AP CSA 4.9 Exercise 1: Removing While You Walk

Unit 4: Data Collections · Lesson 4.9 · Exercise 1

Removing While You Walk

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

Back to the Lesson 4.9 page All Unit 4 lessons

Why this one is worth doing

Removing an element during a forward traversal makes the loop skip the next one, which is the classic ArrayList bug and is on the exam. Doing it backwards avoids the problem entirely, and this exercise asks for both so the difference is visible in the output.

What to write

Read an integer count, then that many integers into an ArrayList of Integer.
Print the sum using an enhanced for loop.
Make a copy of the list. Walk the copy FORWARDS with an index and remove every even value, without adjusting the index after a removal. Print the result, bug and all.
Make another copy. Walk it BACKWARDS and remove every even value. Print that result.
Print true when the two results are equal and false when the forward version missed something.

Your program reads

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

Your program prints

The sum, the forward result, the backward result and whether they matched.

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

6
1 2 4 3 6 5

Example 1 output

21
[1, 4, 3, 5]
[1, 3, 5]
false

Example 2 input

4
1 3 5 7

Example 2 output

16
[1, 3, 5, 7]
[1, 3, 5, 7]
true

Your answer

Main.java

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

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

    // new ArrayList<integer>(list) makes a copy you can damage safely.
  }
}</integer></integer></integer>

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

After remove(i) everything shifts left, so the element that moved into position i never gets tested. Two evens in a row is where it shows.

Hint 2

The backward loop is immune because everything that shifts has already been visited.

Hint 3

Comparing two ArrayLists with equals compares their contents in order, which is what the last line wants.

Where to go next

Rework the Lesson 4.9 page Pick another Unit 4 lesson

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