Loops

Loops allow you to execute a block of code multiple times. Python provides two main types of loops: for loops and while loops.

for Loops

A for loop iterates over a sequence (like a list, tuple, or string) and executes the code block for each item.

Syntax:

for item in sequence:
    # Code to execute for each item

Example:

# Iterating through a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# Iterating through a string
for letter in "Python":
    print(letter)

Using range():

The range() function generates a sequence of numbers.

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

# From 1 to 5
for i in range(1, 6):
    print(i)

# With a step
for i in range(0, 10, 2):
    print(i)

while Loops

A while loop continues to execute as long as its condition is True.

Syntax:

while condition:
    # Code to execute while condition is True

Example:

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

Controlling Loops

break Statement:

Stops the loop immediately.

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

continue Statement:

Skips the rest of the current iteration and moves to the next.

for i in range(10):
    if i % 2 == 0:
        continue
    print(i)  # Prints odd numbers

else Clause with Loops:

Executes after the loop finishes, unless the loop is terminated with break.

for i in range(5):
    print(i)
else:
    print("Loop completed!")

Nested Loops

You can place a loop inside another loop.

Example:

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

Infinite Loops

A loop runs forever if its condition never becomes False. Use with caution and always include a way to exit.

while True:
    user_input = input("Type 'exit' to quit: ")
    if user_input.lower() == "exit":
        break

Common Use Cases

1. Iterating Over a List:

   numbers = [1, 2, 3, 4, 5]
   for num in numbers:
       print(num)
   

2. Counting Down:

   countdown = 5
   while countdown > 0:
       print(countdown)
       countdown -= 1
   print("Blastoff!")
   

3. Finding Factors of a Number:

   number = int(input("Enter a number: "))
   for i in range(1, number + 1):
       if number % i == 0:
           print(i)
   

Practice Exercises

1. Write a program to:
   • Print all numbers from 1 to 20.
   • Skip numbers divisible by 3.
2. Create a program that:
   • Asks the user to guess a secret number.
   • Keep asking until they guess correctly.
3. Generate a multiplication table (from 1 to 10) for a number entered by the user.

Loops are essential for automating repetitive tasks and are a key part of programming in Python!