lab02 : Magic number

num ready? description assigned due
lab02 true Magic number Tue 01/21 12:00AM Tue 01/28 08:59AM

lab02: Magic number

In this lab, you’ll review:

New skills you’ll practice include:

Lab02 Slides

You can view the accompanying slides to lab02 here.

Carefully read and follow all instructions below

Set up your working environment

Before you get started, remember to arrange and resize your windows like you did in the “Quick Detour” in Lab00, so that you can see both the browser (these instructions) and your files side-by-side.

If you need to install pytest refer to Lab01 instructions for how to do it.

Create lab02 directory

It’s important to differentiate between the Python shell >>> vs the terminal $.

Use the Terminal to create a new lab02 directory inside the cs8 directory.

Refer to Lab00 instructions if you need a reminder of how to use the commands.

As a reminder:

With that information, you should be able to determine how to create a ~/cs8/lab02 directory, and make that directory your current working directory (the one that comes up when you type pwd.) Do that now. (Remember that creating and navigating directories happens using the terminal shell prompt, also known as the Unix command line, marked by $.)

Step 0: Warm-up – Using Function Parameters

This step will walk you step-by-step through the instructions for creating a function and adding parameters to it.

This section will also

Remember your very first lab in this course? (We hope so!) Now, let’s turn your "Hello, World!" from that lab into a function that can also greet the user by name they provide.

Create a function to greet the world

Let’s first create a function called hello(), which takes no parameters and prints "Hello, World!". Save it in a file called (one of the advantages of using separate directories for differet labs: you can reuse the file names :-)).

See if you can write this function without looking at the answer below.

Do not copy/paste the code below (it contains errors), and instead, type the correct code into your function.

def hello():
    print(``Hello, World!``)

Run the function and call it on the IDLE prompt to verify it works correctly.

>>> hello()

If you didn’t listen to us and copy/pasted the code anyway, you must have gotten the SyntaxError: invalid syntax error, because print() expects you to print the strings enclosed in quotes.

Now that you have a function hello() (in a file, you can begin changing its behavior, depending on what the user provides as input into this function.

Include an input argument in the function signature

First of all, change the print to return.

Now, to change the behavior of this function, you’ll need to make it accept an argument. Here, we decided to call it name (you can call it something else).

def hello( name ):

Take a look at Step 3 in the previous lab to refresh what each part of the function means/represents.

This argument name will be like a switch, which will tell the function what to do. In this assignment, we want to include the user’s name in the greeting if there’s a name, or if the name is empty, then return a generic “Hello, World!”.

Let’s try to capture it in pseudocode.


# Given an input parameter called name
# Check if name is empty
#     If name is empty, 
#        then return "Hello, World!"
#     If name is not empty,
#        then return "Hello, <name>!"

Note that in the last line, we will be substituting user’s input for <name> inside the return statement.

How would we check if name is empty? What does it mean for name to be empty?

If the string contains no characters, then it is considered to be empty. Below are the examples of empty strings:

empty_s1 = ''
empty_s2 = ""
empty_s3 = str()

If the string contains no characters, then its length will be 0. You can verify it by running the following on the IDLE command prompt:

>>> empty_s1 = ''
>>> len(empty_s1)
>>> empty_s2 = ""
>>> len(empty_s2)
>>> empty_s3 = str()
>>> len(empty_s3) == len(str())

Note: if we accidentally typed str() instead of len(str()), we would have gotten False, because 0 is not the same as ''.

Convert the pseudocode into code

Now that we have most of the pieces that we need, let’s include the pseudocode in our function as comments and start converting it into Python.

def hello( name ):
    """ Given an input parameter called name,
        the function returns "Hello, <name>!"
        if the name is not empty, otherwise,
        the function returns "Hello, World!"
    if len(name) == 0:   # Check if name is empty
    #     If name is empty, 
    #        then return "Hello, World!"
    #     If name is not empty,
    #        then return "Hello, <name>!"

We could have used instead used two if statements:

However, we are taking advantage of the fact that only if the first if statement is False, then the else block will be automatically executed (run).

def hello( name ):
    """ Given an input parameter called name,
        the function returns "Hello, <name>!"
        if the name is not empty, otherwise,
        the function returns "Hello, World!"
    if len(name) == 0:   # Check if name is empty
        return("Hello, World!")
    else:  # the name is not empty,
        return("Hello, "+ name + "!")

