AP CSA 4.2 FRQ Practice: Summarising a Data Set

Unit 4: Data Collections · Lesson 4.2 · FRQ Practice

Summarising a Data Set

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 4.2 pageAll Unit 4 lessons

Why this question is worth four points

The first thing anyone does with a data set is ask for its minimum, maximum and mean, and the first bug anyone writes is seeding the maximum at 0. On data that is entirely negative that reports a maximum which is not in the data at all, and no error is raised.

What you are given

The driver builds an int array and passes it to your methods. Every array has at least one element. Write a class named Summary. Do not write a main method.

The question

  1. (a) public static int max(int[] data) returns the largest value.
  2. (b) public static int min(int[] data) returns the smallest value.
  3. (c) public static int range(int[] data) returns the largest minus the smallest.
  4. (d) public static int mean(int[] data) returns the sum divided by the length, using integer division.

What the reader is looking for

  1. Write class Summary with the four static methods described. No main method.
  2. Seed the maximum and the minimum with the FIRST element, never with 0. The array may be entirely negative or entirely positive.
  3. Parts (a) and (b) should be written once each and reused by part (c).

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
4 9 1 7 3
Example 1 output
9
1
8
4
Example 2 input
4
1 2 3 5
Example 2 output
5
1
4
2

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
Seeding with data[0] and starting the loop at 1 is the standard shape. It works for any data because the seed is guaranteed to be a real element.

    
Hint 2
A maximum seeded at 0 is correct only when the data happens to contain something positive. The hidden cases here do not.

    
Hint 3
Part (c) is one line if parts (a) and (b) are right. Re-scanning the array a third time is another chance to seed one of them wrongly.



  
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 (a) seeds the maximum at 0 instead of the first element
    • 
      
  • part (b) seeds the minimum at 0 instead of the first element
    • 
      
  • part (a) starts the loop at 0 but skips the last element
    • 
      
  • part (c) subtracts the wrong way round
    • 
    

  


  
Where to go next

  

    Rework the Lesson 4.2 page
    Try the from-scratch exercise for this lesson
    Try the debugging exercise for this lesson
  


  
  






  

















  
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