Python Loops

:dart: Learning Objectives

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

• Understand what loops are and why we use them
• Write for loops to repeat code a specific number of times
• Create while loops to repeat code until a condition is met
• Use special statements (else, break, continue) to control loops
• Apply loops to solve real programming problems

:information_source: Definition: A loop is a programming structure that lets you run the same code multiple times automatically.

:emoji: Why Do We Need Loops?

Imagine you want to print "Hello" 100 times. Without loops, you'd have to write print("Hello") 100 times! That's a lot of typing and very inefficient.

Loops solve this problem by letting you write the code once and tell Python to repeat it.

:star2: Benefits of Using Loops:

• Save time: Write code once instead of many times
• Reduce errors: Less code means fewer chances for mistakes
• Easy to change: Update one line instead of hundreds
• Run faster: Python executes loops efficiently

:books: Types of Loops in Python

Python gives us two powerful types of loops:

1. For Loop - Use when you know exactly how many times to repeat
2. While Loop - Use when you want to repeat until something happens

:emoji: For Loop

A for loop helps you go through each item in a collection (like a list) or repeat something a specific number of times.

:bulb: When to use a for loop:

• You know exactly how many times to repeat
• You want to go through each item in a list
• You need to process each character in a string

Basic For Loop Structure

for item in sequence:
    # Your code here (indented!)
    # This runs for each item

:memo: Note The indentation (4 spaces) is very important! It tells Python which code belongs inside the loop.

:dart: How For Loops Work

1. Python looks at the first item in your sequence
2. It runs your code with that item
3. It moves to the next item and repeats
4. When all items are done, the loop ends

Figure 6.2 For loop flows:

         ┌─────────┐
         │  Start  │
         └────┬────┘
              │
              ▼
     ┌─────────────────┐       If no more items
     │  Item from      │      in the sequence
     │  sequence       │───────────────┐
     └────────┬────────┘               │
              │                        │
              │ Next item              │
              │ from sequence          │
              ▼                        │
     ┌─────────────────┐               │
     │ Execute         │               │
     │ Statement(s)    │               │
     └────────┬────────┘               │
              │                        │
              └────────────────────────┘
                            │
                            ▼
                         ┌─────┐
                         │ End │
                         └─────┘

:emoji: While Loop

A while loop keeps running as long as a condition is true. It's like saying "keep doing this while something is happening."

:bulb: When to use a while loop:

• You don't know exactly how many times to repeat
• You want to continue until something specific happens
• You're waiting for user input or a certain result

:warning: Warning Important: Always update your variable inside the loop, or it might run forever!

Basic While Loop Structure

# Set up your variable first
counter = 0

while condition:
    # Your code here
    # Don't forget to update your variable!
    counter = counter + 1

:dart: How While Loops Work

1. Python checks if the condition is true
2. If true, it runs your code
3. It checks the condition again
4. Keeps repeating until the condition becomes false
5. When false, the loop stops

Figure 6.4 While Loop Flows:

         ┌─────────┐
         │  Start  │
         └────┬────┘
              │
              ▼
     ┌─────────────────┐   False
     │     While       ├───────────┐
     │   Condition?    │           │
     └────────┬────────┘           │
              │ True               │
              ▼                    │
     ┌─────────────────┐           │
     │    Codes to     │           │
     │     Execute     │           │
     └────────┬────────┘           │
              │                    │
              └────────────────────┘
                      │
                      ▼
                   ┌─────-┐
                   │ Exit │
                   └─────-┘

:bar_chart: While Loop vs If Statement

:memo: Note Remember: If statements run once, while loops can run many times!

:hammer_and_wrench: Special Loop Control Statements

Python gives us three special tools to control our loops:

1. Else - Run code after the loop finishes normally
2. Break - Stop the loop immediately
3. Continue - Skip to the next iteration

1️⃣ Else Statement

The else statement runs special code after your loop finishes all its work.

for item in sequence:
    Block of codes to execute
else:
    Block of codes to execute if false

Else statement syntax in for loop

while condition:
    Block of codes to execute
else:
    Block of codes to execute if false

Else statement syntax in while loop

2️⃣ Break Statement

The break statement is like an emergency exit - it stops the loop immediately when you need it to.