Woohoo! We now have a new function hello, which takes 1 argument. Since this definition overwrote the previous definition (in which you didn’t have to provide an argument), Python will complain if you forget to provide it with the input argument.

>>> hello()
Traceback (most recent call last):
  File "<pyshell#77>", line 1, in <module>
TypeError: hello() missing 1 required positional argument: 'name'

As long as we provide a string, our function knows what to do.

>>> hello("CS 8")
'Hello, CS 8!'
>>> hello("42")
'Hello, 42!'
>>> hello("")
'Hello, World!'


Note that if we do not provide the input as a string, Python will complain, saying TypeError: object of type 'int' has no len().

>>> hello(42)
Traceback (most recent call last):
    if len(name) == 0:   # Check if name is empty
TypeError: object of type 'int' has no len()

OK, we are done with the warm-up!

“Four is the magic number”

This assignment is based on a brain teaser. Look at the output below is see if you can figure out the pattern. If not, don’t worry – we’ll guide you to the solution.

The riddle starts by telling you “Four is a magic number”. For any number you select, it is possible to “somehow” create a sequence, which will ultimately end in “Four is the magic number”.

For example:

0 =>
  Zero is four and four is the magic number
1 =>
  One is three and three is five and five is four and four is the magic number
2 =>
  Two is three and three is five and five is four and four is the magic number
3 => 
  Three is five and five is four and four is the magic number
4 =>
  Four is the magic number
5 => 
  Five is four and four is the magic number
6 =>
  Six is three and three is five and five is four and four is the magic number
7 =>
  Seven is five and five is four and four is the magic number
8 =>
  Eight is five and five is four and four is the magic number
9 =>
  Nine is four and four is the magic number
10 =>
  Ten is three and three is five and five is four and four is the magic number

Hopefully, by now you are able to see a pattern. If not, no worries! Let’s see if working on the helper functions will give you a better clue.

Step 1.0: Create a file called in your ~/cs8/lab02 directory

Choose “File => New File” in IDLE to bring up an “untitled” window, then type the functions into that window (i.e., in the new file).

IMPORTANT: Make sure to save your file with the exact filename we requested. Naming it with some other name will confuse Gradescope and result in a score of 0, even if your work is correct.

Step 1.1: Add the preliminary information

Write a comment with your name and PERM number in that file (in general, you should write this on each of your submitted source files).

Next, since we will be using pytest to verify that our functions work correctly, let’s import that module:

import pytest

Step 1.2: Add the function definition

Add the following function definition into your file.

def numToStr( num ):
    Checks that the provided `num` is an integer 
    between 0 and 10 (inclusive).
    Returns the spelled-out name of the provided number in English.
    return 42 # TODO: replace this stub with the correct code

Important requirement: If the input is less than 0 or greater than 10, then the function should return None (either explicitly or implicitly, i.e., if no return is specified for those cases).

Remember: None is a special value that has its own type (NoneType), which means it is not the same as "None", which is a string.

Even before you create your solution for this step, you can move to the next step to write test functions (which should originally all fail).

Step 1.3: Add the test functions

Add the following function definitions into your file. These are a special kind of function called a test case. These particular test cases are written in the style used by the pytest testing framework, and they follow these rules:

Take a look at the previous lab’s Step 5 to review how you wrote test cases there.

def test_numToStr_0():
   assert numToStr(0) == "zero"

def test_numToStr_10():
   assert numToStr(10) == "ten"

def test_numToStr_11():
   assert numToStr(11) == None

Finally, run the code, and ensure that you don’t have any syntax errors in your Python code.

Step 1.4: Test your code by hand

Because we want to be sure that you continue to practice the skill, test your code by hand first.

That is, select “Run Module” in IDLE, and then type in a few function calls at the Python Shell Prompt. Here are a few:

>>> numToStr(4)
>>> numToStr(7)

OK, we see that the function will continue to provide us with the same answer, consistent with the return value in our function definition. The point is that

There is a reason for that. We call this a “stub value”. It returns the wrong answer on purpose so that we can check that all of the tests fail. We want to see all of the tests fail, and after we fix the function, we can see all of the tests pass. That’s the general idea:

Step 1.5: Run pytest on the file so far

As a reminder, you run pytest OUTSIDE of IDLE, at the regular Terminal prompt ($).

