While Loops

The Infinite Loop

Take a look at this example:

while 1 < 2:
    print("I'm looooooping...")

Because 1 < 2 is always True and never changes, this loop will run forever. Your program will freeze, your browser tab may crash, and if you are running this in VS Code your terminal will lock up until you force stop it. Be careful.

The same thing happens here:

num = 1
while num < 100:
    print(num)

Because num never changes inside the loop, the condition num < 100 is always True and the loop runs forever.

break

One way to escape an infinite loop is to use break. When Python hits a break statement it immediately stops the loop and moves on. Adding a break after the print in the example above means the loop will only ever run once regardless of the condition.

num = 1
while num < 100:
    print(num)
    break

But break is not the right tool if you want the loop to run a specific number of times. For that we need to update the condition on each iteration.

Updating the Condition

The correct way to control a while loop is to update the variable that the condition depends on inside the loop. This way the condition eventually becomes False and the loop stops naturally.

num = 1
while num < 100:
    print(num)
    num = num + 1

Now on each iteration num increases by 1. Eventually num reaches 100, the condition num < 100 becomes False and the loop stops. This is the standard pattern for a while loop and you will use it constantly. It can also be written using the shorthand num += 1 which means exactly the same thing.

Try each of the following in the pybox below and run them one at a time:

while / else

Just like an if statement, a while loop can have an else block. The else block runs once when the while condition becomes False. It does not run if the loop was exited using break.

This is the key distinction and also the main reason while/else is only useful in specific situations. If you need to know whether a loop completed naturally or was interrupted by a break, else gives you a clean way to handle that.

num = 1
while num < 5:
    print(num)
    num += 1
else:
    print("loop finished naturally")

Outside of this specific use case, while/else is rarely seen in real world Python code. Do not feel the need to use it just because it exists.

while vs for - When to Use Which

Both loops repeat code but they are designed for different situations. Choosing the right one makes your code cleaner and easier to understand.

• Use a for loop when you know exactly what you are iterating over. A list, a string, a range, a dictionary. If you have a collection of items and you want to do something with each one, a for loop is always the right choice.
• Use a while loop when you do not know how many times the loop needs to run. If the loop depends on a condition that changes based on user input, an external event, or some value that is calculated during the loop, a while loop is the right choice.

A simple way to think about it:

• for - "do this for each item in this collection"
• while - "keep doing this until something changes"

In practice you will use for loops far more often than while loops. While loops are most commonly seen in situations like waiting for a user to enter valid input, running a game loop that continues until the player loses, or polling a system until a response is received.

for vs while Side by Side

The best way to understand the difference between a for loop and a while loop is to see them solve the exact same problem side by side.

Both loops below print every item in the same list. The for loop does it in two lines. The while loop does it in four. Python handles all of the index tracking automatically in a for loop. With a while loop you are responsible for managing the index yourself.

The for loop is cleaner, shorter, and easier to read. For any situation where you are iterating over a known collection, it will almost always be the better choice.

The while loop however gives you more control. Because you manage the index and the condition yourself, you can do things a for loop cannot. You can skip items, jump to a specific position, change the step dynamically, or keep looping based on conditions that have nothing to do with the collection itself. This is what makes while loops more dynamic and better suited for situations where the number of iterations is not known in advance.

while True

One of the most powerful and commonly used patterns with a while loop is while True:. Because True is always True, this creates a deliberate infinite loop that runs forever until a break statement is reached.

This pattern is extremely useful when combined with input(). Instead of asking the user for input once and hoping they get it right, you can keep asking until they give you what you need. This is how real applications handle user input validation. We will practice this in the exercises below.

Exercise 1: Basic while Loop

• Task: Create a variable num = 1.
• Goal: Write a while loop that prints every number from 1 to 50. Then modify the loop to print only even numbers between 1 and 50 using the step. Write a comment explaining how you prevented the loop from running infinitely.

Exercise 2: Countdown

• Task: Create a variable num = 10.
• Goal: Write a while loop that counts down from 10 to 1 and prints each number. When the loop finishes print "Blast off!" using the else block. Write a comment explaining when the else block runs and when it would not run.

Exercise 3: while True with input()

• Task: Write a while True: loop that asks the user to enter the secret word using input().
• Goal: If the user types "python" print "Access granted!" and break out of the loop. If they type anything else print "Wrong password, try again." and keep asking. Write a comment explaining why while True is the right choice for this kind of problem.

Exercise 4: while vs for

• Task: Create a list cities = ['Bangkok', 'Tokyo', 'London', 'New York', 'Sydney'].
• Goal: Write two loops that print every city in the list. The first using a for loop and the second using a while loop. Write a comment explaining which one is cleaner and why, and when you might choose the while loop version over the for loop version.

Exercise 5: Input Validation

• Task: Write a while True: loop that asks the user to enter a number between 1 and 10 using input().
• Goal: Convert the input to an integer and check if it falls within the valid range. If it does print "Valid input!" and break. If it does not print "Please enter a number between 1 and 10." and keep asking. Write a comment explaining why this is a more robust solution than asking for input only once.