Class 706 – Unit 2: Function Basics
Learn to write reusable functions with parameters
What Are Functions?
Functions are one of the most powerful tools in programming. They allow you to write code once and reuse it many times, which makes your programs:
- More efficient - Less code to write and maintain
- Easier to read - Named functions describe what the code does
- Easier to debug - Fix problems in one place instead of many
- More flexible - Use parameters to customize behavior
Without Functions (Repeated Code)
# Draw first flower
Circle(100, 300, 20, fill='red')
Circle(80, 280, 15, fill='pink')
Circle(120, 280, 15, fill='pink')
Circle(80, 320, 15, fill='pink')
Circle(120, 320, 15, fill='pink')
Line(100, 320, 100, 380, fill='green')
# Draw second flower
Circle(200, 300, 20, fill='red')
Circle(180, 280, 15, fill='pink')
Circle(220, 280, 15, fill='pink')
Circle(180, 320, 15, fill='pink')
Circle(220, 320, 15, fill='pink')
Line(200, 320, 200, 380, fill='green')
# Draw third flower
Circle(300, 300, 20, fill='red')
Circle(280, 280, 15, fill='pink')
Circle(320, 280, 15, fill='pink')
Circle(280, 320, 15, fill='pink')
Circle(320, 320, 15, fill='pink')
Line(300, 320, 300, 380, fill='green')21 lines - Same code copied 3 times!
With Functions (Reusable Code)
# Define the function once
def drawFlower(x, y):
Circle(x, y, 20, fill='red')
Circle(x-20, y-20, 15, fill='pink')
Circle(x+20, y-20, 15, fill='pink')
Circle(x-20, y+20, 15, fill='pink')
Circle(x+20, y+20, 15, fill='pink')
Line(x, y+20, x, y+80, fill='green')
# Call the function 3 times
drawFlower(100, 300)
drawFlower(200, 300)
drawFlower(300, 300)12 lines - Write once, use many times!
Today's Learning Objectives
- Understand what functions are and why they make programming more efficient
- Write function definitions with clear, descriptive names
- Call functions to execute their code
- Use parameters to pass information into functions
- Understand and use test cases to verify function behavior
- Convert repeated code into reusable functions
- Debug common function errors (incorrect parameters, missing definitions)
Lesson Agenda (90 minutes)
- Introduction (5 min): What are functions and why do we need them?
- Direct Instruction (15 min): Function syntax, parameters, and calling functions
- Teacher Demo (15 min): Live coding - converting repeated drawing code into a reusable function
- Guided Practice (10 min): Work through CMU Function Basics lesson together
- Independent Practice (40 min): Complete CMU exercises on functions, parameters, and debugging
- Wrap-Up (5 min): Review key concepts and assign homework
Key Programming Concepts
Function Definition
Creating a function that can be used later:
def drawStar(x, y):
Star(x, y, 30, 5, fill='gold')
# This function draws a star at position (x, y)Function Call
Using the function you created:
drawStar(100, 200) # Draws a star at (100, 200)
drawStar(300, 150) # Draws another star at (300, 150)Parameters
Parameters are variables that let you pass information into a function. They make functions flexible and reusable for different situations.
Function Definition
Function Call
# Draws a circle at (100, 200) with size 50
Color Key
Test Cases
Test cases help you verify that your function works correctly with different inputs. In CMU CS Academy, you'll use test cases to check your work before submitting.
Teaching Strategy
The "Repeated Code Problem"
Start by showing students code that draws the same shape multiple times with only slight variations. Ask: "What's inefficient about this code?"
Live Coding Demo
Show students how to:
- Identify repeated code patterns
- Create a function definition with a clear name
- Add parameters for the parts that change
- Replace repeated code with function calls
- Test the function with different arguments
Suggested Demo: Draw three different snowmen by writing the code three times, then refactor into a drawSnowman(x, y) function.
What You Must Complete Today
During today's 90-minute class, you should finish:
- CMU CS Academy: Function Basics lesson - Learn to define and call functions
- CMU CS Academy: Debugging lesson - Practice fixing function errors
- Start practice problems - Begin assigned exercises (unfinished work becomes homework)
Remember: Programming requires regular practice. Use your class time productively!
Common Mistakes to Watch For
Functions must be defined before they are called. Always write your
def statements at the top of your program.
If a function expects 2 parameters, you must pass exactly 2 arguments when calling it.
To call a function, you need parentheses:
drawStar(100, 200) not drawStar
The code inside a function must be indented. Python uses indentation to know what belongs to the function.
Discussion Questions
Use these to check student understanding during or after the lesson:
- Why would a programmer choose to use a function instead of copying code?
- What is the difference between defining a function and calling a function?
- How do parameters make functions more flexible?
- When you see the same code repeated multiple times, what should you do?
Homework
Complete any unfinished CMU CS Academy exercises from today's lesson.
Expected time: 30-60 minutes. Functions are a foundational concept - make sure you understand them before moving forward!
Why This Matters
Functions are one of the most important programming concepts you'll learn.
- Functions make code more efficient and easier to understand
- You'll use functions in every program you write from now on
- The AP Create Performance Task requires you to write and use functions
- Professional programmers use functions to organize complex programs
Looking Ahead
Next class (Lesson 707): We'll extend what you learned today by adding multiple parameters to functions and practicing structured design with more complex drawing functions.
Unit 2 Journey: Functions → Mouse Events → Variables → Conditionals → Creative Task