2-Dimensional Array in Java One Page.md

2-Dimensional Array in Java — ONE PAGE

1. Definition

A 2-Dimensional Array (2-D array) is an array arranged in rows and columns, where each element is accessed using two indexes:

array[row][column]

Think of it like a table:

Column 0 1 2 ┌────┬────┬────┐ Row 0 │ 10 │ 20 │ 30 │ ├────┼────┼────┤ Row 1 │ 40 │ 50 │ 60 │ ├────┼────┼────┤ Row 2 │ 70 │ 80 │ 90 │ └────┴────┴────┘

So:

a[0][1] → 20 a[1][2] → 60 a[2][0] → 70

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2. Declaration

int[][] a;

or:

int a[][];

or:

int[] a[];

All three are valid.

The most commonly used form is:

int[][] a;

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3. Creation

int[][] a = new int[3][3];

This creates:

3 rows × 3 columns = 9 elements

Initially:

┌────┬────┬────┐ │ 0 │ 0 │ 0 │ ├────┼────┼────┤ │ 0 │ 0 │ 0 │ ├────┼────┼────┤ │ 0 │ 0 │ 0 │ └────┴────┴────┘

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4. Initialization

You can directly initialize:

int[][] a = { {10, 20, 30}, {40, 50, 60}, {70, 80, 90} };

Structure:

Row 0 → 10 20 30 Row 1 → 40 50 60 Row 2 → 70 80 90

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5. Accessing Elements

Syntax:

array[row][column]

Example:

System.out.println(a[1][2]);

Output:

60

Because:

0 1 2 ┌────┬────┬────┐ 0 │ 10 │ 20 │ 30 │ ├────┼────┼────┤ 1 │ 40 │ 50 │ 60 │ ← [1][2] ├────┼────┼────┤ 2 │ 70 │ 80 │ 90 │ └────┴────┴────┘

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6. Modifying an Element

a[1][2] = 600;

Now:

40 50 600

The original value 60 is replaced by 600.

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7. Number of Rows

For:

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

Number of rows:

a.length

Output:

3

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8. Number of Columns

For a rectangular array:

a[0].length

gives the number of columns in row 0.

For:

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

a.length → 3 a[0].length → 4

Important:

a.length ↓ number of rows

a[i].length ↓ number of columns in row i

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9. Traversing a 2-D Array

A 2-D array generally requires nested loops.

for (int i = 0; i < a.length; i++) {

for (int j = 0; j < a[i].length; j++) {

    System.out.print(a[i][j] + " ");
}

System.out.println();

}

Here:

i → row j → column

Think:

i = 0 → first row j = 0,1,2

i = 1 → second row j = 0,1,2

i = 2 → third row j = 0,1,2

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10. Complete Program

class TwoDArray {

public static void main(String[] args) {

    int[][] a = {
        {10, 20, 30},
        {40, 50, 60},
        {70, 80, 90}
    };

    for (int i = 0; i < a.length; i++) {

        for (int j = 0; j < a[i].length; j++) {

            System.out.print(a[i][j] + " ");
        }

        System.out.println();
    }
}

}

Output:

10 20 30 40 50 60 70 80 90

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11. 2-D Array vs 1-D Array

1-D Array 2-D Array

int[] a int[][] a One index Two indexes a[i] a[i][j] Single sequence Rows and columns Usually one loop Usually nested loops

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12. Most Important Points

2-D Array ↓ Rows + Columns ↓ Two indexes ↓ a[row][column]

Remember these:

Index starts from 0.

First index represents the row.

Second index represents the column.

a.length → number of rows.

a[i].length → number of columns in row i.

Use nested loops to traverse a 2-D array.

Last row index = a.length - 1.

Last column index of row i = a[i].length - 1.

Golden Rule ⭐

a[i][j]

i → Row j → Column

So whenever you see:

a[2][3]

read it as:

Row 2, Column 3.