AP CSA 2.7 FRQ Practice: Digit Machine

Unit 2: Selection and Iteration · Lesson 2.7 · FRQ Practice

Digit Machine

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 2.7 pageAll Unit 2 lessons

Why this question is worth four points

A while loop is the right tool when you do not know the count in advance, which is exactly the case for taking a number apart digit by digit. The trap is the zero case: a loop written as "while there are digits left" runs zero times for the number 0 and reports that 0 has no digits.

What you are given

Nothing is given. Write a class named Digits containing exactly the four static methods described below. Assume every argument is zero or positive. Do not write a main method.

The question

  1. (a) public static int count(int n) returns how many digits n has. The number 0 has one digit.
  2. (b) public static int sum(int n) returns the sum of the digits of n.
  3. (c) public static int lastDigit(int n) returns the final digit of n.
  4. (d) public static int reverse(int n) returns n with its digits reversed. Leading zeros simply disappear, so 100 reverses to 1.

What the reader is looking for

  1. Write class Digits with the four static methods exactly as described. No main method.
  2. Part (a) must return 1 for the number 0. A plain while (n > 0) loop returns 0, which is the bug this part is about.
  3. Every loop here must change n, or it will never end. The verifier gives up after fifteen seconds and reports a timeout, not a wrong answer.

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
4821
Example 1 output
4
15
1
1284
Example 2 input
7
Example 2 output
1
7
7
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
n % 10 gives the last digit and n / 10 removes it. Those two together are the whole technique for every part here.

    
Hint 2
Part (a) is the only one that needs a special case, because 0 has a digit but no digits greater than zero to loop over.

    
Hint 3
Part (d) builds its answer by multiplying by 10 and adding the next digit. Adding first and multiplying after gives a different, wrong number.



  
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) drops the zero special case, so count(0) returns 0
    • 
      
  • part (d) multiplies after adding instead of before
    • 
      
  • part (b) sums by dividing rather than taking the remainder
    • 
      
  • part (c) returns the first digit instead of the last
    • 
    

  


  
Where to go next

  

    Rework the Lesson 2.7 page
    Try the from-scratch exercise for this lesson
    Try the debugging exercise for this lesson
  


  
  






  

















  
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