Array size and bounds in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing array within bounds

What is the output of this C program?

C

#include <stdio.h>

int main() {
    int arr[5] = {10, 20, 30, 40, 50};
    printf("%d\n", arr[3]);
    return 0;
}

A 40

B Runtime error

C 30

D 50

💡 Hint

Remember array indexing starts at 0.

The array has elements indexed 0 to 4. arr[3] is the fourth element, which is 40.

❓ Predict Output

intermediate

Output when accessing array out of bounds

What happens when this C code runs?

C

#include <stdio.h>

int main() {
    int arr[3] = {1, 2, 3};
    printf("%d\n", arr[5]);
    return 0;
}

A Undefined behavior (may print garbage or crash)

B Compile-time error

C 0

D 3

💡 Hint

Accessing outside array size is not safe in C.

Accessing arr[5] is beyond the declared size of 3, causing undefined behavior. It may print garbage or crash.

🧠 Conceptual

advanced

Size of array in bytes

Given this code, what is the output of the program?

C

#include <stdio.h>

int main() {
    int arr[10];
    printf("%zu\n", sizeof(arr));
    return 0;
}

A 10

B 40

C 4

D Depends on compiler, but usually 10

💡 Hint

sizeof returns size in bytes. int is usually 4 bytes.

The array has 10 integers. Each int is 4 bytes on most systems. So total size is 10 * 4 = 40 bytes.

📝 Syntax

advanced

Which array declaration is invalid?

Which of these array declarations will cause a compilation error in C?

A int arr[5];

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

C int arr[1];

D int arr[3] = {1, 2, 3, 4};

💡 Hint

Array size must match initializer elements or be omitted.

Declaring int arr[3] with 4 initializers is invalid and causes a compile error.

🔧 Debug

expert

Why does this array access cause a crash?

Consider this code snippet. Why does it crash when run?

C

#include <stdio.h>

int main() {
    int arr[2] = {5, 10};
    for (int i = 0; i <= 2; i++) {
        printf("%d\n", arr[i]);
    }
    return 0;
}

A Because the loop condition should be i < 3 instead of i <= 2

B Because arr is not initialized properly

C Because the loop accesses arr[2], which is out of bounds causing undefined behavior

D Because printf format specifier is incorrect

💡 Hint

Check the loop condition and array size carefully.

The array has size 2, valid indices are 0 and 1. Accessing arr[2] is out of bounds and causes a crash.