4.5 2D Array Creation and Access

Creating 2D Arrays

A 2D array is an array of arrays, often visualized as a grid of rows and columns. Declare with `int[][] arr = new int[rows][cols];` to create a fixed-size grid, or use array initializer syntax like `{{1,2},{3,4}}`. Java stores 2D arrays as arrays of array references, so rows can technically have different lengths (jagged arrays).

Accessing Elements

Access an element with arr[row][col], where the first index selects the row and the second selects the column. Indices are zero-based, so a grid with `numRows` rows and `numCols` columns has valid indices 0 through `numRows-1` and 0 through `numCols-1`. Going out of these bounds throws an ArrayIndexOutOfBoundsException.

Traversing with Nested Loops

Process every element using nested for loops: the outer loop iterates over rows (`arr.length`), and the inner loop iterates over columns (`arr[row].length`). This ordering matters — using `arr[row].length` handles jagged arrays correctly, since row lengths may vary.

Key terms

2D array

An array of arrays, commonly used to represent a grid of rows and columns.

Syntax to declare a 2D array

type[][] name = new type[rows][cols]; e.g., int[][] arr = new int[3][4];

Accessing an element

Use arr[row][col], where row and col are zero-based indices.

Jagged array

A 2D array where each row can have a different number of columns, since rows are independent arrays.

Nested for loop traversal

Outer loop runs through rows (arr.length), inner loop runs through columns (arr[row].length) to visit every element.

Try an AP-style question

int[][] m = new int[3][4];

What is m.length?

1. 7
2. 4
3. 3
4. 12