AP CSA 4.11 FRQ Practice: Row Then Column

Unit 4: Data Collections · Lesson 4.11 · FRQ Practice

Row Then Column

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 4.11 pageAll Unit 4 lessons

Why this question is worth four points

grid[r][c] is row first and column second, and every grid in this question is deliberately NOT square. On a 3 by 3 grid a swapped pair of indexes quietly transposes the answer and every test still passes; on a 2 by 4 grid it throws immediately. That is the entire reason these dimensions are what they are.

What you are given

The driver builds a rectangular 2D int array and passes it to your methods. Write a class named Cells. Do not write a main method.

The question

  1. (a) public static int rows(int[][] grid) returns how many rows there are.
  2. (b) public static int cols(int[][] grid) returns how many columns each row has. Assume at least one row.
  3. (c) public static int at(int[][] grid, int r, int c) returns the value at row r and column c.
  4. (d) public static int corners(int[][] grid) returns the sum of the four corner values.

What the reader is looking for

  1. Write class Cells with the four static methods described. No main method.
  2. grid.length is the number of ROWS. grid[0].length is the number of COLUMNS.
  3. A grid with one row or one column still has four corners; some of them are the same cell.

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
2 4
1 2 3 4
5 6 7 8
1 2
Example 1 output
2
4
7
18
Example 2 input
3 2
1 2
3 4
5 6
2 1
Example 2 output
3
2
6
14

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
grid.length counts the rows because a 2D array is an array OF rows. Each row is itself an array, so grid[0].length counts the columns.

    
Hint 2
Swapping r and c in part (c) throws on a non square grid rather than returning the wrong number, which is a kinder failure than it looks.

    
Hint 3
On a grid with a single row, the top and bottom corners are the same two cells, so two values get counted twice. That is what the question asks for.



  
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 (a) and part (b) are swapped, invisible on a square grid
    • 
      
  • part (c) indexes column first and row second
    • 
      
  • part (d) uses the row count as a column index
    • 
    

  


  
Where to go next

  

    Rework the Lesson 4.11 page
    Try the from-scratch exercise for this lesson
    Try the debugging exercise for this lesson
  


  
  






  

















  
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