Topic 4.12 — 2D Array Creation and Access

Goal: create 2D arrays, understand default values, and access/update elements using grid[row][col].

The big idea

Creating a 2D array means choosing the number of rows and columns (for rectangular arrays). Access is always [row][col], and indexes start at 0.

When tracing, always identify dimensions first: grid.length (rows) and grid[0].length (cols).

// 3 rows, 4 columns
int[][] grid = new int[3][4];

You can create and fill in one line:

int[][] a = {
  {1, 2, 3},
  {4, 5, 6}
};

int x = a[1][2];     // row 1, col 2  -> 6
a[0][1] = 99;        // change row 0, col 1

Read it as: “in row 1, column 2.”

When you use new to create arrays, Java fills them with defaults.

// ✅ safe for jagged arrays
for (int r = 0; r < grid.length; r++) {
  for (int c = 0; c < grid[r].length; c++) {
    // grid[r][c]
  }
}