What does the AP CSA 4.9 FRQ practice question ask you to do?

Removing While Looping: (a) public static int sum(ArrayList list) returns the total of every element. (b) public static int countAbove(ArrayList list, int threshold) returns how many elements are strictly greater than threshold. (c) public static void removeAllEqual(ArrayList list, int target) removes EVERY element equal to target. (d) public static int firstIndexAbove(ArrayList list, int threshold) returns the first index holding a value above threshold, or -1.

Do you write a whole program or just a method for AP CSA 4.9?

The driver builds an ArrayList of Integer and passes it to your methods. Write a class named Prune. Do not write a main method. A hidden driver calls your code, so the value each method RETURNS is what is scored and nothing you print is measured.

How is the AP CSA 4.9 FRQ practice scored?

Out of 4 points, one per rubric row: (a) public static int sum(ArrayList list) returns the total of every element. (b) public static int countAbove(ArrayList list, int threshold) returns how many elements are strictly greater than threshold. (c) public static void removeAllEqual(ArrayList list, int target) removes EVERY element equal to target. (d) public static int firstIndexAbove(ArrayList list, int threshold) returns the first index holding a value above threshold, or -1. Your answer runs against 5 test cases, 3 of them hidden, and the fraction you pass becomes the score. That is not how a human AP reader marks a rubric, so treat the rows above as the thing being practised.

What is a common mistake on AP CSA 4.9?

part (c) walks forward and skips the element that slides into the gap. It is one of 3 error(s) this question is built to catch.

AP CSA 4.9 FRQ Practice: Removing While Looping

Unit 4: Data Collections · Lesson 4.9 · FRQ Practice

Removing While Looping

A free response question in the shape the exam uses: a stated contract, four rubric parts, and no main method handed to you. Worth 4 points.

Back to the Lesson 4.9 page All Unit 4 lessons

Why this question is worth four points

Removing from a list while walking it forward skips the element that slides into the gap. It is not a crash and not an exception; it is a quietly wrong answer that only shows when two removable elements sit next to each other. Every hidden case here has exactly that.

What you are given

The driver builds an ArrayList of Integer and passes it to your methods. Write a class named Prune. Do not write a main method.

The question

(a) public static int sum(ArrayList list) returns the total of every element.
(b) public static int countAbove(ArrayList list, int threshold) returns how many elements are strictly greater than threshold.
(c) public static void removeAllEqual(ArrayList list, int target) removes EVERY element equal to target.
(d) public static int firstIndexAbove(ArrayList list, int threshold) returns the first index holding a value above threshold, or -1.

What the reader is looking for

Write class Prune with the four static methods described. No main method.
Part (c) must remove every match, including two that are adjacent. Walking backwards, or not advancing after a removal, both work.
Part (d) stops at the first match.

Worked examples

These show what a correct answer prints. There are more cases you cannot see, and they use different values, so an answer built around just these numbers will fail.

Example 1 input

Example 1 output

22
1
4
[1, 2, 9]

Example 2 input

Example 2 output

6
0
-1
[1, 3]

Your answer

Main.java

import java.util.ArrayList;

class Prune {
  // (a) sum(ArrayList<integer> list) -> int

  // (b) countAbove(ArrayList<integer> list, int threshold) -> int

  // (c) removeAllEqual(ArrayList<integer> list, int target)

  // (d) firstIndexAbove(ArrayList<integer> list, int threshold) -> int
}</integer></integer></integer></integer>

Input for the Run button

How this is scored: your answer runs against every test case, and the fraction it passes becomes your score out of 4. That is not how a human AP reader marks a rubric, so treat the score as a check on whether your code works, and the rubric above as the thing you are actually practising.

Stuck?

Hint 1

Walking backwards is the simplest fix: removing an element only shifts the ones AFTER it, and you have already passed those.

Hint 2

Going forwards, the fix is to not increment the index after a removal, because a new element has just slid into that position.

Hint 3

Two adjacent matches are what expose the bug. One match on its own is removed correctly by both the right and the wrong loop.

Before you submit

3 mistake(s) that lose points on this question

Each of these is a real error the grader catches. Check your answer against them before you submit, not instead of trying.

part (c) walks forward and skips the element that slides into the gap
part (b) counts values equal to the threshold as above it
part (d) returns the last index above the threshold rather than the first

Where to go next

Rework the Lesson 4.9 page Try the from-scratch exercise for this lesson Try the debugging exercise for this lesson

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