Lab 2: Tracing and debugging; strings, lists and functions

Task 0: Preliminaries

Policy reminders

• Feel free to ask questions at any point during the lab.

• We may not finish all of the lab exercises during the actual lab session. Every Wednesday, we will post solutions to the current week’s lab on the Labs page of the course website.

• We don’t grade the lab exercises. However, starting with today’s lab, your attendance at lab will count towards your participation grade. If you attend 85% of the lab sessions over the course of the semester, you will get full credit for lab participation. See the syllabus for more details.

• To get full credit for participation in lab, you must work productively throughout the lab session. You will not be penalized if you cannot finish all of the lab exercises.

• Feel free to work on the lab exercises with your classmates, and don’t hesitate to ask a staff member for help!

Creating the necessary folder

1. If you haven’t already created a folder named cs111 for your work in this course, follow these instructions to do so.

2. Then create a subfolder called lab2 within your cs111 folder, and put all of the files for this lab in that folder.

Creating the necessary file

1. Access the template that we have created by clicking on this link and signing into your Google account as needed.

2. When asked, click on the Make a copy button, which will save a copy of the template file to your Google Drive.

3. Select File->Rename, and change the name of the file to lab2.

Task 1: Practice with strings

Consider the following program:

s = 'cs111'
t = 'is amazing!'

u = s[1] + t[-4: ] 
s = s[ :2] + (t[-1]*2)
t = t[1:-2:2]
s[::-2]

print(s, t, u)

1. In the Task 1 section of your lab2 file, we have given you a table that you should fill in to show how the values of the variables change over time. Try to figure out the values on your own, without using Spyder.

2. Based on your tracing, what is the output of this program?

3. In Spyder, open a new editor window using File->New file, and save it to your lab2 folder using the name lab2task1.py.

4. Run the program using the “play” icon or F5. Does the output match your predictions?

5. Why doesn’t the line of the program that uses skip-slicing have any effect on the output of the program?

6. What change could we make so that the skip-slicing changes the value of s?

Task 2: Practice with lists

The problems below ask you to create a few lists using only the following:

• the list num = [8, 6, 7, 5, 3, 0, 9]
• the list info = [4, 1, 1]
• list indexing, such as num[0]
• list slicing, such as num[1:]
• list skip-slicing, such as num[1:6:2]
• list concatenation, such as num[:1] + info[:1] (but don’t use + for addition)
• list construction operator, such as [num[0]] or [num[0], info[0]]

To get started:

1. Download the following file: lab2task2.py

   Make sure to put the file in your lab2 folder. If your browser doesn’t allow you to specify where the file should be saved, try right-clicking on the link above and choosing Save as... or Save link as..., which should produce a dialog box that allows you to choose the correct folder for the file.

2. In Spyder, choose the File->Open... menu option and locate the file. Double-click on the file to open it in Spyder. This should add a tab for the file to your Editor pane.

3. Run lab2task2.py in Spyder. This will make the lists num and info available to you in the IPython console, so that you can experiment with possible answers in the console.

Here are the problems:

1. Use num and/or info to create the list [8, 7, 3, 9], and assign it to the variable list1. This example has been completed for you and is already in lab2task1.py.

2. Use num and/or info to create the list [1, 3, 5, 7], and assign it to a variable named list2. Then add a print statement to print list2. Follow the same format that we gave you for list1.

3. Use num and/or info to create the list [4, 1, 1, 6, 5, 0], and assign it to a variable named list3. Then add a print statement to print list3. Follow the same format that we gave you for list1.

Feel free to ask for help if you get stuck!

Once you have completed these problems, run your program in Spyder to ensure that you get the correct output.

Task 3: Debugging and writing functions

1. In Spyder, open a new editor window using File->New file, and save it to your lab2 folder using the name lab2task3.py.

2. Copy the following (buggy) code into that file:

   def mysum(x, y)
   """ takes two numbers and returns their sum """
       total = x + y
       print(total)
   

3. Save the file using Ctrl-S. Then try to run it. What happens?

4. This function include two syntax errors – errors that violate the rules of the Python language. See if you can find and fix them, continuing until you are able to run the file without any error messages. Feel free to ask for help.

