Can You Find the Bug?

Hello, Python people,

It may be clear to you already that writing code is crucial in the process of learning to program. But there are other important skills that may be less obvious but that we need to practice as well.

For example: reading code written by others. You never know what you’ll learn by reading through other people’s code. You might learn a new bit of Python, or a technique for solving a problem, or a new approach to a problem you’ve already solved. This is why I recommend in the book to look at other code submissions and compare them to yours.

Another important skill: debugging! Finding bugs in code! Many programmers have a negative reaction to bugs… they just want them fixed. And don’t we all :) but there are learning opportunities here. When you finally fix that elusive bug and finish your celebration, take a few minutes to reflect on the process:

• How can I better avoid this class of bugs in the future? Did I make an assumption about the problem that wasn’t justified? Did I misinterpret how a Python feature works?
• How can I more quickly find and fix this bug in the future, if a similar bug does crop up again? Did I (in retrospect) spend time using unproductive debugging strategies?
• What did I learn about programming, Python, or debugging in the process of fixing this bug?

Finding and Fixing a Bug

I’ve included many suggested practice exercises in the book. I thought it would be fun here to provide you some code that doesn’t quite solve one of these exercises. It has a bug. Can you find it?

The exercise that we’re working through here is from Chapter 8… this Utrka problem about marathons.

Here’s the code that doesn’t quite work.

n = int(input())

d = {}

for i in range(2 * n - 1):
    person = input()
    if not person in d:
        d[person] = 1
    else:
        d[person] = d[person] + 1

for person in d:
    if d[person] == 1:
        print(person)

What Do You Think?

Did you have fun with this exercise? Want me to highlight your most devious bugs? :D Please get in touch!