Iteration & Accumulation — Lesson 4: Build: Launch Countdown

New skill: Loops, conditions, strings and return values
Previously learned: Unit 05 — conditional execution

Why this matters

A robotics launch controller needs to report every countdown number and then announce launch.

Study this example

def count_up(stop):
    message = ''
    for n in range(1, stop + 1):
        message += str(n) + '\n'
    return message + 'Done'

Here str(n) turns a number into text. The newline \n separates lines within one returned string. The final message is added after the loop finishes.

Before moving on, predict the result of the program or function. Then run it and compare.

Check your understanding

Why is str(n) necessary before joining it to a string with +?

Common mistake

Building and returning one string is different from printing multiple separate lines.

Practice

Complete create_countdown(start) to return one string containing a countdown from start to 1, each on its own line, followed by Launch!. start is a positive integer. Example for 3: 3\n2\n1\nLaunch!.

Public example 1

Input

create_countdown(3)

Output

'3\n2\n1\nLaunch!'

Public example 2

Input

create_countdown(1)

Output

'1\nLaunch!'

Public example 3

Input

create_countdown(5)

Output

'5\n4\n3\n2\n1\nLaunch!'

Open the Python starter

Hint 1

You'll need a loop that visits numbers in reverse order.

Hint 2

Use `str()` to turn each number into text before appending.

Hint 3

Use `\n` between lines and add `Launch!` only once at the end.

Reflection

Explain why your solution works and describe one boundary case you tried.