4.11 2D Array Creation and Access

1. A 2D array is an array of arrays

A two-dimensional (2D) array stores related data in rows and columns, such as seats in a cinema or scores per player per round. Java stores it as an array of arrays: the outer array holds the rows, and each row is a 1D array 4.11.A.1. So creating and indexing a 2D array works much like a 1D array, with one extra pair of brackets 4.11.A.1.

Like a 1D array, a 2D array’s size is set when it is created and cannot change 4.11.A.1. It can store primitive data (int, double, boolean) or object references (String, or any class) 4.11.A.1.

int[][] seats = new int[2][3];        // 2 rows, 3 columns of int
String[][] board = new String[3][3];  // 3 rows, 3 columns of references
System.out.println(seats.length + " rows, " + seats[0].length + " columns");
System.out.println(board.length * board[0].length + " squares");

Output:

2 rows, 3 columns
9 squares

Picture seats like this:

	column 0	column 1	column 2
row 0	seats[0][0]	seats[0][1]	seats[0][2]
row 1	seats[1][0]	seats[1][1]	seats[1][2]

Scope note: every 2D array on the AP® exam is rectangular (every row has the same length). Arrays with rows of different lengths are outside the course 4.11.A.1.

2. Creating with new: default values

When you create a 2D array with the keyword new, every element starts at the default value for its type 4.11.A.2:

Element type	Default value
int	0
double	0.0
boolean	false
any reference type (String, a class)	null

int[][] counts = new int[2][2];
double[][] prices = new double[2][2];
boolean[][] booked = new boolean[2][2];
String[][] names = new String[2][2];
System.out.println(counts[1][1] + " " + prices[0][1] + " " + booked[1][0] + " " + names[0][0]);

Output:

0 0.0 false null

Watch out: names[0][0].length() would throw a NullPointerException, because the element holds null and not a String object.

3. Creating with an initializer list

An initializer list for a 2D array is a list of initializer lists: each inner list is one row, written like a 1D array 4.11.A.3. Java counts the rows and columns for you.

int[][] scores = { {12, 15, 9, 20},
                   {18, 11, 14, 16},
                   {10, 19, 13, 17} };
String[][] menu = { {"soup", "salad"},
                    {"pasta", "rice"} };
System.out.println(scores.length + " x " + scores[0].length);
System.out.println(menu[1][0]);

Output:

3 x 4
pasta

4. Reading and changing one element: [row][col]

Two indexes in square brackets reach one element: arr[row][col] 4.11.A.4. For the AP® exam, the first index is the row and the second index is the column 4.11.A.4. The same expression reads a value or, on the left of =, changes it.

int[][] scores = { {12, 15, 9, 20},
                   {18, 11, 14, 16},
                   {10, 19, 13, 17} };
System.out.println(scores[1][2]);   // row 1, column 2
scores[2][0] = scores[0][3] + 5;    // change row 2, column 0
scores[0][1]++;
System.out.println(scores[2][0] + " " + scores[0][1]);

Output:

14
25 16

The grid after the two changes:

	col 0	col 1	col 2	col 3
row 0	12	16	9	20
row 1	18	11	14	16
row 2	25	19	13	17

5. One row as a 1D array

Use the array name with a single pair of brackets, arr[r], to get row r as a 1D array 4.11.A.5. This is a reference to the real row, not a copy, so changes through it change the 2D array.

double[][] rain = { {2.5, 0.0, 1.0},
                    {4.0, 3.5, 0.5} };
double[] week2 = rain[1];          // the whole second row
System.out.println(week2.length);
week2[0] = 9.9;                    // changes rain[1][0] too
System.out.println(rain[1][0]);

Output:

3
9.9

Watch out: there is no single-bracket way to get a column. A column must be built with a loop over the rows.

6. Size, valid indexes and ArrayIndexOutOfBoundsException

For a 2D array named grid 4.11.A.6:

What you want	Expression	Valid index values
Number of rows	grid.length	rows 0 to grid.length - 1
Number of columns	grid[0].length (the length of any row)	columns 0 to grid[0].length - 1

Any index outside these ranges throws an ArrayIndexOutOfBoundsException when the program runs 4.11.A.6.

String[][] maze = new String[4][6];
System.out.println("rows: " + maze.length);
System.out.println("columns: " + maze[0].length);
System.out.println("last row index: " + (maze.length - 1));
maze[3][6] = "#";   // column 6 does not exist

Output:

rows: 4
columns: 6
last row index: 3
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 6 out of bounds for length 6

The message says “length 6” because the bad index was the column: each row has length 6, so the last column is 5.