Lesson 8 · 8 of 20
Repeating with while · Python
Repeat code in Python with while as long as a condition holds: save up for a bike week by week and run a countdown.
while repeats its block as long as the condition is true. Before each round, Python checks savings < goal again: with 0, 22, 44, 66 and 88 euros it carries on; at 110 the condition is false and the loop stops.
Something inside the loop must change so that it ends one day. week += 1 is a short way of writing week = week + 1, and savings += 22 adds the week's savings. If nothing changed, the loop would never end (an infinite loop).
After the loop, the variables keep their last value: that is how the program can say it took 5 weeks and there are 10 euros to spare.
The countdown goes the other way: it starts at 3 and subtracts with count -= 1 until count > 0 is no longer true. Then the unindented line after it runs: «Lift-off!».
Code · main.py
# Saving for a bike: add money every week until you have enough.goal = 100savings = 0week = 0while savings < goal: week += 1 # the same as week = week + 1 savings += 22 print(f"Week {week}: {savings} euros saved")print(f"Goal reached in {week} weeks!")print(f"Change left over: {savings - goal} euros")# A countdown: the loop stops when the condition becomes false.count = 3while count > 0: print(count, "...") count -= 1print("Lift-off!")Output · python3 main.py
Week 1: 22 euros saved Week 2: 44 euros saved Week 3: 66 euros saved Week 4: 88 euros saved Week 5: 110 euros saved Goal reached in 5 weeks! Change left over: 10 euros 3 ... 2 ... 1 ... Lift-off!
Key points
while condition:repeats while the condition isTrue.- Something in the loop must bring it closer to the end.
x += 1adds 1 tox;x -= 1subtracts 1.- If the condition is false from the start, the block never runs.
Exercise
Change the savings so that each week you put away 5 euros more than the week before (22, 27, 32…) and find out how many weeks it takes to buy a 250-euro bike.