Introduction to Loops and Iteration (While Loops) in Python

Hello Everyone! Today we are going to learn about Loops and Iteration. In programming, we often need to repeat a block of code multiple times. Instead of copy-pasting the same lines over and over, we use loops to automate repetitive tasks cleanly.

Python primarily features two types of loops. Today, we will master the While Loop.

1. Core Mechanics of a While Loop

A while loop continuously executes its inner code block as long as its conditional expression evaluates to True.

The Infinite Loop Danger

If the condition never becomes False, the program will get stuck in an infinite loop until it crashes or you manually stop it.

# WARNING: This will run forever until manually interrupted!
while True:
     print("Hello World1!")
     print("Hello World2!")
     print("Hello World3!")

Breaking a Loop via State Shifts

To stop a loop, you must change the variables controlling the condition inside the block.

counter = True

while counter:
     print("hello")
     counter = False # The loop runs exactly once because the condition drops to False

2. Controlled Numeric Iterations

To build a structured loop, you need three main components: Initialization, Condition Check, and Updating the Counter.

Counting Down ($5 \rightarrow 0$)

counter = 5

while counter >= 0:
     print(counter)
     counter = counter - 1

Counting Up ($1 \rightarrow 5$)

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

Changing Step Intervals (Odd Numbers)

counter = 1
while counter <= 5:
     print(counter)
     counter = counter + 2 # Jumps by 2: Prints 1, 3, 5

Geometric Multiplication Step Intervals

counter = 1
while counter <= 10:
     print(counter)
     counter *= 2          # Multiplies by 2: Prints 1, 2, 4, 8

3. Foundational Practice Assignments

Here are solutions for common range and direction problems.

A. Counting Up ($1 \rightarrow 50$)

counter = 1
while counter <= 50:
     print(counter)
     counter += 1

B. Reverse Counting ($50 \rightarrow 1$)

counter = 50
while counter >= 1:
     print(counter)
     counter -= 1

C. Negative Range Progression ($-1 \rightarrow -50$)

counter = -1
while counter >= -50:
     print(counter)
     counter -= 1

D. Negative Range Progression ($-50 \rightarrow -1$)

counter = -50
while counter <= -1:
     print(counter)
     counter += 1

E. Odd Numbers ($10 \rightarrow 20$)

counter = 11  # Initialized to the first odd number in the range
while counter <= 20:
     print(counter)
     counter += 2

F. Even Numbers ($50 \rightarrow 80$)

counter = 50
while counter <= 80:
     print(counter)
     counter += 2

G. Interactive Multiplication Table Generator

number = int(input("Enter a number to print its table: "))
counter = 1

while counter <= 10:
     print(f"{number} x {counter} = {number * counter}")
     counter += 1

4. Visual Pattern Printing

Loops can build string layouts dynamically by multiplying characters with the loop index counter.

Right-Angled Triangle Pattern

counter = 1

while counter <= 5:
     print("*" * counter)
     counter += 1

Output:

*
**
***
****
*****

Inverted Number Line Patterns

counter = 5

while counter >= 1:
     print(str(counter) * counter)
     counter -= 1

Output:

55555
4444
333
22
1

Inverted Star Pattern

counter = 5

while counter >= 1:
     print("*" * counter)
     counter -= 1

Output:

*****
****
***
**
*

5. Accumulation, Summation, and Factorials

Accumulators let you add up or multiply values sequentially inside a loop.

Running Sum vs Product ($1 \rightarrow 5$)

# Factorial Logic Simulation
counter = 1
sum = 1

while counter <= 5:
     sum = sum * counter
     counter = counter + 1

Taking 5 Numbers from the Keyboard & Tracking Sums

counter = 1
sum = 0

while counter <= 5:
     number = int(input("Enter A Number: "))
     print(number)
     sum = sum + number
     print(sum) # Prints intermediate running total
     counter += 1

print("Sum is : ", sum)

Factorial Accumulator Script

sum = 1
counter = 1

while counter <= 5:
     sum = sum * counter
     print(sum)
     counter += 1

Skipping Numbers using Conditions inside Loops

counter = 50

while counter > 0:
     if counter % 2 != 0:
          print(counter) # Prints odd numbers moving downwards from 50
     counter -= 1

print(f"Final Counter Value is {counter}")

6. Advanced Practice Workbook

Here are solutions for complex logical problems using combinations of loops, conditions, and termination commands.

Challenge 1: Sum of Even Numbers Up to N

number = int(input("Enter A number: "))
sum = 0
counter = 1

while counter <= number:
     if counter % 2 == 0:
          sum += counter
     counter += 1

print(sum)

Challenge 2: Secure Login Attempt Limiter

This script checks user credentials and terminates the loop early using the break command if details match.

counter = 1

while counter <= 3:
     username = input("Enter Username: ")
     password = input("Enter Password: ")

     if username == "admin" and password == "1234":
          print("Password Accepted.")
          break # Terminate loop instantly upon success
     else:
          print("Incorrect password. Try again.")

     counter += 1

if counter == 4:
     print("Account locked.")

Challenge 3: Smallest Divisor and Prime Tester

counter = 2
number = int(input("Enter A Number: "))

while counter < number:
     if number % counter == 0:
          break # Found a divisor, stop searching
     counter += 1

if number == counter:
     print("Failed to find the smallest divisor as it's a Prime Number.")
else:
     print(f"Number is divisible by {counter}")

Challenge 4: Positive Number Accumulator Sentinel Loop

number = 0
total = 0

while number >= 0:
     number = int(input("Enter A number: "))
     if number > 0:
          total = total + number

print(f"Sum of all values are {total}")