AP CSA 2.1 FRQ Practice: Step Counter

Unit 2: Selection and Iteration · Lesson 2.1 · FRQ Practice

Step Counter

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 2.1 pageAll Unit 2 lessons

Why this question is worth four points

An algorithm that repeats and an algorithm that chooses are the two halves of this unit, and the exam almost always asks for them together: loop over a range, decide something about each value, and report a total. Getting the loop right and the condition backwards still scores zero, which is why each part here isolates one of them.

What you are given

Nothing is given. Write a class named Steps containing exactly the four static methods described below. Do not write a main method: a hidden driver calls your methods.

The question

  1. (a) public static int sumTo(int n) returns 1 + 2 + ... + n, and 0 when n is less than 1.
  2. (b) public static int countEvens(int n) returns how many of 1 through n are even.
  3. (c) public static String verdict(int score) returns "PASS" when score is 60 or more, otherwise "FAIL".
  4. (d) public static int firstMultiple(int start, int step) returns the smallest multiple of step that is at least start. Assume step is at least 1 and start is at least 0.

What the reader is looking for

  1. Write class Steps with the four static methods exactly as described. No main method.
  2. Part (a) must return 0 rather than crash when n is 0 or negative. A loop that never runs already does this, so you may not need a special case at all.
  3. Every method RETURNS its answer. A method that prints it and returns nothing does not compile against the driver.

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
10 60 0 7
Example 1 output
55
5
PASS
0
Example 2 input
1 59 13 5
Example 2 output
1
0
FAIL
15

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
A for loop that starts at 1 and runs while i <= n never executes when n is 0 or negative, so part (a) returns its starting total of 0 with no extra check.

    
Hint 2
Part (c) is a boundary question. 60 itself must be a PASS, so the test is >= and not >.

    
Hint 3
Part (d) already answers correctly when start is 0, because 0 is a multiple of every step. Starting the search at step instead of 0 gets that one case wrong.



  
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) starts the loop at 0 and runs while i < n, dropping the last term
    • 
      
  • part (c) uses > instead of >=, so exactly 60 fails
    • 
      
  • part (d) starts the search at step, so a start of 0 answers step instead of 0
    • 
      
  • part (b) counts odds instead of evens
    • 
    

  


  
Where to go next

  

    Rework the Lesson 2.1 page
    Try the from-scratch exercise for this lesson
    Try the debugging exercise for this lesson
  


  
  






  

















  
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