Welcome to Day 18 of 100 Days of Python! 🐍

Today, we are learning about while loops — one of the fundamental looping mechanisms in Python.

In the previous lesson, we learned about for loops, which are commonly used when iterating over a sequence or when the number of iterations is known.

A while loop works differently. It repeatedly executes a block of code as long as a given condition remains True.

We will also learn:

• How a while loop works

• Updating the loop variable

• Infinite loops and how to avoid them

• while with else

• The break statement

• How to emulate a do-while loop in Python

• Understanding the complete main.py program

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📚 What We Will Learn

1. What is a while loop?

2. Basic syntax

3. How a while loop works

4. Updating the loop variable

5. Avoiding infinite loops

6. while loop with else

7. Understanding break

8. Do-while loops

9. Emulating do-while behavior in Python

10. Complete Day 18 program

11. Important concepts at a glance

12. Quick revision

13. Key takeaways

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1. What is a while Loop?

A while loop repeatedly executes a block of statements as long as its condition evaluates to True.

Basic idea:

Check condition
      ↓
Is it True?
  ↓       ↓
 Yes      No
  ↓        ↓
Execute   Exit loop
body
  ↓
Update variables
  ↓
Check condition again

The loop continues until the condition becomes False.

Basic syntax

while condition:
    # statements

For example:

count = 5

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

Output

5
4
3
2
1

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2. How Does This while Loop Work?

Let's understand the example step by step:

count = 5

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

Initially:

count = 5

Python checks:

count > 0

Since:

5 > 0 → True

the loop body executes.

It prints:

5

Then:

count = count - 1

changes the value to:

4

Python checks the condition again:

4 > 0 → True

The process continues:

5
4
3
2
1

After printing 1, the value becomes:

count = 0

Now:

0 > 0 → False

Therefore, Python exits the loop.

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3. The Importance of Updating the Loop Variable

One of the most important things to understand with a while loop is that you usually need to change something inside the loop that eventually makes the condition false.

For example:

count = 5

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

Here, count decreases every iteration.

5 → 4 → 3 → 2 → 1 → 0

Eventually the condition:

count > 0

becomes false.

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4. What Happens If We Don't Update the Variable?

Consider this:

count = 5

while count > 0:
    print(count)

The value of count never changes.

Therefore:

count = 5

remains true forever.

The condition:

count > 0

will always be:

True

This creates an infinite loop.

Infinite loop concept

5 → 5 → 5 → 5 → 5 → ...

The loop never reaches a state where:

count > 0

becomes False.

Therefore, when designing a while loop, always ask:

What will eventually make the loop condition false?

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5. Incrementing and Decrementing

Depending on the problem, we may either increment or decrement a variable.

Decrementing

count = 5

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

This produces:

5
4
3
2
1

Incrementing

count = 1

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

Output:

1
2
3
4
5

The important point is not whether we increment or decrement.

The important point is that the loop must eventually reach a condition that evaluates to False, unless we intentionally create an infinite loop and exit using something such as break.

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6. while Loop with else

Python also allows an else block to be associated with a while loop.

Example:

x = 5

while x > 0:
    print(x)
    x = x - 1
else:
    print("counter is 0")

Output

5
4
3
2
1
counter is 0

Here, the else block executes after the while condition becomes false.

The flow is:

x = 5
  ↓
x > 0 → True
  ↓
Print 5
  ↓
x = 4
  ↓
...
  ↓
x = 0
  ↓
x > 0 → False
  ↓
Execute else

So:

else:
    print("counter is 0")

runs after normal termination of the while loop.

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7. while + else and break

There is an important detail about while-else.

The else block executes when the loop terminates normally because its condition becomes false.

If the loop exits using break, the else block is skipped.

For example:

x = 5

while x > 0:
    print(x)

    if x == 3:
        break

    x -= 1
else:
    print("Loop completed normally")

Output:

5
4
3

The break statement immediately terminates the loop.

