🚀 Data Science Roadmap 2026

📘 Phase 1: Programming Fundamentals

🐍 Topic 5: Python Loops ("for" & "while")

Welcome back! 👋

In the previous lesson, you learned how Python makes decisions using if, elif, and else. Now it's time to learn how to repeat tasks automatically using loops.

Loops are one of the most powerful concepts in Python. Instead of writing the same code multiple times, you can use loops to execute a block of code repeatedly.

In Data Science, loops are commonly used to process datasets, automate repetitive tasks, iterate through lists, and build machine learning workflows.

🔹 1. What is a Loop?

A loop is used to execute a block of code multiple times until a condition is met.

Without loops:

print("Hello")
print("Hello")
print("Hello")
print("Hello")
print("Hello")

Using a loop:

for i in range(5):
    print("Hello")

Both produce the same output, but the loop is much shorter and easier to maintain.

🔹 2. Types of Loops in Python

Python provides two main types of loops:

✅ "for" Loop

✅ "while" Loop

🔹 3. The "for" Loop ⭐

A "for" loop is used when you know how many times you want to repeat a task.

Syntax

for variable in sequence:
    # Code to execute

Example

for i in range(5):
    print(i)

Output:

0  
1  
2  
3  
4

Notice that range(5) generates numbers from 0 to 4.

🔹 4. The "range()" Function ⭐

The range() function generates a sequence of numbers.

Example 1

for i in range(5):
    print(i)

Output: 0 1 2 3 4

Example 2

for i in range(1, 6):
    print(i)

Output: 1 2 3 4 5

Example 3

for i in range(2, 11, 2):
    print(i)

Output: 2 4 6 8 10

The third argument is called the step size.

🔹 5. Looping Through a List

fruits = ["Apple", "Banana", "Mango"]

for fruit in fruits:
    print(fruit)

Output:

Apple  
Banana  
Mango

🔹 6. The "while" Loop ⭐

A "while" loop continues executing as long as the condition remains True.

Syntax

while condition:
    # Code

Example

count = 1

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

Output: 1 2 3 4 5

🔹 7. Infinite Loop

Be careful when using "while" loops.

while True:
    print("Hello")

This loop never stops unless interrupted.

Always ensure the condition eventually becomes False.

🔹 8. Loop Control Statements ⭐

"break" - Stops the loop immediately

for i in range(10):
    if i == 5:
        break
    print(i)

Output: 0 1 2 3 4

"continue" - Skips the current iteration

for i in range(5):
    if i == 2:
        continue
    print(i)

Output: 0 1 3 4

"pass" - Acts as a placeholder

for i in range(5):
    pass

Useful when writing incomplete code.

🔹 9. Nested Loops

A loop inside another loop.

for i in range(3):
    for j in range(2):
        print(i, j)

Output:

0 0  
0 1  
1 0  
1 1  
2 0  
2 1

🔹 10. Real-World Data Science Example

Calculate the total sales.