Beginner Level • Lesson 3

🔄 Repeating Actions

⏱️ 45 minutes 📚 Beginner 🎯 Loops

🔄

Repeating Actions

Learn how to make your programs repeat actions using loops - this makes your code shorter and more powerful!

What are Loops?

Loops let you repeat code multiple times without writing it over and over. Think of it like repeating a song - instead of singing it 10 times manually, you just say "repeat 10 times"!

💡 Real-World Example: When you scroll through Instagram, the app uses a loop to show each post. Without loops, you'd have to write code for every single post!

The FOR Loop

The for loop repeats code a specific number of times. It's perfect when you know exactly how many times you want to repeat something.

Basic FOR Loop

for i in range(5):
    print("Hello!")
    print("This is loop number", i)

What happens: The code inside the loop runs 5 times. The variable i counts from 0 to 4.

Output:

Hello!
This is loop number 0
Hello!
This is loop number 1
Hello!
This is loop number 2
Hello!
This is loop number 3
Hello!
This is loop number 4

Understanding range()

The range() function creates a sequence of numbers. Here's how it works:

🔢

range(5)

Creates numbers: 0, 1, 2, 3, 4

(5 numbers, starting from 0)

🔢

range(1, 5)

Creates numbers: 1, 2, 3, 4

(starts at 1, stops before 5)

🔢

range(0, 10, 2)

Creates numbers: 0, 2, 4, 6, 8

(starts at 0, stops before 10, steps by 2)

The WHILE Loop

The while loop repeats code as long as a condition is True. It keeps going until the condition becomes False.

WHILE Loop Example

count = 0

while count < 5:
    print("Count is:", count)
    count = count + 1

print("Loop finished!")

What happens: The loop runs while count is less than 5. Each time, it prints the count and adds 1 to it. When count reaches 5, the condition is False and the loop stops.

⚠️ Important: Make sure your while loop has a way to stop! If the condition never becomes False, you'll have an infinite loop that runs forever.

FOR vs WHILE: When to Use Which?

🔁

Use FOR Loop When:

You know how many times to repeat
You're working with a list of items
You want to count something

# Example: Blink LED 5 times
for i in range(5):
    GPIO.output(18, GPIO.HIGH)
    time.sleep(0.5)
    GPIO.output(18, GPIO.LOW)
    time.sleep(0.5)

♾️

Use WHILE Loop When:

You don't know how many times to repeat
You want to keep going until something happens
You're waiting for a condition to change

# Example: Keep blinking until button pressed
while button_not_pressed:
    GPIO.output(18, GPIO.HIGH)
    time.sleep(0.5)
    GPIO.output(18, GPIO.LOW)
    time.sleep(0.5)

Nested Loops

You can put loops inside other loops! This is called "nested loops" and it's useful for creating patterns.

Nested Loop Example

for row in range(3):
    for col in range(3):
        print("Row", row, "Column", col)
    print("---")

What happens: The outer loop runs 3 times. Each time, the inner loop runs 3 times. So you get 9 total prints (3 × 3).

Project: Blinking Pattern Generator

Let's create an LED that blinks in different patterns using loops!

Materials Needed:

Raspberry Pi
LED (any color)
Resistor (220Ω)
Breadboard and jumper wires

Pattern 1: Blink 5 Times

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)

# Blink 5 times
for i in range(5):
    GPIO.output(18, GPIO.HIGH)
    time.sleep(0.3)
    GPIO.output(18, GPIO.LOW)
    time.sleep(0.3)

GPIO.cleanup()

Pattern 2: SOS Pattern (3 short, 3 long, 3 short)

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)

# SOS pattern: ... --- ...
# 3 short blinks
for i in range(3):
    GPIO.output(18, GPIO.HIGH)
    time.sleep(0.2)
    GPIO.output(18, GPIO.LOW)
    time.sleep(0.2)

time.sleep(0.5)  # Pause between groups

# 3 long blinks
for i in range(3):
    GPIO.output(18, GPIO.HIGH)
    time.sleep(0.6)
    GPIO.output(18, GPIO.LOW)
    time.sleep(0.2)

time.sleep(0.5)  # Pause between groups

# 3 short blinks again
for i in range(3):
    GPIO.output(18, GPIO.HIGH)
    time.sleep(0.2)
    GPIO.output(18, GPIO.LOW)
    time.sleep(0.2)

GPIO.cleanup()

Pattern 3: Fade In and Out (using while loop)

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)

# Create PWM (Pulse Width Modulation) for fading
pwm = GPIO.PWM(18, 100)  # 100 Hz frequency
pwm.start(0)  # Start with 0% brightness

# Fade in
brightness = 0
while brightness <= 100:
    pwm.ChangeDutyCycle(brightness)
    time.sleep(0.05)
    brightness = brightness + 5

# Fade out
while brightness >= 0:
    pwm.ChangeDutyCycle(brightness)
    time.sleep(0.05)
    brightness = brightness - 5

pwm.stop()
GPIO.cleanup()

Note: This uses PWM (Pulse Width Modulation) to control LED brightness. We'll learn more about PWM in later lessons!