Therefore:

else:
    print("Loop completed normally")

does not execute.

This behavior is useful when we want to distinguish between:

• the loop finishing normally, and

• the loop being terminated early.

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8. What is break?

The break statement is used to immediately terminate a loop.

Example:

while True:
    number = int(input("Enter a number: "))

    if number < 0:
        break

    print(number)

If the user enters:

5

the loop continues.

If the user enters:

-1

the break statement executes and the loop terminates.

Flow

while True
    ↓
Take input
    ↓
Process input
    ↓
Condition for stopping?
   ↙       ↘
 Yes       No
  ↓         ↓
 break    repeat

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9. Do-While Loop

Some programming languages provide a dedicated do-while loop.

A do-while loop has a special property:

The loop body executes at least once before the condition is checked.

Conceptually:

Execute body
     ↓
Check condition
   ↙       ↘
True      False
 ↓          ↓
Repeat     Exit

This differs from a normal while loop.

A normal while loop checks the condition before executing the body:

Check condition
      ↓
   True?
  ↙     ↘
Yes      No
 ↓        ↓
Body     Exit

Therefore, if the initial condition is already false, a normal while loop may execute zero times.

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10. Does Python Have a do-while Keyword?

Python does not have a dedicated do-while statement.

Instead, we can achieve similar behavior using:

while True:
    # code to execute

    if condition:
        break

This pattern is commonly used to emulate do-while behavior.

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11. Emulating a Do-While Loop in Python

Consider:

while True:
    number = int(input("Enter a positive number: "))
    print(number)

    if not number > 0:
        break

Let's understand it.

Step 1: while True

while True:

True is always true, so this creates a loop that would continue indefinitely.

Step 2: Execute the body

number = int(input("Enter a positive number: "))
print(number)

The user is asked for a number.

Importantly, this code executes before the stopping condition is checked.

Step 3: Check the stopping condition

if not number > 0:
    break

If the entered number is not positive, break terminates the loop.

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12. Understanding not number > 0

This condition:

not number > 0

means:

not (number > 0)

For example:

If:

number = 4

then:

number > 0 → True
not True → False

So the break statement does not execute.

The loop continues.

If:

number = -1

then:

number > 0 → False
not False → True

Therefore:

break

executes.

The loop terminates.

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13. Example Execution

Suppose the user enters:

1
4
-1

The program behaves like this:

First iteration

Enter a positive number: 1
1

Since 1 > 0, the loop continues.

Second iteration

Enter a positive number: 4
4

Since 4 > 0, the loop continues.

Third iteration

Enter a positive number: -1
-1

Now:

-1 > 0 → False

Therefore:

not -1 > 0

is true, and:

break

terminates the loop.

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14. Day 18 main.py – Complete Program

Here is the complete program from today's lesson:

i = int(input("Enter a number: "))
print(i)

while i <= 38:
    i = int(input("Enter a number: "))
    print(i)

print("Done with the loop")

count = 5

while count > 0:
    print(count)
    count = count - 1
else:
    print("I am inside else block")

while True:
    number = int(input("Enter a positive number: "))
    print(number)

    if not number > 0:
        break

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15. Understanding the First while Loop

The first section is:

i = int(input("Enter a number: "))
print(i)

while i <= 38:
    i = int(input("Enter a number: "))
    print(i)

print("Done with the loop")

First, the program asks the user for a number.

Then it checks:

while i <= 38:

If i is less than or equal to 38, the loop runs.

Inside the loop, another number is requested:

i = int(input("Enter a number: "))

This updates the value of i.

The loop continues until the user enters a value greater than 38.

For example:

Enter a number: 10
10
Enter a number: 20
20
Enter a number: 38
38
Enter a number: 50
50
Done with the loop

When 50 is entered:

50 <= 38 → False

Therefore, the loop terminates.

Important observation

The first input is taken before entering the loop.

