Multi-dimensional arrays in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested loop accessing 2D array

What is the output of the following C++ code?

C++

#include <iostream>
int main() {
    int arr[2][3] = {{1, 2, 3}, {4, 5, 6}};
    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < 3; j++) {
            std::cout << arr[i][j] << " ";
        }
    }
    return 0;
}

A 1 4 2 5 3 6

B 1 2 3 4 5 6

C 6 5 4 3 2 1

D 1 2 3 6 5 4

💡 Hint

Think about how the nested loops iterate rows then columns.

The outer loop goes through rows (0 and 1), and the inner loop goes through columns (0 to 2). So it prints all elements in row 0, then all in row 1.

❓ Predict Output

intermediate

Value of element after assignment in 3D array

What is the value of arr[1][0][2] after running this code?

C++

#include <iostream>
int main() {
    int arr[2][2][3] = {{{1,2,3},{4,5,6}}, {{7,8,9},{10,11,12}}};
    arr[1][0][2] = arr[0][1][1] + arr[1][1][0];
    std::cout << arr[1][0][2];
    return 0;
}

A 15

B 11

C 9

D Error

💡 Hint

Check the values at arr[0][1][1] and arr[1][1][0] and add them.

arr[0][1][1] is 5 and arr[1][1][0] is 10, so 5 + 10 = 15.

🔧 Debug

advanced

Identify the error in 2D array initialization

What error will this code produce?

C++

#include <iostream>
int main() {
    int arr[2][2] = {1, 2, 3, 4, 5};
    std::cout << arr[1][1];
    return 0;
}

A Runtime error: out of bounds access

B Outputs 5

C Outputs 4

D Compilation error: too many initializers

💡 Hint

Check how many values are allowed for a 2x2 array.

The array size is 2x2=4 but 5 values are given, causing a compilation error.

❓ Predict Output

advanced

Sum of elements in jagged 2D array simulation

What is the output of this code simulating a jagged array using pointers?

C++

#include <iostream>
int main() {
    int row1[] = {1, 2};
    int row2[] = {3, 4, 5};
    int* arr[2] = {row1, row2};
    int sizes[2] = {2, 3};
    int sum = 0;
    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < sizes[i]; j++) {
            sum += arr[i][j];
        }
    }
    std::cout << sum;
    return 0;
}

A 12

B 14

C 15

D Error

💡 Hint

Add all elements in both rows carefully.

Sum is 1+2+3+4+5 = 15.

🧠 Conceptual

expert

Memory layout of multi-dimensional arrays

In C++, how are multi-dimensional arrays stored in memory?

A Row-major order: rows stored contiguously, then columns

B Each element stored at random memory locations

C Random order depending on compiler

D Column-major order: columns stored contiguously, then rows

💡 Hint

Think about how arrays are laid out in C++ standard.

C++ stores multi-dimensional arrays in row-major order, meaning all elements of a row are stored next to each other in memory.

Practice

1. What is a multi-dimensional array in C++?

easy

A. A variable that holds a single number

B. An array that stores only one value

C. A function that returns multiple values

D. An array with more than one index, like rows and columns

Solution

1. Step 1: Understand array dimensions

   A multi-dimensional array uses multiple indexes to access elements, such as row and column in a table.

2. Step 2: Compare options

   Only An array with more than one index, like rows and columns describes an array with more than one index, matching the concept of multi-dimensional arrays.

3. Final Answer:

   An array with more than one index, like rows and columns -> Option D

4. Quick Check:

   Multi-dimensional array = multiple indexes [OK]

Hint: Think of rows and columns in a table for multi-dimensional arrays [OK]

Common Mistakes:

• Confusing arrays with single values
• Thinking functions return multiple values
• Mixing variables with arrays

2. Which of the following is the correct way to declare a 2D array of integers with 3 rows and 4 columns in C++?

easy

A. int arr[3][4];

B. int arr(3,4);

C. int arr[4][3];

D. int arr{3,4};

Solution

1. Step 1: Recall C++ 2D array syntax

   The correct syntax uses square brackets with sizes: int arr[rows][columns];

2. Step 2: Check each option

   int arr[3][4]; uses int arr[3][4]; which means 3 rows and 4 columns, correct syntax. Others are invalid or reversed.

3. Final Answer:

   int arr[3][4]; -> Option A

4. Quick Check:

   Use square brackets for dimensions [OK]

Hint: Use square brackets for rows and columns in declaration [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Swapping row and column sizes
• Using curly braces for declaration

3. What is the output of the following C++ code?

int arr[2][3] = {{1, 2, 3}, {4, 5, 6}};
std::cout << arr[1][2];

medium

A. 6

B. 3

C. 4

D. 2

Solution

1. Step 1: Understand array initialization

   The array has 2 rows and 3 columns: first row {1,2,3}, second row {4,5,6}.

2. Step 2: Access element arr[1][2]

   Row index 1 is second row, column index 2 is third element: value is 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   arr[1][2] = 6 [OK]

Hint: Remember indexes start at 0; arr[1][2] is second row, third column [OK]

Common Mistakes:

• Confusing row and column indexes
• Forgetting zero-based indexing
• Reading wrong element from array

4. Identify the error in this C++ code snippet:

int matrix[2][2] = {{1, 2}, {3}};
std::cout << matrix[1][1];

medium

A. Missing value in second row initialization

B. Array size mismatch causes compile error

C. No error, code runs and prints 0

D. Incorrect use of std::cout syntax

Solution

1. Step 1: Check array initialization

   The second row has only one value {3}, missing one element for 2 columns.

2. Step 2: Understand C++ behavior for missing elements

   Missing elements are initialized to 0 automatically, so matrix[1][1] is 0.

3. Step 3: Check output statement

   std::cout syntax is correct, so it prints 0 without error.

4. Final Answer:

   No error, code runs and prints 0 -> Option C

5. Quick Check:

   Missing elements default to 0 in arrays [OK]

Hint: Missing elements in array init become zero, no error [OK]

Common Mistakes:

• Assuming missing elements cause errors
• Thinking std::cout syntax is wrong
• Expecting garbage values instead of zero

5. How can you sum all elements of a 3x3 integer array named grid in C++?

hard

A. Use std::accumulate directly on the 2D array

B. Use nested loops to add each element to a sum variable

C. Use a single loop from 0 to 8 to sum elements

D. Add only the first row elements

Solution

1. Step 1: Understand 2D array traversal

   To access all elements in a 3x3 array, use two loops: one for rows, one for columns.

2. Step 2: Sum elements with nested loops

   Inside inner loop, add each element grid[i][j] to a sum variable to get total sum.

3. Final Answer:

   Use nested loops to add each element to a sum variable -> Option B

4. Quick Check:

   Nested loops sum all elements [OK]

Hint: Use two loops: outer for rows, inner for columns [OK]

Common Mistakes:

• Using single loop ignoring 2D structure
• Summing only part of the array
• Trying to use std::accumulate on 2D array directly