AP CSA 4.16 Exercise 1: The Base Case First

Unit 4: Data Collections · Lesson 4.16 · Exercise 1

The Base Case First

Recursion. Write real Java, run it, and submit it to be graded against hidden test cases.

Back to the Lesson 4.16 page All Unit 4 lessons

Why this one is worth doing

Every recursive method needs a base case and a call that gets closer to it. Write the base case first and the rest usually falls out; write it last and the program runs forever.

What to write

Read one integer n, which is 0 or more, and then a word.
Write a recursive method that returns the factorial of n, and print it. The factorial of 0 is 1.
Write a recursive method that returns the sum of the digits of n, and print it.
Write a recursive method that returns the word reversed, and print it.
Write a recursive method that returns how many times the method was called to compute the factorial, counting the base case, and print it.
Every one of these must be recursive. Do not use a loop.

Your program reads

An integer of 0 or more, then a word.

Your program prints

Four lines: the factorial, the digit sum, the reversed word and the call count.

Worked examples

These are the cases you can see. There are more you cannot, and they use different values, so an answer that prints these numbers as constants will fail.

Example 1 input

5 stack

Example 1 output

120
5
kcats
5

Example 2 input

0 a

Example 2 output

Your answer

Main.java

import java.util.Scanner;

public class Main {
  public static int factorial(int n) {
    return 0;
  }

  public static int digitSum(int n) {
    return 0;
  }

  public static String reverse(String s) {
    return "";
  }

  public static int calls(int n) {
    return 0;
  }

  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int n = input.nextInt();
    String word = input.next();

    // Four printed lines.
  }
}

Input for the Run button

Run sends whatever is in the input box below. Submitting runs your program against every test case, including hidden ones with different input.

Stuck?

Hint 1

factorial stops at 1 or below and returns 1, which covers both 0 and 1 with one test.

Hint 2

Reversing a String recursively is the reverse of everything after the first character, with the first character stuck on the end.

Hint 3

The call count for factorial(5) is 5, because the base case call counts too. Trace factorial(1) and factorial(0) by hand if the number is off by one.

Where to go next

Rework the Lesson 4.16 page Pick another Unit 4 lesson

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