Then another input is taken inside the loop.

This means the first value determines whether the loop starts at all.

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16. Understanding the Second while Loop

The second section is:

count = 5

while count > 0:
    print(count)
    count = count - 1
else:
    print("I am inside else block")

Output:

5
4
3
2
1
I am inside else block

The counter follows:

5 → 4 → 3 → 2 → 1 → 0

When:

count > 0

becomes false, the loop ends normally and the else block executes.

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17. Understanding the Third while Loop

The final section is:

while True:
    number = int(input("Enter a positive number: "))
    print(number)

    if not number > 0:
        break

This is the do-while-style portion of today's program.

The body executes first, and then the program checks whether it should stop.

For example:

Enter a positive number: 5
5
Enter a positive number: 10
10
Enter a positive number: 3
3
Enter a positive number: -2
-2

When -2 is entered:

not number > 0

becomes true.

Therefore:

break

terminates the loop.

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18. while Loop vs for Loop

Both are used for repetition, but they are often used in different situations.

for Loop	while Loop
Commonly iterates over an iterable	Runs while a condition is true
Useful when iterating through sequences	Useful when repetition depends on a condition
Often used when iteration is determined by a collection or range	Often used when the number of iterations is not known beforehand
Example: for x in range(5)	Example: while x < 5

Example:

for i in range(5):
    print(i)

versus:

i = 0

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

Both produce:

0
1
2
3
4

But the logic controlling the repetition is expressed differently.

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19. Common Mistakes with while Loops

Mistake 1: Forgetting to update the variable

count = 5

while count > 0:
    print(count)

This can result in an infinite loop.

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Mistake 2: Updating in the wrong direction

Suppose we write:

count = 5

while count > 0:
    print(count)
    count += 1

The values become:

5 → 6 → 7 → 8 → ...

The condition:

count > 0

never becomes false.

Again, this creates an infinite loop.

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Mistake 3: Forgetting break in an intentional infinite loop

If we write:

while True:
    print("Hello")

there is no natural stopping condition.

If we intentionally use while True, we generally need a suitable exit mechanism such as:

break

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20. Important Concepts at a Glance

Concept	Meaning
while	Repeats code while a condition is True
Condition	Controls whether the loop continues
Loop variable	A variable whose value often changes during the loop
while True	Creates an intentionally infinite loop
break	Immediately exits the loop
else with while	Runs when the loop ends normally because its condition becomes false
Do-while	Executes the body at least once before checking the continuation condition
Python do-while	Emulated using while True and break
Infinite loop	A loop that never reaches a terminating condition

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21. Quick Revision

Basic while

while condition:
    # code

Decrementing

count = 5

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

Incrementing

count = 1

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

while-else

while condition:
    # code
else:
    # runs after normal loop termination

Exit using break

while True:
    # code

    if condition:
        break

Do-while-style pattern

while True:
    # execute at least once

    if stop_condition:
        break

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22. Key Takeaways

• A while loop executes code repeatedly while its condition is True.

• The condition is checked before each iteration.

• The loop variable usually needs to be updated so that the loop can eventually terminate.

• Forgetting to update the relevant variable can create an infinite loop.

• A while loop can have an else block.

• The else block runs when the loop terminates normally because its condition becomes false.

• If the loop exits through break, its else block does not execute.

• Python does not have a dedicated do-while keyword.

• A do-while-style loop can be created using while True and break.

• A do-while-style structure guarantees that the loop body executes at least once.

• break immediately terminates the current loop.

Today we learned how to control repetition using conditions with while loops, how while-else works, and how Python can emulate the behavior of a do-while loop.

The next step is to keep practicing these loop patterns until the flow becomes intuitive — especially the relationship between conditions, variable updates, and loop termination.

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📂 Day 18 Resources

https://github.com/SriteshSuranjan/100-Days-of-Python/tree/main/18-Day18-While-Loops

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