Two-dimensional arrays in C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a 2D array of integers with 3 rows and 4 columns.

C

int arr[3][[1]];

Drag options to blanks, or click blank then click option'

A4

B3

C2

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 instead of 4 for the number of columns.

Confusing rows and columns sizes.

The second dimension size is 4, so the array has 3 rows and 4 columns.

2fill in blank

medium

Complete the code to access the element in the second row and third column of a 2D array named matrix.

C

int value = matrix[[1]][2];

Drag options to blanks, or click blank then click option'

A1

B2

C3

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 2 instead of 1 for the row index.

Confusing row and column indices.

Array indices start at 0, so the second row is index 1.

3fill in blank

hard

Fix the error in the code to correctly initialize a 2D array with 2 rows and 3 columns.

C

int arr[2][3] = {{ {1, 2, 3}, [1] }};

Drag options to blanks, or click blank then click option'

A{3, 4, 5, 6}

B{7, 8}

C{1, 2}

D{4, 5, 6}

Attempts:

3 left

💡 Hint

Common Mistakes

Using fewer or more elements than columns in a row.

Missing braces around the second row.

The second row must have exactly 3 elements, so {4, 5, 6} is correct.

4fill in blank

hard

Fill both blanks to complete the nested loop that prints all elements of a 2D array named grid with 3 rows and 2 columns.

C

for(int i = 0; i < [1]; i++) {
    for(int j = 0; j < [2]; j++) {
        printf("%d ", grid[i][j]);
    }
    printf("\n");
}

Drag options to blanks, or click blank then click option'

A3

B2

C4

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping rows and columns counts.

Using wrong loop limits causing out-of-bounds access.

The outer loop runs over rows (3), the inner loop over columns (2).

5fill in blank

hard

Fill all three blanks to create a 2D array named matrix with 2 rows and 3 columns, initialize it, and print the element at first row, second column.

C

int matrix[[1]][[2]] = {{ {1, 2, 3}, {4, 5, 6} }};
printf("%d\n", matrix[[3]][1]);

Drag options to blanks, or click blank then click option'

A2

B3

C0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong sizes for rows or columns.

Using 1 instead of 0 for the first row index.

The array has 2 rows and 3 columns. The first row index is 0, so matrix[0][1] prints the second element of the first row.

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