← Back to course
Code Crew G6+ / Week 04 / Friday
5/6
Week 04 · Loops

Friday

Countdown clock
// for, while, break and a countdown clock
⏱ about 30 min

Friday: Countdown clock

Launch day is coming, and Rocket wants a countdown on the big screen. His first try prints all ten numbers in a flash, and the word Liftoff appears before anyone can blink.

"That was the fastest countdown in history," he says, laughing. "The computer does not know it should wait."

Raven checks her watch. "What do you notice?" she asks. "A real countdown waits one second between numbers. The waiting is part of the job."

Nova taps a small clock icon. "Would you like a hint?" she asks. "Python comes with a module called time. One of its tools pauses your program for as many seconds as you ask."

Rocket adds one new line at the top: import time.

Importing the time module

A module is a file of ready-made Python tools. The line import time loads the time module, which comes with Python. After that, time.sleep(1) pauses the program for one second.

Programmers reuse modules all the time instead of writing everything from scratch.

# countdown.py
import time

start = int(input("Countdown from: "))
for n in range(start, 0, -1):
    print(n)
    time.sleep(1)
print("Liftoff!")

With 5, it shows one number each second:

Countdown from: 5
5
4
3
2
1
Liftoff!
If the user types 10, how many seconds does the countdown take, roughly?
WHY THIS EXERCISErange(10, 0, -1) gives ten numbers, and each one is followed by a one-second sleep.

A minutes and seconds clock

Longer countdowns look better as minutes and seconds. This version uses // and % from week 2. The :02d in the f-string shows a whole number with at least two digits, adding a 0 in front if needed.

# launch_clock.py
import time

total = int(input("Countdown seconds: "))
while total > 0:
    minutes = total // 60
    seconds = total % 60
    print(f"{minutes:02d}:{seconds:02d}")
    time.sleep(1)
    total = total - 1
print("Liftoff!")
Countdown seconds: 3
00:03
00:02
00:01
Liftoff!
The user types 125. What is the first line the clock shows?
The user types 60. What is the first line the clock shows?
At your computer
1. Type countdown.py, save it in your mission folder and run it with 5.
2. Change time.sleep(1) to time.sleep(0.5). Run it and notice the speed.
3. Type launch_clock.py, save it and run it with 3.
4. Run launch_clock.py with 65 and watch it cross from 01:00 to 00:59.
5. If you want to stop early, click in the shell and press Ctrl+C.

Practice: a loading bar

A loop that grows a string makes a loading bar. Each round prints one more # than the round before, with a short pause.

# loading.py
import time

for step in range(1, 6):
    bar = "#" * step + "." * (5 - step)
    print(f"[{bar}] {step * 20}%")
    time.sleep(0.5)
print("Systems loaded")
READ THE LOADING BAR
  • Read the question.
  • Tap your answer.
What does the line for step 2 look like?
About how long does the whole bar take to fill?
What does import time let the program do?
f"{7:02d}" shows ____
95 seconds on the launch clock shows ____
In loading.py, what percent shows at step 4?
StatementTrue or false?
time.sleep(2) pauses for two seconds.?
You must import time before using time.sleep.?
time.sleep(0.5) pauses for five seconds.?
The time module has to be downloaded from the internet.?
WHY THIS EXERCISEtime is part of the standard library, and sleep takes seconds, so 0.5 is half a second.

You used your first module and built a real countdown clock. That is a big week of loops. Tomorrow is a Crew Quest.

← Thursday