AP CSA 4.6 FRQ Practice: Parsing Lines

Unit 4: Data Collections · Lesson 4.6 · FRQ Practice

Parsing Lines

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 4.6 pageAll Unit 4 lessons

Why this question is worth four points

Reading a file and reading standard input are the same problem once the data is in hand: text arrives as Strings and has to become numbers before it can be added up. "12" + "3" is "123", and that is the bug this topic exists to prevent.

What you are given

The driver reads whitespace separated tokens and passes them to your methods as a String array. Write a class named Lines. Do not write a main method.

The question

  1. (a) public static int count(String[] tokens) returns how many tokens there are.
  2. (b) public static int sum(String[] tokens) returns the sum of the tokens read as integers.
  3. (c) public static String join(String[] tokens) returns the tokens separated by commas, with no comma at the start or end.
  4. (d) public static int longest(String[] tokens) returns the length of the longest token, and 0 when there are none.

What the reader is looking for

  1. Write class Lines with the four static methods described. No main method.
  2. Every token is a valid integer, so Integer.parseInt will not throw.
  3. Part (b) must ADD the numbers. Concatenating the Strings and parsing at the end is a different, much larger number.

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
3
12 3 40
Example 1 output
3
55
12,3,40
2
Example 2 input
3
1000 5 7
Example 2 output
3
1012
1000,5,7
4

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
Integer.parseInt turns one token into a number. Parse each token as you go and add the numbers, rather than gluing the text together first.

    
Hint 2
Part (d) can seed at 0 safely, because a length is never negative. That is the one place in this unit where a zero seed is correct.

    
Hint 3
The comma in part (c) goes between tokens. Adding it after every token leaves one on the end, which the driver compares.



  
Before you submit

  
3 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 (b) concatenates the text instead of adding the numbers
    • 
      
  • part (c) puts a comma before every token, including the first
    • 
      
  • part (d) returns the length of the LAST token rather than the longest
    • 
    

  


  
Where to go next

  

    Rework the Lesson 4.6 page
    Try the from-scratch exercise for this lesson
    Try the debugging exercise for this lesson
  


  
  






  

















  
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