Revise Python in 15 days for ML (2025) : Day 4 (Loop)

Welcome to Day 4 of our Python journey! If you missed Day 3 where we explored operators and conditionals, make sure to check it out first!

🔁 What Are Loops?

Loops are like having a super-efficient assistant who can repeat the same task multiple times without getting tired. Instead of writing the same code over and over, we can wrap it in a loop and let Python handle the repetition!

Today we'll master two powerful loop types:

• 🔢 For loops - when you know exactly how many times you want to repeat something
• ⏱️ While loops - when you want to keep going until a condition changes

🔢 For Loops: The Counting Champion

A for loop works like this:

1. Specify what you want to iterate through
2. For each item, execute the indented code block
3. Move to the next item and repeat until you've gone through everything

💫 Magic of Iteration: Three Ways to Use For Loops

1️⃣ Iterating Through Each Character

name = "Ridwan"
for i in name:
    print(i, 'i')

Output:

R i
i i
d i
w i
a i
n i

What's happening?

• Python takes each character from "Ridwan" one by one
• It temporarily stores each character in the variable i
• Then it runs the indented code (printing the character and 'i')
• It automatically moves to the next character

2️⃣ Using Range and Indexing

name = "Rid wan"
for i in range(len(name)):
    print(name[i], 'i')

Output:

R i
i i
d i
  i
w i
a i
n i

What's happening?

• range(len(name)) creates a sequence: 0, 1, 2, 3, 4, 5, 6
• Each number is stored in i during each loop iteration
• We use i as an index to access each character with name[i]

💡 Pro Tip: This approach gives you the index position, which can be really useful when you need to know where you are in the sequence!

3️⃣ Processing List Items

L = [1, 2, 3, 4, 5]
sum = 0
for i in L:
    sum += i
    print(i, sum)
print('sum is = ', sum)

Output:

1 1
2 3
3 6
4 10
5 15
sum is = 15

What's happening?

• Python takes each number from the list one by one
• It temporarily stores each number in the variable i
• It adds i to our running total sum
• It prints both the current number and the running total

⏱️ While Loops: The Patient Guardian

While loops work differently:

1. Check if a condition is True
2. If True, execute the code block
3. Check the condition again
4. Keep repeating until the condition becomes False

name = "Rid wan"
i = 0
while i < len(name):
    print(name[i], 'i')
    i += 1

Output:

R i
i i
d i
  i
w i
a i
n i

What's happening?

• We start with i = 0
• The loop checks if i < len(name) (which is 7)
• If True, it prints the character at position i
• Then it increases i by 1 with i += 1
• This process repeats until i reaches 7, at which point the condition is False

🎭 For Loops vs While Loops: The Key Differences

⚠️ Warning! Always make sure your while loop condition will eventually become False, or your program will run forever!

🌀 Nested Loops: Loops Inside Loops

Nested loops are like wheels within wheels - an inner loop runs completely for each iteration of the outer loop.

🧩 Basic Nested Loop

for i in range(1, 5):
    for j in range(1, 5):
        print(i, j)
    print()

Output:

1 1
1 2
1 3
1 4

2 1
2 2
2 3
2 4

3 1
3 2
3 3
3 4

4 1
4 2
4 3
4 4

What's happening?

• The outer loop runs 4 times (i = 1, 2, 3, 4)
• For each value of i, the inner loop runs 4 times (j = 1, 2, 3, 4)
• That's a total of 16 print statements!
• The empty print() adds a blank line after each inner loop completes

🧮 Practical Example: Multiplication Table

for i in range(1, 5):
    for j in range(1, 5):
        print(i, '*', j, '=', i*j)
    print()

Output:

1 * 1 = 1
1 * 2 = 2
1 * 3 = 3
1 * 4 = 4

2 * 1 = 2
2 * 2 = 4
2 * 3 = 6
2 * 4 = 8

3 * 1 = 3
3 * 2 = 6
3 * 3 = 9
3 * 4 = 12

4 * 1 = 4
4 * 2 = 8
4 * 3 = 12
4 * 4 = 16

Visualization of nested loops:

Outer loop (i=1) → Inner loop (j=1,2,3,4) → Print blank line
Outer loop (i=2) → Inner loop (j=1,2,3,4) → Print blank line
Outer loop (i=3) → Inner loop (j=1,2,3,4) → Print blank line
Outer loop (i=4) → Inner loop (j=1,2,3,4) → Print blank line

🛠️ Practical Loop Applications

For loops are great for:

• Processing each item in a list, tuple, string, or dictionary
• Executing code a specific number of times
• Iterating with precise control over the sequence

While loops excel at:

• Processing user input until it's valid
• Reading file data until end-of-file
• Running games until the player quits
• Implementing algorithms that run until convergence

🧠 Loop Selection Guide

Choose a for loop when:

• You can count the iterations in advance
• You're working with a collection (list, string, etc.)
• You need automatic iteration

Choose a while loop when:

• The number of iterations depends on user input
• You need to check a condition before each iteration
• You might need to skip iterations or break early based on conditions

🚀 Challenge Yourself!

Try these exercises to cement your understanding:

1. Create a program that uses a for loop to find all even numbers between 1 and 20
2. Write a while loop that asks for names until the user enters "quit"
3. Build a nested loop to create a simple pattern of asterisks forming a triangle

🔗 Resources

• Complete Day 4 Notebook on GitHub
• GeeksforGeeks Python Loops Tutorial

🔮 Coming Up Next...

Tomorrow we'll dive into Python data structures - the fundamental building blocks for organizing and storing your data efficiently!

← Day 3: Operators & Conditionals | Day 4: Mastering Loops | [Day 5: Data Structures →]

Which loop do you find more useful in your coding projects? Share your thoughts in the comments below! 💬