AP CSA 4.12 FRQ Practice: Row Major and Column Major

Unit 4: Data Collections · Lesson 4.12 · FRQ Practice

Row Major and Column Major

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 4.12 pageAll Unit 4 lessons

Why this question is worth four points

Row major walks each row left to right; column major walks each column top to bottom. They visit the same cells in a different ORDER, and on a non square grid swapping the loop bounds is not merely a different order but an immediate crash.

What you are given

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

The question

  1. (a) public static String rowMajor(int[][] grid) returns every value in row major order, separated by single spaces.
  2. (b) public static String columnMajor(int[][] grid) returns every value in column major order, separated by single spaces.
  3. (c) public static int rowSum(int[][] grid, int r) returns the sum of row r.
  4. (d) public static int colSum(int[][] grid, int c) returns the sum of column c.

What the reader is looking for

  1. Write class Walk with the four static methods described. No main method.
  2. Part (b) needs the column loop on the OUTSIDE and must still bound each index by its own dimension.
  3. Separate the values with single spaces and do not start with one.

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 3
1 2 3
4 5 6
1 2
Example 1 output
1 2 3 4 5 6
1 4 2 5 3 6
15
9
Example 2 input
3 2
1 2
3 4
5 6
2 0
Example 2 output
1 2 3 4 5 6
1 3 5 2 4 6
11
9

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
The outer loop decides the order. Row major has rows outside; column major has columns outside. The index bounds do not swap with them.

    
Hint 2
A row sum walks the columns and a column sum walks the rows. That is easy to write backwards, and on a non square grid it throws rather than answering wrongly.

    
Hint 3
Both traversals visit every cell exactly once, so the two Strings contain the same numbers in a different order. If yours do not, one of them has a bound wrong.



  
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) is written the same as row major, so the order never changes
    • 
      
  • part (c) sums a column instead of a row
    • 
      
  • part (d) stops one row short
    • 
    

  


  
Where to go next

  

    Rework the Lesson 4.12 page
    Try the from-scratch exercise for this lesson
    Try the debugging exercise for this lesson
  


  
  






  

















  
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