2D Arrays in C++ | A Detailed Guide

What You’ll Learn

By the end of this post, you’ll be able to:

• Understand what a 2D array is
• Declare, initialize, and access elements of a 2D array
• Use nested loops to work with 2D arrays
• Solve simple problems using 2D arrays like matrix input/output and element operations

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What is a 2D Array?

A 2D array is simply an array of arrays. Think of it like a table with rows and columns.

If a regular array looks like this:

[1, 2, 3]

Then a 2D array could look like this:

[
  [1, 2, 3],
  [4, 5, 6]
]

This array has 2 rows and 3 columns — we call it a 2x3 (2 by 3) array.

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Declaring a 2D Array

Syntax:

type arrayName[rows][columns];

Example:

int matrix[2][3]; // 2 rows, 3 columns

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Initializing a 2D Array

Manual Initialization:

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

You can also omit the row size if you’re providing all data:

int matrix[][3] = {
    {7, 8, 9},
    {10, 11, 12}
};

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Accessing Elements

To access elements, you need two indices:

• First index = row
• Second index = column

Example:

cout << matrix[1][2]; // Prints 6 (2nd row, 3rd column

Arrays are zero-indexed, so matrix[0][0] is the top-left corner.

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Input and Output with 2D Arrays

Use nested loops to handle row-wise or column-wise operations.

Example: Input and Output of a 2×3 matrix

int matrix[2][3];

cout << "Enter values:\n";
for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 3; j++) {
        cin >> matrix[i][j];
    }
}

cout << "You entered:\n";
for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 3; j++) {
        cout << matrix[i][j] << " ";
    }
    cout << endl;
}

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Example: Sum of All Elements

int matrix[3][3] = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

int sum = 0;
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
        sum += matrix[i][j];
    }
}

cout << "Total sum: " << sum << endl;

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Example: Transpose of a Matrix

The transpose of a matrix swaps rows with columns.

If this is the original:

1 2 3
4 5 6

The transpose will be:

1 4
2 5
3 6

Code:

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

int transpose[3][2];

for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 3; j++) {
        transpose[j][i] = original[i][j];
    }
}

// Output the transpose
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 2; j++) {
        cout << transpose[i][j] << " ";
    }
    cout << endl;
}

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Example: Diagonal Elements of a Square Matrix

int matrix[3][3] = {
    {10, 20, 30},
    {40, 50, 60},
    {70, 80, 90}
};

// Primary diagonal: matrix[i][i]
cout << "Primary Diagonal: ";
for (int i = 0; i < 3; i++) {
    cout << matrix[i][i] << " ";
}

// Secondary diagonal: matrix[i][n-i-1]
cout << "\nSecondary Diagonal: ";
for (int i = 0; i < 3; i++) {
    cout << matrix[i][2 - i] << " ";
}

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Common Mistakes to Avoid

• Confusing row and column order ([row][col])
• Accessing out-of-bounds indexes
• Forgetting to initialize all elements if required

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Mini Exercises

1. Create a 3×3 matrix and calculate the sum of each row.
2. Write a program to find the largest element in a 2D array.
3. Create a 4×4 identity matrix (1s on diagonal, 0s elsewhere).
4. Ask the user to enter a matrix and print it in reverse row order.

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Summary

In this post, you learned:

• What 2D arrays are and how they are structured
• How to declare, initialize, and access them
• How to use nested loops for input/output
• Common operations like sum, transpose, and diagonals

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What’s Next?

In the next post, we’ll explore Input/Output in C++, including standard input/output, file reading and writing, and simple graphics.