F. Drawing a pie chart with the turtle

← E3. Multi-color bars ⏱️ 8 min List basics →

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🎯 Learning Goals

By the end of this lesson, you'll be able to:

• ○ Understand what a pie chart is
• ○ Convert percentages into angles
• ○ Draw a pie slice with the turtle

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Core Concepts

What is a Pie Chart?

Pie graphshows how much of the whole each part takes up, its Proportion.

🍕 Analogy: Sharing a pizza

Imagine sharing a whole pizza with your friends!

• If I eat half (50%) → a half-circle size
• If a friend eats 1/4 (25%) → a quarter size
• The whole pizza = 100% = 360 degrees

Percentage → angle conversion

Proportion	Calculation	Angle
50%	360 × 0.5	180°
25%	360 × 0.25	90°
10%	360 × 0.1	36°

Formula: angle = 360 × (percentage / 100)

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Drawing a sector with the turtle

Steps	Action
1	Start at the center
2	Move forward by the radius
3	Draw the arc
4	Return to the center

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Try Visual Coding

Let's start by drawing a circle!

🎯 Mission

○ Try drawing a circle with the turtle

Python Code Flowchart

# Add blocks and the Python code will show up here...

Add blocks and the flowchart will show up here...

100 90 90 50 4

▶ Run

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Example Code

Drawing a pie slice with a function

Let's turn the code that draws a pie slice into a Functionso we can reuse it!

import turtle

def draw_pie_slice(t, radius, angle, color):
    """A function that draws one pie slice"""
    t.fillcolor(color)
    t.begin_fill()

    t.forward(radius)          # Move forward by the radius
    t.left(90)                 # Left 90 degrees
    t.circle(radius, angle)    # Draw the arc
    t.left(90)                 # Left 90 degrees
    t.forward(radius)          # Return to the center
    t.left(180 - angle)        # Starting position for the next slice

    t.end_fill()

t = turtle.Turtle()
t.shape("turtle")
t.speed(3)

# Draw a half-circle (50%) with a function call!
draw_pie_slice(t, 80, 180, "red")

What Happens

A red half-circle (50%) is drawn! 🔴

💡 Advantages of functions

• Reusable: Make it once, use it many times
• Clean: call complex code in a single line
• Parameter: radius, angle, colorcan be freely changed

A 2-slice pie chart with functions

import turtle

def draw_pie_slice(t, radius, angle, color):
    """A function that draws one pie slice"""
    t.fillcolor(color)
    t.begin_fill()

    t.forward(radius)
    t.left(90)
    t.circle(radius, angle)
    t.left(90)
    t.forward(radius)
    t.left(180 - angle)

    t.end_fill()

t = turtle.Turtle()
t.shape("turtle")
t.speed(5)

radius = 80

# Data: Like 70%, Dislike 30%
data = [70, 30]
colors = ["green", "red"]

# Use functions to keep it clean!
for i in range(2):
    angle = 360 * data[i] / 100  # Percentage → angle
    draw_pie_slice(t, radius, angle, colors[i])

What Happens

A pie chart split into green (70%) and red (30%)! Just 2 lines inside the loopmake it very clean.

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🎯 Quiz

Quiz: Pie chart

How many degrees does 25% take up in a pie chart?

A) 25 degrees B) 90 degrees C) 180 degrees

Show Answer

Answer: B) 90 degrees

Calculation: 360 × 0.25 = 90 degrees

25% is 1/4 of the whole, so it's 90 degrees, a quarter of the circle!

Pie chart angle calculation:

• Angle for 50%: degrees
• Angle for 10%: degrees
• Full circle: degrees

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🎯 Fill-in-the-Blank Mission

Fill in the blanks to complete the code!

Check answers

for percent in data:
    angle = 360 * percent / 100
    print(f'{percent}% = {angle}degrees')

Result: 60% = 216 degrees, 40% = 144 degrees

Check answers

t.circle(radius, angle)

- circle(radius, angle)draws an arc

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⚠️ Try Breaking It (Let's Break It!)

The pie chart in the following code looks wrong. What's the problem?

import turtle

t = turtle.Turtle()

# Data: Like, Neutral, Dislike
data = [50, 30, 10]  # Sum is 90%?!
colors = ["green", "yellow", "red"]

for i in range(3):
    angle = 360 * data[i] / 100
    # Draw the pie slices...

Show Hint

In a pie chart, what should all the percentages add up to?

Show Answer

The percentages don't add up to 100%!

50 + 30 + 10 = 90% (10% short!)

# Correct data (sums to 100%)
data = [50, 30, 20]  # 50 + 30 + 20 = 100%

# Or
data = [50, 30, 10, 10]  # Add Other 10%

In a pie chart, the percentages of all slices must add up to 100%! Otherwise the circle won't be completely filled, or it will overflow.

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🔧 Debug Clinic

Problem: the slices overlap!

❌ Wrong Code

t.forward(radius)
t.circle(radius, angle)
# Doesn't set the starting position for the next slice!

✅ Correct Code

t.forward(radius)
t.left(90)
t.circle(radius, angle)
t.left(90)
t.forward(radius)
t.left(180 - angle)  # Starting position for the next slice!

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Try Coding It Yourself

Making a 2-slice pie chart with functions

draw_pie_slice() Complete the function to draw a 2-slice pie chart!

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🧩 Key Takeaways

What you learned in this lesson:

Concept	Description
Pie graph	Represent percentages as circle slices
Angle calculation	360 × (percentage/100)
circle(r, angle)	Draw an arc with radius r and angle angle
draw_pie_slice() function	Make pie slice drawing reusable
Using lists	data[i], colors[i]to apply the data in order

Review Checklist

• [ ] Can you convert percentages into angles?
• [ ] draw_pie_slice() Can you make a function?
• [ ] Can you use loops and functions together to draw multiple slices?

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← E3. Multi-color bars ⏱️ 8 min List basics →