The focus of this week's lab is on:

• Strings and performing important operations on them (e.g. splitting into substrings, joining, concatenation)
• File IO (reading/writing to a text file)

• Lab 7: Strings
  • Tasks
    • Notes
    • L0. The setup
    • L1. Reading Files, manipulating strings and writing to a file
      • L1a. Read from file
      • L1b. Split and join
      • L1c. Write to file
    • L2. Creating a Decoder
      • L2a: Create a list of lists based on text from a file
      • L2b: Build ABCs string from phrases
      • L2c: Create a cipher decoder

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Tasks

Notes

• Before you turn in your code: make sure that running each one of your code files prints the expected outputs.
• Use if __name__ == '__main__ to organize your code.

L0. The setup

Create a Lab07 folder on your computer where you store CS151 projects and labs, hopefully somewhere that is backed up (e.g. in Dropbox, Google Drive, Microsoft One Drive, etc.)

Open VS Code. Open up a Terminal (e.g. Control+` in VS Code) and change the directory (cd) to the Lab07 folder.

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L1. Reading Files, manipulating strings and writing to a file

Download digits.txt and place it in your lab folder.

L1a. Read from file

Write code in a new source code file lab7_digits.py that reads in the data and stores it as one single-line string. When you print out the string, you should get:
1 2 3 4 5 6 7 8 9 10 11 12

Try doing this two different ways:

1. concatenation (+ operator)
2. using lists and the join method

Hint: Each line ends with an invisible new line character. You probably want to discard each one. Recall: you can use either indexing or the strip string method to help with this.

L1b. Split and join

Starting from your string from L2a, create a new string using both split and join methods (without using loops) that when printed, gives (again, not in a loop: print(yourString)):

1
2
3
4
5
6
7
8
9
10
11
12

L1c. Write to file

Write your string from L1b to a new text file called digits2.txt. There should be one number per line of the file.

Look at the python documentation for an example of how to write a string to a file.

This can be done by creating a new file in write mode ("w")

newFile = open("digits2.txt", "w")

Use the file object's write method with a string as the argument, to add that string to your file.

Make sure to close the file using the close method.

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L2. Creating a Decoder

L2a: Create a list of lists based on text from a file

Create a new file called lab7_letters.py. Download abcs.txt and place it in your lab folder.

Open up the text file in VS Code. For each of the 26 letters in the alphabet, there is a phrase for each of the 26 letters of the alphabet. For example:

a is for apple
b is for banana

Your goal is to create a function called decryption_table which creates a list of lists and returns it. Each sublist should contain:

• first item: Single lowercase letters of the alphabet (the first character of each line). For example, 'a', 'b', etc.
• second item: The last word (without new lines at the end). For example, 'apple', 'banana', etc.

Your list of lists should have 26 items.

After you create your function decryption_table which returns the list of lists, create a test function table_test that runs the following test code. Running lab7_letters.py should produce the expected output below:

letters=decryption_table()
print(len(letters))  # 26
print(letters[0])  # ['a', 'apple']
print(letters[1][0])  # b
print(letters[-1][1])  # zebra

Hints:

• Remember to remove the new line character as you read each line from the file.

L2b: Build ABCs string from list of lists

Now that you have your list of lists set up, your next goal is to create a function abcs which creates a single line string with the 26 letters of the alphabet (all next to each other, without spaces) using your list of lists and prints it.

Calling your function abcs should yield:

abcdefghijklmnopqrstuvwxyz

L2c: Create a cipher decoder

Write a function decrypt that takes in the parameter message, a string, and prints the message decoded.

After you create your function decrypt, create a test function decryption_test that runs the following test code.

decrypt('happy everyone lamb lamb open') # hello
decrypt('water open rooster lamb donut') # world

Call your test function in your main guard (if __name__ == '__main__). Running lab7_letters.py should produce the expected output below:

hello
world

Submit your lab on Classroom

Upload and submit your lab7_letters.py file on Google Classroom.

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