Level 9 · How many weeks until we run out?

for or while?

A for loop walks through something that already exists: a list of orders, the keys of a dictionary, a range. A while loop repeats as long as a condition is true, which is what you need when you don’t know in advance how many steps there will be.

while stock >= demand:
    # one more week can be covered
    ...

Python checks the condition, runs the body, checks again, and stops the first time the condition is false. The body must change something the condition depends on, or the loop never ends.

Counting weeks with growing demand

If demand were flat, 260 // 38 would answer the question: 6 weeks. But holiday demand grows 10% each week, so every week needs more than the last. The loop handles that by updating three things on every pass:

weeks = 0
while stock >= demand:
    stock = stock - demand            # ship this week's units
    demand = demand * (1 + growth)    # next week needs 10% more
    weeks += 1                        # one more week covered

Tracing it shows how quickly growth eats the buffer. The weekly needs are 38, 41.8, 45.98, about 50.6 and about 55.6. After five weeks 232 units have gone out and 28 are left. Week six would need about 61.2, more than what remains, so the condition is false and the loop stops with weeks at 5. That is a full week less than the flat-demand estimate, which matters for a January delivery.

Make sure it can stop

Every while loop needs a way out. Here the stock falls and the demand rises on each pass, so stock >= demand must eventually become false. If you forget to subtract the demand, the stock never changes and the loop runs until the page stops it. When you write a while, ask “what in the body moves the condition towards false?” before you run it.

Common mistakes

• Using floor division on the starting numbers and ignoring growth.
• Forgetting one of the updates, which gives an endless loop.
• Testing stock > 0 instead of stock >= demand. That counts a sixth week the stock cannot cover.
• Rounding demand on every pass, which slowly drifts away from the real numbers.