Python Loops – For, While and Nested Loops

 we’ll cover:

• What are loops in Python?
• Types of loops.
• Examples of each type of loop.

What are Loops?

Loops execute a block of code repeatedly as long as a condition is true. They are especially useful when working with repetitive tasks or iterating through data structures like lists, dictionaries, or strings.

Types of Loops in Python

Python provides two main types of loops:

1. for loop: Used for iterating over a sequence (e.g., list, tuple, string).
2. while loop: Repeats a block of code as long as a condition is true.

1. for Loop

The for loop iterates over elements in a sequence (e.g., list, string, or range).

Syntax:

for item in sequence:
    # Code to execute

Example: Loop through a list

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)

Output:

apple
banana
cherry

Example: Using range()

The range() function generates a sequence of numbers.

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

Output:

1
2
3
4
5

2. while Loop

The while loop continues executing as long as its condition is True.

Syntax:

while condition:
    # Code to execute

Example: Countdown

count = 5

while count > 0:
    print(count)
    count -= 1  # Decrease the count by 1

Output:

5
4
3
2
1

3. Nested Loops

A loop inside another loop is called a nested loop.

Example: Multiplication Table

for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} x {j} = {i * j}")

Output:

1 x 1 = 1
1 x 2 = 2
1 x 3 = 3
2 x 1 = 2
2 x 2 = 4
2 x 3 = 6
3 x 1 = 3
3 x 2 = 6
3 x 3 = 9

4. Control Statements in Loops

Python provides control statements to manage the flow of loops:

• break: Exits the loop prematurely.
• continue: Skips the rest of the loop’s body for the current iteration.
• pass: Does nothing; acts as a placeholder.

break Example

Exit the loop when a condition is met.

for number in range(1, 6):
    if number == 3:
        break
    print(number)

Output:

1
2

continue Example

Skip the current iteration and proceed with the next.

for number in range(1, 6):
    if number == 3:
        continue
    print(number)

Output:

1
2
4
5

pass Example

Do nothing in the loop (used as a placeholder).

for number in range(1, 6):
    if number == 3:
        pass
    else:
        print(number)

Output:

1
2
4
5

5. Iterating Over Data Structures

Strings

You can iterate over each character in a string.

word = "Python"

for char in word:
    print(char)

Output:

P
y
t
h
o
n

Lists

numbers = [10, 20, 30]

for num in numbers:
    print(num)

Output:

10
20
30

Dictionaries

Iterate through keys and values.

person = {"name": "Alice", "age": 25}

for key, value in person.items():
    print(f"{key}: {value}")

Output:

name: Alice
age: 25

6. Else with Loops

Both for and while loops can have an else clause that executes after the loop finishes.

Example: Using else with a for Loop

for number in range(1, 4):
    print(number)
else:
    print("Loop completed.")

Output:

1
2
3
Loop completed.

Example: Using else with a while Loop

count = 3

while count > 0:
    print(count)
    count -= 1
else:
    print("Countdown finished.")

Output:

3
2
1
Countdown finished.

Common Mistakes with Loops

1. Infinite Loops: Forgetting to update the condition in a while loop can result in an infinite loop.

   count = 3
   while count > 0:
       print(count)  # No decrement leads to infinite loop!
   

2. Misusing break and continue: Understand when to exit or skip iterations carefully.

3. Improper Indentation: Python relies on indentation to define loop bodies.