While Loops

A while loop repeats its body as long as a condition stays True:

n = 1
while n <= 4:
    print(n)
    n += 1

Output:

1
2
3
4

The condition is checked before every pass. When n reaches 5, the check fails and the loop ends.

Use for when you know what to loop over — a list, a range. Use while when you only know the stopping condition:

def divide_until_small(n):
    steps = 0
    while n > 1:
        n = n // 2
        steps += 1
    return steps

How many passes this makes depends on n — a for loop cannot express that directly.

Something in the body must move the condition toward False. If nothing does, the loop never ends and the program hangs — this editor stops runaway code after 10 seconds.

Exercise

An account earns 5% interest per year: each year the balance is multiplied by 1.05.

1. Define a function named years_to_reach with two parameters: balance and target.
2. It returns how many whole years it takes for balance to reach or pass target. Use a while loop.
3. If balance already meets the target, return 0.

years_to_reach(1000, 1100) returns 2 — after one year the balance is 1050.0, after two 1102.5.

Run your code to see the output, then press Submit.

Tests

import unittest


class TestYearsToReach(unittest.TestCase):
    def test_1000_to_1100_takes_2_years(self):
        self.assertEqual(years_to_reach(1000, 1100), 2)

    def test_already_at_target_takes_0_years(self):
        self.assertEqual(years_to_reach(500, 500), 0)

    def test_1000_to_2000_takes_15_years(self):
        self.assertEqual(years_to_reach(1000, 2000), 15)