AP CSA 4.10 Debugging: Building the Wrong List

Unit 4: Data Collections · Lesson 4.10 · Debugging

Building the Wrong List

Algorithms with ArrayLists. 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 4.10 page All Unit 4 lessons

Why debugging is its own skill

The filter builds a result list and then reports on the original, so the filtering is real, invisible, and completely wasted. The second bug is subtler: an empty result divides by its own size, and a list that filtered nothing out crashes on the average.

What is wrong with it

The evens are collected into a result list, but the printing loop and the size report both use the ORIGINAL list. Report on the result list instead.
The average divides by the result size without checking whether anything survived the filter. When no value is even, that is a division by zero. Print NONE instead when the result list is empty.
The filtering condition itself is correct.

The program reads

A count, then that many integers.

The program should print

Each even value on its own line, then how many there were, then their integer average, or NONE when there are none.

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
1 2 3 4 5

Example 1 output

Example 2 input

3
1 3 5

Example 2 output

0
NONE

Your answer

Main.java

import java.util.ArrayList;
import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int n = input.nextInt();
    ArrayList<integer> all = new ArrayList<integer>();
    for (int i = 0; i < n; i++) {
      all.add(input.nextInt());
    }

    ArrayList<integer> evens = new ArrayList<integer>();
    for (int v : all) {
      if (v % 2 == 0) {
        evens.add(v);
      }
    }

    for (int v : all) {
      System.out.println(v);
    }
    System.out.println(all.size());

    int total = 0;
    for (int v : evens) {
      total = total + v;
    }
    System.out.println(total / evens.size());
  }
}</integer></integer></integer></integer>

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

Two lists are in play and the variable names are one character apart in meaning. Read every loop header and ask which list it walks. The filter fills evens; everything after it should read evens.

Hint 2

Any time you divide by the size of a collection you just built, ask what happens when the build produced nothing. An all-odd input makes that size 0.

Hint 3

Guard the empty case before doing the arithmetic, not after. Checking afterward means the crash has already happened.

Where to go next

Rework the Lesson 4.10 page Try the from-scratch version of this lesson

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