Mistake #1

Swapping Row and Column Bounds

This is the #1 2D array error. grid.length is the number of rows. grid[0].length is the number of columns. Swap them and your loop crashes on non-square grids.

int[][] grid = new int[3][5]; // 3 rows, 5 cols
for (int r = 0; r < grid[0].length; r++)  // 5, but only 3 rows
{
    for (int c = 0; c < grid.length; c++)  // 3, but 5 cols
    {
        System.out.print(grid[r][c] + " ");
        // crashes when r = 3
    }
}

for (int r = 0; r < grid.length; r++)       // rows
{
    for (int c = 0; c < grid[0].length; c++) // cols
    {
        System.out.print(grid[r][c] + " ");
    }
}

Mistake #2

Accessing [col][row] Instead of [row][col]

In Java 2D arrays, the first index is always the row and the second is always the column. Writing grid[c][r] reads the wrong cell.

// grid has 3 rows, 5 columns
// To access row 1, column 3:
int val = grid[1][3];  // CORRECT
int val = grid[3][1];  // WRONG — accesses row 3 (may be out of bounds)

Mistake #3

Assuming the Grid Is Square

Students assume grid.length == grid[0].length and use a single variable for both bounds. This works for square grids but crashes for rectangular ones.

int size = grid.length;
for (int r = 0; r < size; r++)
{
    for (int c = 0; c < size; c++)  // wrong if grid is 3x5
    {
        // ...
    }
}

Mistake #4

Off-by-One in Neighbor Access

Accessing grid[r-1][c] or grid[r][c+1] without checking bounds first. The edges and corners have fewer neighbors.

for (int r = 0; r < grid.length; r++)
{
    for (int c = 0; c < grid[0].length; c++)
    {
        int above = grid[r - 1][c];  // crashes when r = 0
    }
}

for (int r = 0; r < grid.length; r++)
{
    for (int c = 0; c < grid[0].length; c++)
    {
        if (r > 0)
        {
            int above = grid[r - 1][c];  // safe
        }
    }
}

Mistake #5

Using Enhanced For Loop and Losing Indices

The enhanced for loop on a 2D array gives you each row as a 1D array. You lose access to the row index, which many FRQ problems require.

// Enhanced for on 2D array:
for (int[] row : grid)
{
    for (int val : row)
    {
        // You have the value but NOT the row or column index
        // Cannot do grid[r][c] = something
    }
}

Mistake #6

Column-Major Traversal When Row-Major Is Expected

Row-major (outer loop = rows) visits all columns in row 0, then all columns in row 1, etc. Column-major (outer loop = columns) visits all rows in column 0 first. The exam specifies which order to use — using the wrong one produces different output.

for (int c = 0; c < grid[0].length; c++)
{
    for (int r = 0; r < grid.length; r++)
    {
        System.out.print(grid[r][c] + " ");
    }
}
// Visits: (0,0) (1,0) (2,0) (0,1) (1,1) ...

for (int r = 0; r < grid.length; r++)
{
    for (int c = 0; c < grid[0].length; c++)
    {
        System.out.print(grid[r][c] + " ");
    }
}
// Visits: (0,0) (0,1) (0,2) (1,0) (1,1) ...