:bulb: Use break when you find what you're looking for and don't need to continue.

python

for item in sequence:
    if condition:
        break
    codes to execute

Break statement syntax in for loop

python

while condition1:
    codes to execute
    if condition2:
        break
    codes to execute

Break statement syntax in while loop

3️⃣ Continue Statement

The continue statement skips the rest of the current loop and jumps to the next item. tip Use continue when you want to skip certain items but keep the loop running.

for item in sequence:
    if condition:
        continue
    codes to execute

Continue statement syntax in for loop

while condition1:
    codes to execute
    if condition2:
        continue
    codes to execute

Continue statement syntax in while loop

:bulb: Examples and Practice

Let's explore each concept with real examples you can try yourself!

:emoji: For Loop Examples

Example One: Say Good Morning 3 Times

for i in range(3):
    print("Good morning")

Expected output:

Good morning
Good morning
Good morning

Example 2: Count from 0 to 2

The range(3) function gives us numbers 0, 1, and 2:

for i in range(3):
    print(i)

Expected output:

0
1
2

Example 3: Print Multiples of 5

Let's create the multiplication table for 5:

x = 5
for i in range(5):
    print(x)
    x = x + 5

Expected output:

5
10
15
20
25

Example 4: Loop Through a List

For loops can go through each item in a list:

fruits = ["apple", "banana", "cherry"]
for i in fruits:
    print(i)

Expected output:

apple
banana
cherry

Example 5: Loop Through Letters in a Word

You can even loop through each letter in a word:

string = "python"
for x in string:
    print(x)

Expected output:

p
y
t
h
o
n

:emoji: While Loop Examples

Example 6: Count from 1 to 5

With while loops, remember to set up your counter first:

x = 1
while x <= 5:
    print(x)
    x = x + 1

Expected output:

1
2
3
4
5

Example 7: Print All Multiples of 5 up to 50

x = 5
while x <= 50:
    print(x)
    x = x + 5

Expected output:

5
10
15
20
25
30
35
40
45
50

1️⃣ Else Statement Examples

Example 8: Using Else with For Loop

fruits = ['apple', 'banana', 'cherry']
for i in fruits:
    print(i)
else:
    print("No items left.")

Expected output:

apple
banana
cherry
No items left.

Example 9: Using Else with While Loop

x = 5
while x <= 50:
    print(x)
    x = x + 5
else:
    print("That's all for multiples of 5 up to 50")

Expected output:

5
10
15
20
25
30
35
40
45
50
That's all for multiples of 5 up to 50

2️⃣ Break Statement Examples

Example 10: Stop a For Loop Early

This loop stops when it reaches 3:

for i in range(5):
    if i == 3:
        break
    print(i)

Expected output:

0
1
2

Example 11: Stop a While Loop Early

x = 1
while x <= 5:
    print(x)
    if x == 3:
        break
    x = x + 1

Expected output:

1
2
3

3️⃣ Continue Statement Examples

Example 12: Skip Number 3 in For Loop

This loop skips printing 3 but continues with other numbers:

for i in range(5):
    if i == 3:
        continue
    print(i)

Expected output:

0
1
2
4

Example 13: Skip Number 3 in While Loop

x = 0
while x < 5:
    x = x + 1
    if x == 3:
        continue
    print(x)

Expected output:

1
2
4
5

:memo: Summary

Key Points to Remember:

• Loops help us repeat code without writing it multiple times
• For loops are perfect when you know how many times to repeat
• While loops are great when you want to repeat until something happens
• Break stops the loop completely
• Continue skips to the next iteration
• Else runs code after the loop finishes normally

:dart: Quick Reference:

# For loop - repeat 5 times
for i in range(5):
    print(i)

# While loop - count to 10
count = 1
while count <= 10:
    print(count)
    count = count + 1

:movie_camera: Video Lesson

:emoji: Practice with AI

Code with AI

Try using AI to help you create loops and practice what you've learned!

:dart: Practice Prompts:

Beginner Level:

• "Write a Python for loop that prints 'Hello World' 5 times"
• "Create a while loop that counts from 1 to 10"
• "Show me how to use a for loop to print each letter in my name"

Intermediate Level:

• "Write a Python program that prints the first 10 Fibonacci numbers"
• "Generate a Python loop to calculate the factorial of a number"
• "Create a loop that finds all even numbers between 1 and 50"

Challenge Level:

• "Write a program using nested loops to create a multiplication table"
• "Create a while loop that plays a number guessing game"
• "Show me how to use break and continue in a loop that processes a list"

:bulb: Tips for Learning:

1. Start with simple loops and gradually make them more complex
2. Always test your code with different inputs
3. If your loop runs forever, use Ctrl+C to stop it
4. Practice writing the same program with both for and while loops