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Java 2D Arrays: Grids, Matrices and Jagged Arrays
Work with tables and grids in Java: creating 2D arrays, rows and columns, nested loops, printing with Arrays.deepToString, row and column totals, transposing a matrix, and jagged arrays whose rows have different lengths.
Beginner11 min readUpdated
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Many problems involve grids: a seating plan, a game board, a timetable, a spreadsheet or a matrix. In Java, a 2D array is an array of arrays: each element of the outer array is one row.
Creating a 2D array#
import java.util.Arrays;
public class Main {
public static void main(String[] args) {
int[][] grid = {
{1, 2, 3, 4},
{5, 6, 7, 8},
{9, 10, 11, 12},
};
System.out.println(grid[0][0] + " " + grid[1][2] + " " + grid[2][3]);
System.out.println(grid.length + " rows, " + grid[0].length + " columns");
int[][] empty = new int[2][3]; // 2 rows of 3 zeros
System.out.println(Arrays.deepToString(empty));
}
}1 7 12
3 rows, 4 columns
[[0, 0, 0], [0, 0, 0]]grid[row][col]reaches one cell; both indexes start at 0.grid.lengthis the number of rows;grid[r].lengthis the length of rowr.Arrays.deepToStringprints nested arrays (Arrays.toStringwould print the rows' references).
Looping over a grid#
The outer loop walks the rows; the inner loop walks the columns of the current row:
public class Main {
public static void main(String[] args) {
int[][] grid = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
for (int r = 0; r < grid.length; r++) {
for (int c = 0; c < grid[r].length; c++) {
System.out.print(grid[r][c] + "\t");
}
System.out.println();
}
int sum = 0;
for (int[] row : grid) {
for (int value : row) {
sum += value;
}
}
System.out.println("Sum: " + sum);
}
}1 2 3
4 5 6
7 8 9
Sum: 45Row and column totals#
public class Main {
public static void main(String[] args) {
int[][] sales = {
{120, 150, 110},
{90, 95, 130},
{200, 180, 210},
};
for (int r = 0; r < sales.length; r++) {
int rowTotal = 0;
for (int v : sales[r]) rowTotal += v;
System.out.println("Shop " + (r + 1) + ": " + rowTotal);
}
for (int c = 0; c < sales[0].length; c++) {
int colTotal = 0;
for (int r = 0; r < sales.length; r++) colTotal += sales[r][c];
System.out.println("Month " + (c + 1) + ": " + colTotal);
}
}
}Shop 1: 380
Shop 2: 315
Shop 3: 590
Month 1: 410
Month 2: 425
Month 3: 450For column totals, the column loop goes on the outside.
A game board#
public class Main {
public static void main(String[] args) {
char[][] board = {
{'X', 'O', 'X'},
{' ', 'X', 'O'},
{'O', ' ', 'X'},
};
for (char[] row : board) {
StringBuilder line = new StringBuilder();
for (char cell : row) line.append('[').append(cell).append(']');
System.out.println(line);
}
boolean diagonal = board[0][0] == 'X' && board[1][1] == 'X' && board[2][2] == 'X';
System.out.println("X wins on the diagonal: " + diagonal);
}
}[X][O][X]
[ ][X][O]
[O][ ][X]
X wins on the diagonal: trueTransposing a matrix#
Transposing swaps rows and columns: [r][c] moves to [c][r]:
import java.util.Arrays;
public class Main {
public static void main(String[] args) {
int[][] m = {{1, 2, 3}, {4, 5, 6}};
int[][] t = new int[m[0].length][m.length];
for (int r = 0; r < m.length; r++) {
for (int c = 0; c < m[0].length; c++) {
t[c][r] = m[r][c];
}
}
System.out.println(Arrays.deepToString(t));
}
}[[1, 4], [2, 5], [3, 6]]Jagged arrays#
Because each row is a separate array, rows can have different lengths. Pascal's triangle is a classic example:
public class Main {
public static void main(String[] args) {
int rows = 5;
int[][] pascal = new int[rows][];
for (int r = 0; r < rows; r++) {
pascal[r] = new int[r + 1];
pascal[r][0] = 1;
pascal[r][r] = 1;
for (int c = 1; c < r; c++) {
pascal[r][c] = pascal[r - 1][c - 1] + pascal[r - 1][c];
}
}
for (int[] row : pascal) {
StringBuilder line = new StringBuilder();
for (int v : row) line.append(v).append(' ');
System.out.println(line.toString().trim());
}
}
}1
1 1
1 2 1
1 3 3 1
1 4 6 4 1new int[rows][] creates the outer array with empty (null) rows; each row is then created with its own length. Always use grid[r].length for the inner loop when rows may differ.
Common mistakes#
Practice#
Practice exercise
Check whether {{2, 7, 6}, {9, 5, 1}, {4, 3, 8}} is a magic square: every row, column and both diagonals have the same sum.
Show solution
public class MagicSquare {
public static void main(String[] args) {
int[][] m = {{2, 7, 6}, {9, 5, 1}, {4, 3, 8}};
int n = m.length;
int target = 0;
for (int v : m[0]) target += v;
boolean magic = true;
int diag1 = 0;
int diag2 = 0;
for (int i = 0; i < n; i++) {
int row = 0;
int col = 0;
for (int j = 0; j < n; j++) {
row += m[i][j];
col += m[j][i];
}
if (row != target || col != target) magic = false;
diag1 += m[i][i];
diag2 += m[i][n - 1 - i];
}
if (diag1 != target || diag2 != target) magic = false;
System.out.println("Magic square: " + magic + " (sum " + target + ")");
}
}Magic square: true (sum 15)Key takeaway#
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