py -add 1 2 3 4 5
15   # prints the sum of all the numbers typed in
          
        

You may assume that you are provided with a main function that takes as a parameter the list of arguments typed in the console, as below:

          
import sys

def main(args):
    # your code here
    pass

if __name__ == "__main__":
    main(sys.argv)
          
        

Thus, your job is to decompose and implement the main function so that your program produces the sample output above.

Notes

• You may assume that the correct number of parameters are passed in to the program: specifically, -square will be followed by two numbers, -exp will be followed by two numbers and -add will be followed by at least one number.
• You will need to deal with the fact that the command line parameters are passed to your program as strings, and you will need to convert them to ints.