AP CSA 4.10 FRQ Practice: Build a New List

Unit 4: Data Collections · Lesson 4.10 · FRQ Practice

Build a New List

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 4.10 pageAll Unit 4 lessons

Why this question is worth four points

Filtering into a NEW list is safer than editing in place and is what most free response questions ask for. The trap is returning the list you were given, or building the new one but never returning it, both of which look finished on the page.

What you are given

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

The question

  1. (a) public static ArrayList above(ArrayList list, int threshold) returns a NEW list of the elements strictly greater than threshold, in their original order.
  2. (b) public static ArrayList doubled(ArrayList list) returns a NEW list with every element multiplied by 2, leaving the original alone.
  3. (c) public static ArrayList reversed(ArrayList list) returns a NEW list in the opposite order.
  4. (d) public static int countDistinctValues(ArrayList list) returns how many different values appear.

What the reader is looking for

  1. Write class Build with the four static methods described. No main method.
  2. The original list must be unchanged after every one of these calls. The driver prints it again at the end to check.
  3. Part (d) counts VALUES, so a list of 4, 4, 7 has two distinct values.

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
5
3 8 3 1 9
4
Example 1 output
[8, 9]
[6, 16, 6, 2, 18]
[9, 1, 3, 8, 3]
4
[3, 8, 3, 1, 9]
Example 2 input
4
1 2 3 4
2
Example 2 output
[3, 4]
[2, 4, 6, 8]
[4, 3, 2, 1]
4
[1, 2, 3, 4]

Your answer

Main.java

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
Start each method with a brand new ArrayList and add into that. Returning the parameter hands back the caller's own list.

    
Hint 2
The driver prints the ORIGINAL list last. If any of your methods edited it in place, that final line is wrong even when the earlier ones looked right.

    
Hint 3
contains asks whether a value is already in a list, which is all part (d) needs.



  
Before you submit

  
4 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 (b) doubles the original list in place and returns it
    • 
      
  • part (a) keeps the values at or below the threshold instead
    • 
      
  • part (d) counts every element rather than the distinct values
    • 
      
  • part (c) copies in the original order
    • 
    

  


  
Where to go next

  

    Rework the Lesson 4.10 page
    Try the from-scratch exercise for this lesson
    Try the debugging exercise for this lesson
  


  
  






  

















  
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