AP CSA 4.11 Debugging: Rows and Columns, Swapped

Unit 4: Data Collections · Lesson 4.11 · Debugging

Rows and Columns, Swapped

2D Array Creation and Access. This is not a blank editor: it is someone else's attempt. Find what is wrong, fix it, and submit to be graded against hidden test cases.

Back to the Lesson 4.11 page All Unit 4 lessons

Why debugging is its own skill

The first index is the row and the second is the column, always. Swap them and a rectangular grid crashes the moment it is not square, which means every test on a square grid passes and tells you nothing.

What is wrong with it

The grid is filled with grid[c][r] instead of grid[r][c], so on any non-square grid it reaches outside the array and throws. Fix the index order.
The column count is taken from grid.length, which is the number of ROWS. Use the length of a row instead.
The allocation on the line with new int[rows][cols] is correct.

The program reads

A row count, a column count, then that many integers in row-major order.

The program should print

The number of rows, the number of columns, then each row as its values separated by single spaces.

Worked examples

These show what the FIXED program should print. There are more cases you cannot see, and they use different values, so patching around just these numbers will fail.

Example 1 input

2 3
1 2 3 4 5 6

Example 1 output

Example 2 input

1 4
7 8 9 10

Example 2 output

1
4
7 8 9 10

Your answer

Main.java

import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int rows = input.nextInt();
    int cols = input.nextInt();
    int[][] grid = new int[rows][cols];

    for (int r = 0; r < rows; r++) {
      for (int c = 0; c < cols; c++) {
        grid[c][r] = input.nextInt();
      }
    }

    System.out.println(grid.length);
    System.out.println(grid.length);

    for (int r = 0; r < grid.length; r++) {
      String line = "";
      for (int c = 0; c < grid[r].length; c++) {
        if (c > 0) {
          line = line + " ";
        }
        line = line + grid[r][c];
      }
      System.out.println(line);
    }
  }
}

Input for the Run button

Run sends whatever is in the code box below, bugs and all, so you can see the crash or the wrong answer for yourself before you fix anything.

Stuck?

Hint 1

Read int[][] grid as an array of rows, where each row is itself an array of values. grid.length counts the rows. grid[0].length counts the values in the first row, which is the column count.

Hint 2

A 2 by 3 grid has no grid[2], so writing grid[c][r] with c reaching 2 throws immediately. On a square grid the same bug quietly transposes your data and never crashes, which is why square test cases are useless here.

Hint 3

The printing loop is already written the right way. Copy its index order into the filling loop.

Where to go next

Rework the Lesson 4.11 page Try the from-scratch version of this lesson

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