   Note: You should not change the print statement at this point, since it does not have any syntax errors.

5. Once you have eliminated the syntax errors, test the function from the IPython console as follows:

   >>> mysum(44, 67)
   
   >>> print(mysum(10, 7))
   

   What happens? Why?

6. Fix the function so that both of the above tests work as expected. Hint: The function is supposed to return the sum, not print it!

   Don’t forget that you need to re-run the lab2task3.py file after making changes to the function.

7. In lab2task3.py, write a function named evens_odds that accepts a string s as its only parameter. The function should create and return a new string that begins with all of the characters in the even positions of s (positions 0, 2, ...) followed by all of the characters in the odd positions of s (positions 1, 3, ...). For example:

   >>> evens_odds('python!')
   result: 'pto!yhn'
   

   Note that the result begins with the characters in the even positions of python ('pto!') followed by the characters in the odd positions ('yhn').

   Here is a suggested template:

   def evens_odds(s):
       """ takes a string s and returns a new string in which 
           the characters in the even positions of s are followed 
           by the characters in the odd positions of s
       """
       evens = ____________________
       odds = _________________
       return _________________
   

8. After writing the function, make sure to test it by making function calls from the console like the one shown above.

   Here again, don’t forget to re-run the file after making any changes.

Task 4: Tracing a function call with conditional execution

Consider the following function:

def mystery(a, b):
    """ takes two numbers, a and b, and does something with them! """
    if a > b:
        a = a * 2
        c = a + 3
    elif b > a:
        b = b * 3
        c = b - 2
    elif b == 3:
        c = a + 1
    else:
        c = -1
    if a % 2 == 0:
        c = c // 2
    return c

Consider the following call to this function:

mystery(5, 3)

On the second page of the lab2 template, we have given you a table that you should fill in to show how the values of the variables change over time. You may not need all of the rows.

If you have extra time!

Complete any of the problems listed under the Warmup-1 collection of problems on CodingBat. These problems provide good practice with writing functions with lists and strings.

Task 5: Practice submitting a PDF file on Gradescope

As mentioned last week, we won’t typically ask you to submit the work that you complete in lab. Rather, there will be an attendance sheet to sign, and staff members will check that you are working steadily on the assigned tasks during the lab.

However, this week we are asking you to submit a PDF formed from the work that you did in the lab2 file on Google Drive. That’s because we want you to practice the procedure for submitting PDFs that contain answers to multiple problems. We won’t actually grade the work that you submit for this lab.

Here are the steps:

1. Open your copy of the lab2 file on Google Drive.

2. Choose File->Download->PDF document, and save the resulting file (lab2.pdf) to your machine.

   Make sure to put the file in your lab2 folder. If your browser doesn’t allow you to specify where the file should be saved, try right-clicking on the link above and choosing Save as... or Save link as..., which should produce a dialog box that allows you to choose the correct folder for the file.

3. Login to Gradescope by clicking the link in the left-hand navigation bar.

4. Once you are in logged in, click on the box for CS 111.

5. Click on the name Lab 2 in the list of assignments. You should see a pop-up window labeled Submit Assignment. (If you don’t see it, click the Submit or Resubmit button at the bottom of the page.)

6. Choose the Submit PDF option, and then click the Select PDF button and find the lab2.pdf that you created in step 2. Then click the Upload PDF button.

7. You should see an outline of the tasks along with thumbnails of the pages from your uploaded PDF. For each problem in the outline:

   • Click the title of the task.
   • Click the page(s) on which your work for that task can be found.

   If this were an actual homework submission, you would click on the magnifying glass icon for each page and doublecheck that the pages that you see contain the work that you want us to grade.

8. Once you have assigned pages to all of the tasks in the question outline, click the Submit button in the lower-right corner of the window.

9. You should see a box saying that your submission was successful. Click the (x) button to close that box.

10. You can use the Resubmit button at the bottom of the page to resubmit your work as many times as needed before the final deadline.

Last updated on September 16, 2024.