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AP CSAJava2D ArraysAlgorithmsJune 3, 2026

AP CSA 2D Array Grid Tracing

A detailed AP CSA guide to rows, columns, nested loops, neighbor checks, and grid mutation in 2D array problems.

2D array questions test whether students can separate row logic from column logic. The syntax is not the hard part. The hard part is knowing what each nested loop visits.

Scenario: classroom seating grid

A grid stores whether each seat is occupied. The method counts occupied seats in each row.

public static int[] rowCounts(boolean[][] seats)
{
    int[] counts = new int[seats.length];

    for (int r = 0; r < seats.length; r++)
    {
        for (int c = 0; c < seats[r].length; c++)
        {
            if (seats[r][c])
            {
                counts[r]++;
            }
        }
    }

    return counts;
}

Why seats[r].length matters

Some 2D arrays can be ragged, meaning rows have different lengths. Using seats[0].length for every row can fail if another row is shorter.

Neighbor checks

For grid problems, students often need to check up, down, left, and right. Every neighbor check needs a boundary test before indexing.

if (r + 1 < grid.length && grid[r + 1][c] == target)
{
    count++;
}

Practice prompt

Write a method that counts how many cells in a grid are greater than their left neighbor. Decide what should happen in column 0 before writing code.