Two-dimensional arrays in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested loops filling 2D array

What is the output of this C program that fills a 2D array and prints it?

C

#include <stdio.h>

int main() {
    int arr[2][3];
    int val = 1;
    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < 3; j++) {
            arr[i][j] = val++;
        }
    }
    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < 3; j++) {
            printf("%d ", arr[i][j]);
        }
        printf("\n");
    }
    return 0;
}

A 1 2 3 4 5 6 \n

B 1 3 5 \n2 4 6 \n

C 0 1 2 \n3 4 5 \n

D 1 2 3 \n4 5 6 \n

💡 Hint

Look at how the nested loops assign values row by row.

The outer loop goes over rows, inner loop over columns. Values increment from 1 to 6 filling row-wise.

🧠 Conceptual

intermediate

Memory layout of 2D arrays in C

In C, how is a two-dimensional array like int arr[2][3] stored in memory?

A As a contiguous block in row-major order (rows stored one after another)

B As a contiguous block in column-major order (columns stored one after another)

C As separate blocks for each row scattered in memory

D As pointers to pointers with each row allocated separately

💡 Hint

Think about how C stores arrays by default.

C stores 2D arrays in row-major order, meaning all elements of the first row are stored first, then the second row, and so on.

🔧 Debug

advanced

Identify the error in 2D array initialization

What error will this code produce when compiled? int arr[2][3] = { {1, 2}, {3, 4, 5, 6} };

A Runtime error: array index out of bounds

B Compilation error: missing semicolon

C Compilation error: too many initializers for array

D No error, compiles and runs fine

💡 Hint

Check the number of elements in each inner initializer list.

The second inner initializer has 4 elements but the array row size is 3, causing a compilation error.

📝 Syntax

advanced

Correct syntax to declare a pointer to 2D array

Which option correctly declares a pointer to a 2D array of integers with 3 columns?

A int (*ptr)[3];

B int *ptr[3];

C int **ptr;

D int ptr(*)[3];

💡 Hint

Remember the parentheses placement for pointer to array.

int (*ptr)[3]; declares ptr as a pointer to an array of 3 integers, which matches a 2D array row.

🚀 Application

expert

Sum of diagonal elements in a square 2D array

Given a 3x3 integer array, which code snippet correctly calculates the sum of the main diagonal elements?

A

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

B

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

C

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

D

int sum = 0;
for (int i = 0; i < 3; i++) {
    sum += arr[i][i + 1];
}

💡 Hint

Main diagonal elements have equal row and column indices.

The main diagonal elements are arr[0][0], arr[1][1], arr[2][2]. Option B sums these correctly.

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