AP CSA 2.9 FRQ Practice: Running Statistics

Unit 2: Selection and Iteration · Lesson 2.9 · FRQ Practice

Running Statistics

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 2.9 pageAll Unit 2 lessons

Why this question is worth four points

This is the shape of a real free response question: loop over values, keep several running answers at once, and report them. The maximum here is seeded with the first value rather than with zero, because the data can be entirely negative and a zero seed would report a maximum that never appeared in the input.

What you are given

The driver reads a count and then that many integers, and passes them to your methods one call at a time is NOT how this works: each method receives the count and reads nothing. Instead each method is given the values it needs as parameters. Write a class named Stats containing exactly the four static methods described below. Do not write a main method.

The question

  1. (a) public static int betterMax(int running, int next) returns whichever of the two is larger.
  2. (b) public static int rangeSum(int low, int high, int skip) returns the sum of low through high inclusive, leaving out any value that divides evenly by skip. Assume skip is at least 2.
  3. (c) public static int countAbove(int low, int high, int threshold) returns how many integers from low to high inclusive are strictly greater than threshold.
  4. (d) public static int firstAbove(int low, int high, int threshold) returns the first integer from low upward that is strictly greater than threshold, or high + 1 if there is none.

What the reader is looking for

  1. Write class Stats with the four static methods exactly as described. No main method.
  2. Part (d) must stop at the FIRST match. A loop that keeps going and overwrites its answer returns the last match instead.
  3. "Strictly greater" excludes the threshold itself in both part (c) and part (d).

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
1 10 3 5
Example 1 output
10
37
5
6
Example 2 input
1 6 2 6
Example 2 output
6
9
0
7

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
Part (a) is where a running maximum comes from. Returning next when it is strictly greater keeps the EARLIER value on a tie, which is usually what a question means by "the first largest".

    
Hint 2
Part (d) returns from inside the loop. That is what makes it stop at the first match rather than the last.

    
Hint 3
Part (b) skips a value when it divides evenly, so the condition to KEEP a value is i % skip != 0.



  
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) keeps looping and returns the LAST match instead of the first
    • 
      
  • part (c) counts values equal to the threshold as above it
    • 
      
  • part (b) keeps the multiples and skips everything else
    • 
      
  • part (a) keeps the smaller value instead of the larger
    • 
    

  


  
Where to go next

  

    Rework the Lesson 2.9 page
    Try the from-scratch exercise for this lesson
    Try the debugging exercise for this lesson
  


  
  






  

















  
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