Problem Set 2: Middle Letter – Extracting Characters from a String Learning Objective

How to Approach Problem Sets

When you start a new problem set, your first instinct might be to open your computer and begin typing code right away. While this can feel productive, it often leads to frustration when things don't work as expected. Instead, take a few minutes to slow down and plan.

Here are some helpful strategies:

Understand the problem clearly
- Read the instructions carefully — twice if needed.
- Ask yourself: What exactly is being asked?
Break the problem into smaller steps
- Think about the smallest possible actions the computer will need to perform.
- For example: If the task is to find the first recurring letter in a word, what steps must happen first?
Try solving it on paper first
- Write out your steps in plain language (pseudocode).
- Test your steps with a simple example before touching the keyboard.
Translate your steps into code
- Start small — write only a few lines at a time and test often.
- Don't worry about perfection at first; get a working version, then improve it.
Check your solution
- Run it with different examples, including edge cases.
- Ask: Does this solve the problem in all situations?

Read the question carefully (twice).
Break the task into the smallest steps.
Sketch or write pseudocode before coding.
Start small — test as you go.
Check your solution with different cases.

You will learn how to work with strings in Python, including how to find the length of a string, use indexing, and extract characters. You will also practice thinking about odd and even numbers.

Background

A string is a sequence of characters, like "hello" or "computer". Each character in a string has a position called an index.

Python starts counting at 0, so in "hello":
- "h" is at index 0
- "e" is at index 1
- "l" is at index 2
- "l" is at index 3
- "o" is at index 4

The function len(string) tells us how many characters are in the string.

The middle character(s) are found like this:

If the string length is odd → return one middle character.
- Example: "cat" → middle letter is "a".
If the string length is even → return the two middle characters.
- Example: "test" → middle letters are "es".

Tasks

Task 1: Trace It

Trace what the following code will print:

word = "python"
length = len(word)
middle = length // 2
print(word[middle-1:middle+1])

Question: What will the output be? Show your working.

Task 2: Write a Function

Write a function middle_letter(word) that:

Finds the length of the string.
If the length is odd, returns the single middle character.
If the length is even, returns the two middle characters.

Example:

print(middle_letter("cat"))   # Output: a
print(middle_letter("dogs"))  # Output: og

Task 3: Test Your Function

Test your function with at least five different strings, including:

A one-letter word (e.g. "a")
A three-letter word (e.g. "sun")
A four-letter word (e.g. "code")
A longer odd-length word (e.g. "banana")
A longer even-length word (e.g. "school")

Write down the output of your program for each test case.

Task 4: Explain

Question:
Why do we need to treat odd-length and even-length strings differently when finding the middle letter(s)?

Success Criteria

By the end of this task you should be able to:

Correctly find the length of a string using len().
Use indexing and slicing to extract specific characters from a string.
Write a Python function that works with both odd and even length strings.
Explain why odd and even lengths produce different results for the middle.