Multi-dimensional arrays in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

A multi-dimensional array is an array of arrays. It allows storing data in a table-like structure with rows and columns, such as a 2D array.

beginner

How do you declare a 2D array with 3 rows and 4 columns in C++?

You declare it like this: int arr[3][4]; This creates 3 rows and 4 columns of integers.

beginner

How do you access the element in the 2nd row and 3rd column of a 2D array named arr?

Use arr[1][2]. Remember, indexing starts at 0, so row 2 is index 1 and column 3 is index 2.

intermediate

How can you initialize a 2D array with values at the time of declaration?

You can write: int arr[2][3] = {{1, 2, 3}, {4, 5, 6}}; This sets the first row to 1,2,3 and the second row to 4,5,6.

intermediate

What happens if you try to access an element outside the bounds of a multi-dimensional array?

Accessing outside the array bounds causes undefined behavior. This can lead to wrong data, crashes, or security issues.

How do you declare a 3D array with dimensions 2x3x4 in C++?

A int arr[2,3,4];

B int arr[3][4][2];

C int arr[2][3][4];

D int arr(2,3,4);

The correct syntax uses square brackets for each dimension: int arr[2][3][4];

What is the index of the first element in any dimension of a C++ array?

A 1

B Depends on the array

C -1

D 0

C++ arrays start indexing at 0 for every dimension.

Which of these is a valid way to access an element in a 2D array named arr?

A arr[2][3]

B arr(2,3)

C arr[2,3]

D arr{2}{3}

Use square brackets for each dimension: arr[2][3].

What does the following code do?
int arr[2][2] = {{1, 2}, {3, 4}};

A Causes a syntax error

B Creates a 2x2 array with values 1,2 in first row and 3,4 in second row

C Creates an empty 2D array

D Creates a 1D array with 4 elements

The code initializes a 2x2 array with the given values in each row.

What is the risk of accessing arr[5][5] if arr is declared as int arr[3][3];?

A Undefined behavior, possible crash or wrong data

B Compiler error

C No risk, it returns 0

D It will resize the array automatically

Accessing outside declared bounds causes undefined behavior.

Explain how multi-dimensional arrays are stored in memory in C++ and how indexing works.

Think about how a 2D table is stored as a long list in memory.

Describe how to declare, initialize, and access elements in a 2D array in C++.

Use examples with small arrays to explain.

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