AP CSA 4.17 FRQ Practice: Recursion That Returns

Unit 4: Data Collections · Lesson 4.17 · FRQ Practice

Recursion That Returns

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 4.17 pageAll Unit 4 lessons

Why this question is worth four points

The last topic of the course closes the thread opened in 1.10: a call whose result is discarded does nothing. In recursion that mistake is deadly, because the answer is built entirely out of returned values. Writing helper(...) on a line by itself instead of return helper(...) compiles, terminates, and answers wrongly every time.

What you are given

The driver builds a SORTED int array and passes it to your methods. Write a class named RecSearch. Do not write a main method.

The question

  1. (a) public static int find(int[] data, int target, int low, int high) searches recursively between low and high inclusive and returns an index holding target, or -1.
  2. (b) public static int find(int[] data, int target) starts that search across the whole array.
  3. (c) public static int maxFrom(int[] data, int i) returns the largest value at or after index i, recursively. Assume i is a valid index.
  4. (d) public static int countFrom(int[] data, int target, int i) returns how many elements at or after i equal target, recursively.

What the reader is looking for

  1. Write class RecSearch with the four methods described. No main method.
  2. Every recursive call's result must be RETURNED or combined into the answer. Calling and discarding compiles and answers wrongly.
  3. Part (a) returns -1 when low passes high, which is what makes the search end when the target is absent.

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
7
1 3 5 7 9 11 13
11
Example 1 output
true
true
13
1
Example 2 input
6
2 4 4 4 8 10
4
Example 2 output
true
true
10
3

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
Two methods with the same name and different parameter lists are overloads, and the short one exists only to supply the starting bounds to the long one.

    
Hint 2
Part (c) and part (d) both compute the answer for the REST of the array first, then fold the current element into it. That returned value is the whole answer.

    
Hint 3
The driver prints whether the search succeeded rather than the index, because on repeated values a correct recursive search may land on any of them.



  
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 (d) discards the recursive result, the thread opened back in 1.10
    • 
      
  • part (c) discards the recursive result and returns the current element
    • 
      
  • part (a) recurses into the wrong half
    • 
      
  • part (b) searches only the first half of the array
    • 
    

  


  
Where to go next

  

    Rework the Lesson 4.17 page
    Try the from-scratch exercise for this lesson
    Try the debugging exercise for this lesson
  


  
  






  

















  
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