You may find it helpful to bring up a second terminal window and use

cd ~/cs8/lab02

to get into the correct directory. Then use:

python3 -m pytest

You should see three test failures. If you do, then you ready to fix the code so that it works, which is the next step.

(If you need a refresher on how to interpret the output of pytest, refer back to lab01.)

If you got a SyntaxError: invalid syntax, then verify that you are running the command not in IDLE (which has the >>> prompt), but in Unix using the Terminal (which has the $ prompt).

Step 1.6: Fixing the code for numToStr

As usual, if you have failing test cases, the thing to do is fix the code so that the test cases pass.

There are many ways to approach the solution of this problem. We recommend you to use a list that includes the spelled-out name of each number from 0 to 10 (inclusive) in English.

numbers = [ "one", "two", "three", ...] # TODO: update it to include all correct values

Note: you will need to change numbers shown above.

Keeping in mind that list indexing starts at 0, how could you use this to your advantage to return the correct value? (E.g., numToStr(1) should return "one".) Implement your answer by updating your definition of the function numToStr(). Hint: You would need to complete the list of numbers and include the example list numbers in your code, before the return statement.

Once you have the code correct, try testing both using interactive testing as well as by running pytest.

At each step, you should first try to get the test cases to pass by running pytest at the Unix command line as discussed above.

Step 1.7: Seeing the magic in action

Below is the skeleton code for printing the riddle for any number between 0 and 10.

def print_magic( num ):
    if num < 0 or num > 10:
        return # exit the function

    num_str = ... # TODO call numToStr with num as an input

    while num != 4:
        print(num, "is", ...) # TODO: figure out the last piece of the puzzle
        num = ... # TODO: num should become the new value from above
        num_str = # TODO update num_str: call numToStr using new num 
    print("four is the magic number") # output once the loop is finished

When you complete the code correctly and run it in IDLE, you should see the following output.

>>> print_magic(1)
1 is 3
3 is 5
5 is 4
four is the magic number

>>> print_magic(10)
10 is 3
3 is 5
5 is 4
four is the magic number

Note that if we give it a number outside of the expected range, the function should not print anything.

>>> print_magic(11)


If you are getting IndexError: list index out of range, then double-check that the list in the numToStr function has the correct number of elements (should be 11 total, since we are starting at zero).

Another error you might run into is TypeError: object of type 'int' has no len(). If you see it, verify that when you call len(), the variable you give it contains a string object. If you accidentally gave it an integer object or None, it will produce this error. (Note that you can check the type by printing the value or type() of that object as part of debugging your code.)

Some of you might get the error Test Failed: None != Hello, 'World!' when trying to submit on Gradescope. Make sure that the function that is failing is actually returning something, not just printing. Remember, return and print do not function the same way.

Bonus (optional) TO BE ADDED

Last Step: Submit your files to Gradescope

Once you see your tests are passing, THEN submit a version to Gradescope.

Even if you are not finished with this lab, make a submission to Gradescope now anyway. Here is why:

  1. After you upload your file, it will provide a backup copy of your work in case something goes wrong with your computer (yes, this happens and you want to make sure there is a backup somewhere).

  2. You also will be able to see some progress towards completion of the lab— partial credit for completion of this step.

Once you’ve submitted and you see that you got more than 0 points, you are ready to continue with the rest of the lab.

Before you submit your code to Gradescope, make sure that your files

You are ready to submit your work to Gradescope.

Navigate to the Lab assignment on Gradescope and upload your .py files, similar to how you submitted for Lab00. Important: you need to upload both files at the same time. You can hold down Ctrl (or Cmd/command) key while clicking on the files to select them to upload. Or, you can drag them one by one into the submission box on Gradescope.

Gradescope will run a few tests to check if your functions are correct. If your tests do not pass, go back to these functions and double-check your logic and function syntax.

Passing Gradescope tests doesn’t guarantee that your solution is correct. Make sure to test your code thoroughly yourself. We will be including additional hidden tests after the due date in Gradescope to verify that your code is running as was specified in the lab instructions.

Final Step: Log Out

Actually, this is the final step of every lab.

In fact, you should do this every time you walk away from a lab computer, either in Phelps 3525 or CSIL.

Here’s how:

Acknowledgment: This lab was designed at UCSB by YKK for CS 8 (W20).