Instructions for debugMathPractice

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Background

It is rarely the case that code works the first time you test it. It often contains one or more so-called bugs that cause it to behave incorrectly. Accordingly, much of the time you spend programming in the course will involve debugging, which is the process of finding and removing bugs.

Here's a simple example of a program that does not behave as expected:

username = input('What is your name? ')  # line 1
print('Hello, username')                 # line 2
n = input('Enter an integer n: ')        # line 3
print('n is', n, '; 2*n is', 2*n)        # line 4

For example:

What is your name? Wendy
Hello, username
Enter an integer n: 23
n is 23 ; 2*n is 2323

(Important Note: In this pset, we use blue text to highlight text entered by the user to distinguish it from text printed by the program, which is in black. When you actually run the program in Thonny, the text entered by the user will also appear in blue.)

This program misbehaves in two ways:

1. Rather than displaying the name entered by the user (Wendy in this case), it always displays the string username.
2. Rather than showing the double of the integer n for the expression 2*n (46 in this case) it shows the digits repeated twice ("2323" in this case).
   
   

The first bug is due to the fact that in line 2, the occurrence of username inside the printed string denotes the literal string U-S-E-R-N-A-M-E and not the value of the variable named username. This can be fixed by changing line 2 to be

print('Hello, ' +  username)       # fixed line 2, version 1, for 1st bug

or

print('Hello,', username)          # fixed line 2, version 2, for 1st bug

The second bug occurs because in the expresion 2*n the value of n is the string of digits entered by the user (e.g., the 2-digit string "23") rather than the integer 23. This can be fixed by using the int() function to convert the string of digits "23" to the integer 23. One way to apply this fix is to change line 4 to be:

print('n is', n, '; 2*n is', 2*int(n))     # fixed line 4 for 2nd bug

Alternatively, we could leave line 4 unchanged, and instead change line 3 to be

n = int(input('Enter an integer n: '))     # fixed line 3 for 2nd bug. 

With either combination of fixes, the program now works as expected:

What is your name? Wendy
Hello, Wendy
Enter an integer n: 23
n is 23 ; 2*n is 46

This example illustrates that (1) even a very short program can contain multiple bugs and (2) often there is more than one way to fix a bug.

A Note on print

Most examples involving print that you have seen in the course up to this point involve calling it with a single parenthesized argument that's a string. For example, suppose that:

• i is a variable holding the integer 42
• f is a variable holding the floating point number 6.023
• s is a variable holding the string "bunny"

Then:

print('i is ' + str(i) + '; f is ' + str(f) + '; s is ' + s)

displays the characters

i is 42; f is 6.023; s is bunny

However, it turns out that the print function in Python is special in two ways:

• print can accept any number of arguments.

• Each argument to print is automatically converted to a string, and spaces are automatically inserted between them.

For example:

print('i is', i, '; f is', f, '; s is', s)

will also displays the characters

i is 42; f is 6.023; s is bunny

Passing multiple arguments with arbitrary types to print is often more convenient that having to convert the values to strings and concatenating them into one big string.

The mathPractice.py Program

For task 1, we will work with a Python program that quizzes the user on basic math operations (although it does not check their answers). This program is supposed behave as shown in the first and second examples below, where text highlighted in blue is entered by the user.

If you open the file mathPractice.py (which is included in the starter code for ps01) in Thonny, you will see an attempted implementation of this program.

Unfortunately, this example code has numerous bugs that prevent it from working properly. Your goal in this subtask is to fix the program so that it behaves as indicated by the above examples.

You should modify mathPractice.py by finding and fixing all the bugs it contains, so that ultimately it has the correct behavior as shown by the examples. You may find it useful to make comment every time you fix a bug so that you can more easily review you progress against the rubric.

Notes:

• Most changes require modifying/replacing existing lines in the program. It is never necessary to completely remove any lines from the program. It is not necessary to add any new lines to the program (although you might need to change a blank line).

• You should use an incremental strategy for debugging. This means that, at every step, you should focus on the first line of the program that is incorrect (i.e., either it prints the wrong output or the program generates an error at that line). Then attempt to fix this line, and test your modified program to see if it improves the behavior of the program. If your bug fix doesn't work, try again. If it does work, focus on the next broken line. Keep doing this until the behavior is correct.

• Thonny's assistant feature will give you tips about any errors that happen while running the code, as well as helping to point you at lines that may have bugs. But it does not always identify the best way to fix a bug, and sometimes an issue it identifies is actually caused by a problem that occurs earlier in the program. Try to really understand the cause of each problem before unthinkingly making changes.

• There are some bugs in the program that don't cause it to crash with an error message, and which Thonny's assistant cannot detect. Make sure you actually check whether the output looks correct in addition to fixing any errors and warnings that Thonny displays. If you want to check your output carefully, you can use this free difference checker website to compare the example outputs above with what your code actually produces and highlight any differences.

• Testing the program is time-consuming because you have to type in your values over and over again. This is not required, but if you want to test things more thoroughly, we have provided a file named test_mathPractice.py as part of the starter code. When you run this file, you should see two lines of ouptut in red that look like this:

  txt ✓ test_mathPractice.py:20 ✓ test_mathPractice.py:51

  (You may see different specific filenames.) After those two lines, you should see the output of the program, with 10 selected for each input. However, you may not see those messages if there are still bugs in your program! This test file runs your mathPractice.py file three times. The first two times it checks the results (and will tell you about any differences it finds) using the same inputs as the exmaples shown here. The third time, it uses custom inputs defined in the file, but it does not know how to check the results, so it just prints them out. You can edit test_mathPractice.py to change the num1 and num2 variables near the bottom of the file in order to run the last test with different numbers, but you will still have to inspect it manually to see if it's correct or not.