AP CSA 4.4 Debugging: Find the Off-By-One

Unit 4: Data Collections · Lesson 4.4 · Debugging

Find the Off-By-One

Array Traversals. This is not a blank editor: it is someone else's attempt. Find what is wrong, fix it, and submit to be graded against hidden test cases.

Back to the Lesson 4.4 page All Unit 4 lessons

Why debugging is its own skill

This program is not a blank editor. It is somebody else's attempt, and it almost works: two real bugs are hiding in it, the kind that show up as a crash and the kind that shows up as a wrong answer nobody notices until a tie happens. Finding a bug in code you did not write is its own skill, distinct from writing code that has none yet.

What is wrong with it

Run this program on the sample input. It crashes with an ArrayIndexOutOfBoundsException while printing the array backward. Find the loop bound that reads one index past the end of the array and fix it.
Once it runs without crashing, it still disagrees with the expected output whenever the largest value appears more than once: it reports the LAST index that value appears at instead of the FIRST. Fix the comparison so a tie keeps the earlier index.
The forward print and the sum of the even-index elements are already correct. Do not rewrite them; the fix is smaller than it looks.

The program reads

An integer count of at least 1, then that many integers.

The program should print

Every element forwards, then backwards, then the FIRST index of the largest value, then the sum of the elements at even indexes.

Worked examples

These show what the FIXED program should print. There are more cases you cannot see, and they use different values, so patching around just these numbers will fail.

Example 1 input

5
3 9 4 9 1

Example 1 output

Example 2 input

1
42

Example 2 output

Your answer

Main.java

import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int count = input.nextInt();
    int[] data = new int[count];
    for (int i = 0; i < count; i++) {
      data[i] = input.nextInt();
    }

    for (int value : data) {
      System.out.println(value);
    }
    for (int i = data.length; i >= 0; i--) {
      System.out.println(data[i]);
    }

    int best = 0;
    for (int i = 1; i < data.length; i++) {
      if (data[i] >= data[best]) {
        best = i;
      }
    }
    System.out.println(best);

    int evenSum = 0;
    for (int i = 0; i < data.length; i = i + 2) {
      evenSum = evenSum + data[i];
    }
    System.out.println(evenSum);
  }
}

Input for the Run button

Run sends whatever is in the code box below, bugs and all, so you can see the crash or the wrong answer for yourself before you fix anything.

Stuck?

Hint 1

The crash happens on the very first line of the backward loop, before anything prints. Valid indexes run from 0 to length - 1; length itself is one past the end.

Hint 2

A tie for the largest value should keep the FIRST index it appeared at. A strict > only replaces best when a later value is bigger, so an equal later value leaves the earlier index alone. >= replaces it anyway.

Hint 3

The forward print and the even-index sum never needed a fix. If your diff touches those lines, you changed something that was not broken.

Where to go next

Rework the Lesson 4.4 page Try the from-scratch version of this lesson

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