Two-dimensional arrays in C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a two-dimensional array in C?

A two-dimensional array in C is like a table with rows and columns. It stores data in a grid format, where each element is accessed by two indexes: one for the row and one for the column.

beginner

How do you declare a two-dimensional array of integers with 3 rows and 4 columns in C?

You declare it like this: int arr[3][4]; This means the array has 3 rows and 4 columns.

beginner

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

You use arr[1][2]. Remember, counting starts at 0, so the 2nd row is index 1 and the 3rd column 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 declared size of a 2D array?

Accessing outside the declared size causes undefined behavior. This means the program might crash, show wrong data, or behave unpredictably. Always stay within the array bounds.

How do you declare a 2D array with 5 rows and 2 columns in C?

A int arr[5][2];

B int arr[2][5];

C int arr(5,2);

D int arr[5,2];

The correct syntax uses square brackets for each dimension: int arr[5][2];.

What is the index of the first element in a 2D array in C?

A [0][0]

B [1][1]

C [0][1]

D [1][0]

C arrays start at index 0, so the first element is at [0][0].

How do you access the element in the 3rd row and 1st column of arr?

A arr[0][1]

B arr[2][0]

C arr[1][2]

D arr[3][1]

Indexing starts at 0, so 3rd row is index 2 and 1st column is index 0.

Which of these initializes a 2x2 array with all zeros?

A int arr[2][2] = {{0, 0}, {0, 0}};

B int arr[2][2] = {1};

C int arr[2][2] = 0;

D int arr[2][2] = {0};

Using {0} initializes all elements to zero.

What is the risk of accessing arr[4][5] if arr is declared as int arr[3][4]?

A It returns zero

B No risk, it's valid

C Undefined behavior, possible crash

D It automatically resizes the array

Accessing outside the declared size causes undefined behavior and can crash the program.

Explain how to declare, initialize, and access elements in a two-dimensional array in C.

Think of a table with rows and columns.

What are the dangers of accessing elements outside the bounds of a two-dimensional array in C?

Imagine trying to open a door that doesn't exist.

Practice

1. What is a two-dimensional array in C?

easy

A. An array of arrays, storing data in rows and columns

B. A single list of numbers stored in a line

C. A function that returns multiple values

D. A variable that holds only one value

Solution

1. Step 1: Understand array structure

   A two-dimensional array stores data in a grid format with rows and columns.

2. Step 2: Compare options

   Only An array of arrays, storing data in rows and columns describes this structure as an array of arrays.

3. Final Answer:

   An array of arrays, storing data in rows and columns -> Option A

4. Quick Check:

   2D array = array of arrays [OK]

Hint: Think of a table with rows and columns [OK]

Common Mistakes:

• Confusing 2D arrays with single arrays
• Thinking 2D arrays are functions
• Assuming 2D arrays hold only one value

2. Which of the following is the correct way to declare a 3x3 integer two-dimensional array in C?

easy

A. int arr(3,3);

B. int arr[3][3];

C. int arr[3,3];

D. int arr{3}{3};

Solution

1. Step 1: Recall C array declaration syntax

   In C, arrays use square brackets with sizes inside, like arr[rows][columns].

2. Step 2: Check each option

   Only int arr[3][3]; uses correct syntax: int arr[3][3];

3. Final Answer:

   int arr[3][3]; -> Option B

4. Quick Check:

   Use square brackets for array sizes [OK]

Hint: Use square brackets for array sizes, not parentheses [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Using commas inside brackets
• Using curly braces for declaration

3. What will be the output of the following C code?

int arr[2][2] = {{1, 2}, {3, 4}};
printf("%d", arr[1][0]);

medium

A. 3

B. 1

C. 2

D. 4

Solution

1. Step 1: Understand array initialization

   The array arr has two rows: first row {1, 2}, second row {3, 4}.

2. Step 2: Access element arr[1][0]

   Row index 1 is second row, column index 0 is first element in that row, which is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   arr[1][0] = 3 [OK]

Hint: Remember indexing starts at 0 for rows and columns [OK]

Common Mistakes:

• Confusing row and column indexes
• Starting index at 1 instead of 0
• Mixing up elements in initialization

4. Identify the error in the following C code snippet:

int matrix[2][3];
matrix[2][1] = 5;

medium

A. Wrong data type for matrix

B. No error, code is correct

C. Syntax error due to missing semicolon

D. Index out of bounds error on matrix[2][1]

Solution

1. Step 1: Check array size and valid indexes

   matrix is declared with 2 rows (0 and 1) and 3 columns (0 to 2).

2. Step 2: Analyze matrix[2][1]

   Row index 2 is invalid because valid rows are 0 and 1 only, causing out of bounds error.

3. Final Answer:

   Index out of bounds error on matrix[2][1] -> Option D

4. Quick Check:

   Row index must be less than 2 [OK]

Hint: Array indexes start at 0 and go to size-1 [OK]

Common Mistakes:

• Using index equal to array size
• Ignoring zero-based indexing
• Assuming no error for out-of-range access

5. Given the 2D array declaration int grid[3][3] = {{1,2,3},{4,5,6},{7,8,9}};, which code snippet correctly calculates the sum of all elements?

hard

A. int sum=0; for(int i=1;i<3;i++) for(int j=1;j<3;j++) sum += grid[i][j]; printf("%d", sum);

B. int sum=0; for(int i=0;i<=3;i++) for(int j=0;j<=3;j++) sum += grid[i][j]; printf("%d", sum);

C. int sum=0; for(int i=0;i<3;i++) for(int j=0;j<3;j++) sum += grid[i][j]; printf("%d", sum);

D. int sum=0; for(int i=0;i<3;i++) sum += grid[i]; printf("%d", sum);

Solution

1. Step 1: Understand array size and loops

   grid has 3 rows and 3 columns, so indexes go from 0 to 2.

2. Step 2: Check loop bounds and summation

   int sum=0; for(int i=0;i<3;i++) for(int j=0;j<3;j++) sum += grid[i][j]; printf("%d", sum); uses correct loop bounds (i<3, j<3) and sums all elements correctly.

3. Step 3: Analyze other options

   int sum=0; for(int i=0;i<=3;i++) for(int j=0;j<=3;j++) sum += grid[i][j]; printf("%d", sum); uses <=3 causing out-of-bounds access; the option starting loops at 1 misses first row and column elements; the option trying sum += grid[i] is invalid for 2D arrays.

4. Final Answer:

   int sum=0; for(int i=0;i<3;i++) for(int j=0;j<3;j++) sum += grid[i][j]; printf("%d", sum); -> Option C

5. Quick Check:

   Use correct loop bounds and access grid[i][j] [OK]

Hint: Loop from 0 to size-1 for both rows and columns [OK]

Common Mistakes:

• Using <= instead of < in loops
• Starting loops at 1 instead of 0
• Trying to add entire row instead of elements