Recursive Function: Countdown

Part of the Logic & Flow section of Coddy's Dart journey — lesson 48 of 65.

Now it's time to put recursion into practice with a simple countdown function. This exercise will help you understand how the base case and recursive step work together in real code.

A countdown function starts with a number and prints each number down to zero. Here's how recursion makes this work: if the number is greater than zero, print it and then call the same function with a smaller number. When the number reaches zero, stop.

void countdown(int number) {
  if (number <= 0) {
    print("Done!");
    return; // Base case - stop here
  }
  
  print(number);
  countdown(number - 1); // Recursive step
}

The base case checks if number <= 0 - this is when we stop calling the function. The recursive step calls countdown(number - 1), which is the same problem but with a smaller input, gradually moving us toward the base case.

When you call countdown(3), it prints 3, then calls countdown(2), which prints 2, then calls countdown(1), which prints 1, then calls countdown(0), which prints "Done!" and stops.

Challenge

Easy

Create a program that implements a recursive countdown timer with custom messages. Your program will demonstrate recursion by counting down from a given number and displaying personalized messages at each step.

Read a string input representing the starting number for the countdown
Read a string input representing a custom message prefix (e.g., "Launch in", "Timer", or "Countdown")
Convert the first input to an integer using int.parse()
Create a recursive function called customCountdown that takes two parameters:

An integer number representing the current countdown value
A string messagePrefix for the custom message

The function should implement the following logic:

Base case: If the number is less than or equal to 0, print "Countdown complete!" and return
Recursive step: Print the message prefix followed by the current number, then call itself with number - 1

Call the customCountdown function with the converted number and message prefix
Display the results in the exact format shown below

For example, if the inputs are "5" and "Launch in", your program should output:

Starting countdown from: 5
Message prefix: Launch in
========================
Launch in 5
Launch in 4
Launch in 3
Launch in 2
Launch in 1
Countdown complete!

If the inputs are "3" and "Timer", your program should output:

Starting countdown from: 3
Message prefix: Timer
========================
Timer 3
Timer 2
Timer 1
Countdown complete!

If the inputs are "0" and "Ready", your program should output:

Starting countdown from: 0
Message prefix: Ready
========================
Countdown complete!

Your program must implement the recursive customCountdown function that calls itself with a decremented number until it reaches the base case. The function should print the custom message with the current number at each recursive call, demonstrating how recursion breaks down the problem into smaller, identical subproblems. Use string interpolation to format the countdown messages as "$messagePrefix $number".

Cheat sheet

A recursive countdown function demonstrates how recursion works by counting down from a number to zero:

void countdown(int number) {
  if (number <= 0) {
    print("Done!");
    return; // Base case - stop here
  }
  
  print(number);
  countdown(number - 1); // Recursive step
}

The base case checks if number <= 0 to stop the recursion. The recursive step calls the same function with number - 1, moving toward the base case.

When calling countdown(3), it prints 3 → 2 → 1 → "Done!" by making successive recursive calls with decremented values.

Try it yourself

import 'dart:io';

// TODO: Create your customCountdown function here
// Remember to handle the base case (number <= 0) and recursive step

void main() {
  // Read input
  String? startingNumber = stdin.readLineSync();
  String? messagePrefix = stdin.readLineSync();
  
  // Convert string to integer
  int number = int.parse(startingNumber!);
  
  // Display initial information
  print('Starting countdown from: $number');
  print('Message prefix: $messagePrefix');
  print('========================');
  
  // TODO: Call your customCountdown function here
  // Remember to handle null safety for messagePrefix
}