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The following guide pages cover material needed for this assignment:

Lab 4e — Split and join


1 minute

Download the zip file for this lab, located above. This zip file has code that you will use for this assignment. Extract the files and put them in your cs110 directory in a folder called lab4e.


10 minutes

Review this `” code from the guide on split and join:

def cats_to_cougars(line: str) -> str:
    words = line.split()
    new_words = []
    for word in words:
        if word == 'cat':
            word = 'cougar'
    return ' '.join(new_words)

def main():
    lines = ["BYU's mascot is cool, like a cat or something.",
             "Just like a cat can, he can catch stuff."]
    for line in lines:

if __name__ == '__main__':

Discuss with the TA:

  • How does cats_to_cougars() work? Be sure everyone understands every line of this code.
  • What does split() do in this code?
  • What does join() do in this code?
  • What changes would we make to use replace()?
  • Would using replace() change the functionality?
  • Could we use string concatenation instead of a list?
  • Note that split() is not needed to iterate for each character in a string, or for each string in a list. split() is used to convert a string into a list of tokens.

Cats and dogs

20 minutes

Write a program that:

  • reads lines from input
  • changes the lines so that
    • words containing ‘cat’ are surrounded with a cat emoji
    • words containing ‘dog’ are surrounded with a dog emoji
  • prints out the new lines

You can find starter code in, which also has comments guiding you through the exerise.

When you run the finished program, you should see something like this:

> I like cats and dogs.
I like 🐈cats🐈 and 🐕dogs.🐕
> You should doggedly concatenate your strings!
You should 🐕doggedly🐕 🐈concatenate🐈 your strings!
> There is peace today -- dogs are sleeping with cats.
There is peace today -- 🐕dogs🐕 are sleeping with 🐈cats.🐈

This program has two pieces:

  • The main() function handles the input loop. You should recognize this from Unit 3. Review the guide on input loops as needed.

  • The do_subs() function takes the line and changes it as needed. This follows a new pattern, where you:

    • split() the line into a list of words
    • change some of the words
    • accumulate original and changed words in a new list
    • convert the list of words back into a string using join()

You can review the code and guide links above to see how to do this.

Discuss with the TA:

  • How do you modify a word to have something before and after it?
  • Share a solution and see if anyone did it differently.

Doubling a recipe

20 minutes

Write a program to double a recipe. This program will take two arguments:

  • the input file
  • the output file

The program reads every line of the recipe, finds any words that are numeric and doubles them, then saves all the lines to the output file. You can call the program like this:

python banana-bread.txt doubled-banana-bread.txt

Don’t worry too much about whether doubling makes sense. So if the recipe says:

1 cup of flour

Bake for 40 minutes

The doubled recipe should say

2 cup of flour

Bake for 80 minutes

This is OK!

You can also ignore any floats, so 1.3 cups can remain undoubled.

We have provided code in that has decomposed the problem and has some comments to guide you. There are two main parts:

  • double_quantities(lines) should go through all the lines and call a function to do the doubling. It returns the new lines.

  • double_quantity(line) takes just one line and does the modifications needed to double any quantities in that line (i.e. anything starting with a digit).

Discuss with the TA:

  • How do you modify a word that has a quantity in it?
  • How do you make sure a newline is at the end of each line?
  • Share a solution and see if anyone did it differently.
  • Though not required for the class, the TA might show how to double floats using try/except


To finish this lab and receive a grade, take the canvas quiz.


We are providing a solution so you can check your work. Please look at this after you complete the assignment. 😊