Loop Control: break and continue

Sometimes you want to stop a loop early or skip an iteration. Python has two special keywords for this:

⏹️

break

Stops the loop immediately

for i in range(10):
    if i == 5:
        break  # Stop at 5
    print(i)
# Output: 0, 1, 2, 3, 4

⏭️

continue

Skips the rest of this iteration

for i in range(5):
    if i == 2:
        continue  # Skip 2
    print(i)
# Output: 0, 1, 3, 4

Common Mistakes to Avoid

⚠️ Watch Out For:

Infinite Loops: Make sure your while loop has a way to stop
Missing Colon: Don't forget the colon (:) after for and while
Wrong Indentation: Code inside loops must be indented
Off-by-One Errors: Remember range(5) gives 0-4, not 1-5
Forgetting to Update: In while loops, make sure you change the condition variable

Summary

You've learned:

✅ Loops let you repeat code without writing it multiple times
✅ for loops repeat a specific number of times
✅ while loops repeat as long as a condition is True
✅ range() creates sequences of numbers for loops
✅ You can nest loops inside other loops for complex patterns
✅ break stops a loop early, continue skips an iteration
✅ Loops are essential for creating LED patterns and animations

🎉 Excellent Progress! You've learned how to make programs repeat actions. In the next lesson, you'll learn about functions to organize your code better!

🎮 Try It: Practice with Loops!

Practice writing loops. Try these challenges:

📝 Challenge 1: Count to 10

Write a for loop that prints numbers from 1 to 10:

📝 Challenge 2: Countdown

Write a loop that counts down from 10 to 1, then prints "Blast off!":

📝 Challenge 3: Sum Numbers

Write a loop that adds numbers from 1 to 5 and prints the total:

💡 Tip: Remember that range(5) gives 0-4, but range(1, 6) gives 1-5. Use the second form when you want to start from 1!

🎯 Activity: Blinking Pattern Generator

What You'll Build:

Create an LED that blinks in different patterns using loops!

Step-by-Step Instructions:

Set Up Hardware: Connect LED to GPIO 18 with resistor
Pattern 1: Write code to blink the LED exactly 5 times using a for loop
Pattern 2: Create a pattern: blink 3 times fast, pause, blink 3 times slow
Pattern 3: Use nested loops to create a pattern (e.g., 3 groups of 2 blinks each)
Test: Run each pattern and observe the LED behavior
Experiment: Try changing the number of blinks and timing

Pattern Ideas:

✅ SOS pattern: 3 short, 3 long, 3 short
✅ Heartbeat pattern: 2 quick blinks, pause, repeat
✅ Countdown pattern: 5 blinks, pause, 4 blinks, pause, etc.
✅ Random pattern: Use nested loops for complex sequences

🏆 Bonus Challenge: Can you create a pattern that blinks faster and faster each time? (Hint: use a variable to change the sleep time inside the loop)

💪 Practice Challenges

Challenge 1: Multiplication Table

Write a loop that prints the 5 times table (5 × 1 = 5, 5 × 2 = 10, etc.) up to 5 × 10:

# Example:
for i in range(1, 11):
    result = 5 * i
    print("5 ×", i, "=", result)

Challenge 2: LED Blink Counter

Write code that blinks an LED 10 times, but prints the count each time:

Blink 1, Blink 2, Blink 3, etc.
After 10 blinks, print "Done!"

Challenge 3: Stop at 7

Write a loop that counts from 1 to 10, but stops (using break) when it reaches 7:

# Example:
for i in range(1, 11):
    if i == 7:
        break
    print(i)
# Output: 1, 2, 3, 4, 5, 6

Challenge 4: Skip Even Numbers

Write a loop that prints numbers 1-10, but skips even numbers (use continue):

# Example:
for i in range(1, 11):
    if i % 2 == 0:  # If number is even
        continue  # Skip it
    print(i)
# Output: 1, 3, 5, 7, 9

Challenge 5: Nested Loop Pattern

Use nested loops to create this pattern:

*
**
***
****
*****

Click to see the answer

for row in range(1, 6):
    for col in range(row):
        print("*", end="")
    print()  # New line after each row