Python while Loop Exercises: 5 Patterns That Cover Most Problems

Pattern 1: count down with a changing condition

Start with a value, test it, and change it inside the loop. The update is not optional: it is what makes the loop finish. Predict the output before running the example.

count = 3
while count > 0:
    print(count)
    count -= 1
print("Liftoff")

A predictable countdown

Pattern 2: stop on a sentinel

A sentinel is a special value that means stop. Check both the index and the value so the loop cannot run past the end of the sequence. This is the same idea as reading input until the user types quit.

commands = ["start", "check", "quit", "ignored"]
i = 0
while i < len(commands) and commands[i] != "quit":
    print(commands[i])
    i += 1

Stop before the sentinel

Pattern 3: break for an early exit

Use break when the stopping condition is easier to express inside the body than in the while header. Keep the exit close to the condition that triggers it, so a reader does not have to search for it.

prices = [3.0, 6.0, 4.5]
i = 0
while i < len(prices):
    if prices[i] > 5:
        print(prices[i])
        break
    i += 1

Find the first match

Pattern 4: accumulate until a limit

Create the accumulator before the loop, update it inside, and print it after. A limit in the condition prevents an accidental endless total. Trace the first two iterations by hand before trusting the result.

total = 0
n = 1
while total < 10:
    total += n
    n += 1
print(total)

A bounded accumulation

Debug an infinite loop

If a loop never ends, print the loop variable at the top of the body. Check that the update is inside the loop, that the condition uses the right operator, and that no branch skips the update. Then run again with a small input.

n = 0
while n < 3:
    print("top", n)
    n += 1
print("done", n)

Inspect the changing value

Your practice checklist

• Name the pattern before writing the condition.
• Make sure the loop body changes the tested value.
• Use break only for a clear early exit.
• Trace a small input by hand before running it.

Common questions

Keep reading:

• Python while Loops
• Python Loop Exercises: Five Patterns That Cover Most Beginner Problems
• Python Practice for Beginners: A Simple 4-Week Plan