COS 100: Introduction to Programming

Fall 2019

Lab 7: Writing functions

In-class practice labs will not directly impact your grade (although you'll get your standard participation point for staying on task), and will serve to help you with homework projects.

Lab 7A: That's how functions function

• Goal: Get started with writing simple functions.
• Task 1: Write a function that takes an integer as input and returns a boolean:
  • If the input is even, return True.
  • Otherwise, return False.
• Task 2: Write another function that takes an integer as input and returns a boolean:
  • If the input is a prime number, return True.
  • Otherwise, return False.
  • Remember that we already have some code for determining whether a number is prime or not. You can just adapt that code to fit this lab (package it up as a function).
• Task 3:
  • Prompt the user for an integer and report whether it is even or odd and whether it is prime.
  • You should call the two functions you've written above.
  • Make your reports to the user friendly; don't use jargons like "True" and "False."
• Important note: When you have a function that returns a boolean value, the convention is to name the function starting with is. In this case, is_even and is_prime would be appropriate names. Then when you read your if statement out loud, it should almost be in English, which is helpful for understanding your code.

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Lab 7B: What's the function's function?

• Goal: Conform to Python programming conventions.
• Start with Lab 7A code and do the following.
• Task 1: Move all the "main" code (whatever's not part of the two functions) into a third function called main. This new function takes no input and returns nothing. At the end of your program, outside of the main function, call the main function.
• In general, this will be what our programs look like from now on. All the code will be inside functions, with the exception of:
  • comment at the beginning of the file with authorship and brief description of the file
  • import statements at the beginning of the file
  • a call to the main function at the end of the file
• Task 2: Add a comment to describe what each function does, like this:

  def is_prime(number):
      """Take a number and return if it is a prime or not."""
      here be your code that you already wrote
  

  The description should be short and fit on one line. These are known as docstrings.
• All the functions you write, except the "main" function, should have these docstrings. (The purpose of the "main" function is (should be) already described by comments at the top of the file.)
• Task 3: Write a new function that takes an integer number and prints all prime numbers up to number. You should (of course) give the function a reasonable name, have a docstring, and call is_prime.
• To test this function, simply add a call to this function at the end of the file.
• It may be useful to comment out the main() call at the very end of the file so you can focus on this function. See how convenient it is once code is packaged in functions? Even the main function can be skipped!
• Challenge Task 4: Write a new function that takes an integer number and prints the first number primes. (How is this different from the previous function?)

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Challenge Lab 7C

• Goal: Evolve your code.
• Task: Create a system to let the user interactively test properties about numbers. Ask the user for a number, list some actions, and then repeatedly let the user choose actions until the user quits. (See below for a sample run.)
• Start with your code above. Write a new function, perhaps creatively named main2. Change the main() call at the end of your code to main2() in order to call this new main function.
• You can (and should, for now) leave the old main function in the file. This is another convenience of functions.
• Sample run:

  Available actions:
  A. Determine if number is even.
  B. Determine if number is prime.
  C. Change the number.
  D. List primes up to this number.
  E. List this number of primes.
  Q. Quit.
  
  Integer, please: 23
  Action? A
  23 is odd.
  Action? B
  23 is prime.
  Action? A
  23 is odd.
  Action? D
  2 is a prime
  3 is a prime
  5 is a prime
  7 is a prime
  11 is a prime
  13 is a prime
  17 is a prime
  19 is a prime
  Action? C
  Integer, please: 1
  Action? A
  1 is odd.
  Action? B
  1 is not prime.
  Action? C
  Integer, please: 2
  Action? A
  2 is even.
  Action? B
  2 is prime.
  Action? X
  Sorry, I don't understand.
  Action? Q
  Bye.
  

• Challenge: Most likely you have two lines that both prompt for integer from the user. Try to make it so there's only one.