Loops in Python (for, while)

Loops are fundamental control structures in programming that allow you to execute a block of code repeatedly. Instead of writing repetitive code, loops make programs efficient, concise, and easier to maintain. In Python, the primary loop constructs are for and while, supported by control keywords like break, continue, and else.

This chapter explores Python loops in depth — including syntax, flow, nesting, loop control statements, iterables, and common mistakes. We’ll use practical examples to help you understand how to apply loops in real-world programming tasks.

1. Introduction to Looping

Loops enable you to repeat an action multiple times. This is particularly useful when dealing with collections (like lists or strings), generating sequences, or automating tasks.

1.1 Use Cases:

• Iterating over elements in a list
• Repeating a task until a condition is met
• Processing user input until ‘exit’
• Generating multiplication tables, patterns

2. The for Loop

Used to iterate over a sequence (such as a list, tuple, string, or range).

2.1 Syntax:

for item in iterable:
    # block of code

2.2 Example:

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

3. The range() Function

Often used with for to generate a sequence of numbers.

Syntax:

range(start, stop, step)

Examples:

for i in range(5):       # 0 to 4
    print(i)

for i in range(1, 10, 2):  # 1 to 9, step 2
    print(i)

4. Looping Through Strings

Strings are sequences, so you can loop through each character.

for char in "Python":
    print(char)

5. Nested for Loops

A loop inside another loop.

for i in range(3):
    for j in range(2):
        print(f"i={i}, j={j}")

Common in:

• Matrices
• Pattern printing
• Cartesian products

6. The while Loop

Executes a block as long as a condition is true.

6.1 Syntax:

while condition:
    # block of code

6.2 Example:

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

Use Cases:

• Unknown iteration counts
• Waiting for user input
• Infinite loops (with breaks)

7. Infinite Loops and Safety

Be careful not to create infinite loops:

# Dangerous!
while True:
    print("Running...")

Break condition is essential:

while True:
    command = input("Enter 'exit' to stop: ")
    if command == 'exit':
        break

8. break Statement

Exits the loop prematurely.

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

9. continue Statement

Skips the current iteration and continues with the next.

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

10. else with Loops

Python supports an optional else block after loops, executed when the loop completes normally (not via break).

for i in range(3):
    print(i)
else:
    print("Loop completed")

# Example with break
for i in range(5):
    if i == 3:
        break
    print(i)
else:
    print("This won't be printed")

11. Looping Through Dictionaries

d = {"a": 1, "b": 2, "c": 3}

# Keys
for key in d:
    print(key)

# Keys and values
for key, value in d.items():
    print(key, value)

12. Looping with Index: enumerate()

names = ["Alice", "Bob", "Charlie"]
for index, name in enumerate(names):
    print(index, name)

13. Looping Over Multiple Sequences: zip()

names = ["Alice", "Bob"]
ages = [25, 30]

for name, age in zip(names, ages):
    print(f"{name} is {age} years old")

14. Common Mistakes and Debugging Tips

• Forgetting to increment in while loop (causes infinite loop)
• Incorrect indentation
• Using = instead of ==
• Forgetting break condition

Debugging Tips:

• Use print statements to trace loop flow
• Use small ranges for testing
• Add manual exit options for infinite loops

15. Best Practices

• Keep loop blocks short and focused
• Avoid deep nesting
• Use functions to encapsulate logic
• Use enumerate() and zip() when needed
• Prefer for loops when iterating over sequences

16. Exercises

1. Write a for loop to print numbers from 1 to 10
2. Write a while loop that prints even numbers less than 20
3. Use nested loops to print a multiplication table
4. Loop through a dictionary and print key-value pairs
5. Use zip() to pair student names and their grades
6. Write a program to accept user input until ‘done’ is typed

17. Summary

Loops are powerful constructs that save time and reduce code duplication. Python provides for loops for sequence iteration and while loops for condition-based execution. With control statements like break, continue, and else, Python makes looping expressive and flexible.

Understanding how and when to use each type of loop — along with tools like enumerate() and zip() — empowers you to write more efficient, readable, and Pythonic code.

✅ Next Chapter: Functions in Python – Learn how to create reusable blocks of code using functions.