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.