9. Loops in Python (for, while) with Practical Examples

Loops allow you to repeat actions automatically. Instead of writing the same code multiple times, Python loops execute a block of code repeatedly. This is essential for processing data, handling user input, and automating tasks.

1) The for Loop

The for loop is commonly used to iterate over a sequence such as a list, string, or range of numbers.

Basic Example

for i in range(5):
    print("Number:", i)
    

range(5) generates numbers from 0 to 4.

2) Looping Through a List

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

for fruit in fruits:
    print("Fruit:", fruit)
    

The loop automatically takes each item from the list one by one.

3) The while Loop

The while loop repeats as long as a condition is True. It is useful when you don’t know how many times the loop should run.

count = 0

while count < 5:
    print("Count:", count)
    count += 1
    

4) Practical Example: User Login System

password = "1234"
attempt = ""

while attempt != password:
    attempt = input("Enter password: ")

print("Access granted!")
    

The loop continues until the correct password is entered.

5) break and continue

Using break

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

break stops the loop completely.

Using continue

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

continue skips the current iteration.

6) Nested Loops

A loop inside another loop is called a nested loop.

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

Nested loops are often used for working with tables or matrices.

7) Looping Through a Dictionary

student = {"name": "Ali", "age": 22, "grade": "A"}

for key, value in student.items():
    print(key, ":", value)
    

8) Common Beginner Mistakes

• Forgetting to update the counter in a while loop
• Incorrect indentation
• Creating infinite loops accidentally
• Using break incorrectly

Conclusion

Loops are powerful tools that allow you to automate repetition. The for loop is best when iterating over sequences, while the while loop is useful for condition-based repetition.

Mastering loops will allow you to process data, build interactive programs, and solve real-world problems efficiently.