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++ code snippet?

C++

#include <iostream>
int main() {
    int arr[5] = {10, 20, 30, 40, 50};
    std::cout << arr[3] << std::endl;
    return 0;
}

A 50

B 40

C 30

D Compilation error

💡 Hint

Remember that 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 <iostream>
int main() {
    int arr[3] = {1, 2, 3};
    std::cout << arr[5] << std::endl;
    return 0;
}

A Compilation error

B 3

C Undefined behavior, may print garbage value

D Runtime error: out of bounds exception

💡 Hint

C++ does not check array bounds at runtime.

Accessing arr[5] is out of bounds and causes undefined behavior. It may print garbage or crash.

❓ Predict Output

advanced

Size of array using sizeof operator

What is the output of this C++ program?

C++

#include <iostream>
int main() {
    int arr[10];
    std::cout << sizeof(arr) / sizeof(arr[0]) << std::endl;
    return 0;
}

A 10

B 40

C 4

D Compilation error

💡 Hint

sizeof(arr) gives total bytes, sizeof(arr[0]) gives bytes per element.

The array has 10 elements. Dividing total size by element size gives 10.

❓ Predict Output

advanced

Behavior of pointer arithmetic on arrays

What does this C++ code print?

C++

#include <iostream>
int main() {
    int arr[4] = {5, 10, 15, 20};
    int* p = arr;
    std::cout << *(p + 2) << std::endl;
    return 0;
}

A 5

B 10

C Compilation error

D 15

💡 Hint

Pointer arithmetic moves by element size.

p points to arr[0]. Adding 2 moves to arr[2], which is 15.

🧠 Conceptual

expert

Why does accessing out-of-bounds array cause undefined behavior?

Which statement best explains why accessing an array out of its declared bounds in C++ leads to undefined behavior?

A Because C++ does not perform automatic bounds checking, so accessing invalid memory can cause unpredictable results.

B Because the compiler automatically resizes the array to fit the access.

C Because the operating system blocks any out-of-bounds access and throws an exception.

D Because arrays in C++ are dynamically sized and always safe to access.

💡 Hint

Think about what happens when you access memory not allocated for the array.

C++ arrays do not check bounds at runtime, so accessing outside the array can read or write invalid memory, causing undefined behavior.