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.

Practice

1. What is the valid range of indexes for an array declared as int arr[10]; in C++?

easy

A. 1 to 9

B. 1 to 10

C. 0 to 10

D. 0 to 9

Solution

1. Step 1: Understand array size declaration

   The array arr is declared with size 10, meaning it holds 10 elements.

2. Step 2: Determine valid index range

   In C++, array indexing starts at 0, so valid indexes are from 0 up to size-1, which is 9.

3. Final Answer:

   0 to 9 -> Option D

4. Quick Check:

   Array size 10 means indexes 0 to 9 [OK]

Hint: Array indexes start at 0 and end at size minus 1 [OK]

Common Mistakes:

• Thinking indexes start at 1
• Including size as a valid index
• Confusing size with last index

2. Which of the following is the correct way to declare an array of 5 integers in C++?

easy

A. int arr[5];

B. int arr = 5;

C. int arr(5);

D. int arr{5};

Solution

1. Step 1: Review array declaration syntax

   In C++, arrays are declared using square brackets with the size inside, like int arr[5];.

2. Step 2: Check each option

   int arr(5); uses parentheses which is invalid for arrays. int arr = 5; assigns an int, not an array. int arr{5}; uses brace initialization but without specifying size properly.

3. Final Answer:

   int arr[5]; -> Option A

4. Quick Check:

   Array declaration uses square brackets [OK]

Hint: Use square brackets [] to declare array size [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Assigning a single int instead of array
• Confusing brace initialization with size

3. What will be the output of the following code?

int arr[3] = {10, 20, 30};
std::cout << arr[3] << std::endl;

medium

A. Undefined behavior (may print garbage or crash)

B. 0

C. 30

D. Compilation error

Solution

1. Step 1: Identify array size and index used

   The array has size 3, valid indexes are 0, 1, 2. The code accesses arr[3], which is out of bounds.

2. Step 2: Understand consequences of out-of-bounds access

   Accessing outside array bounds in C++ causes undefined behavior. It may print garbage, crash, or behave unpredictably.

3. Final Answer:

   Undefined behavior (may print garbage or crash) -> Option A

4. Quick Check:

   Out-of-bounds access causes undefined behavior [OK]

Hint: Access only indexes 0 to size-1 to avoid undefined behavior [OK]

Common Mistakes:

• Assuming arr[3] returns last element
• Expecting zero instead of garbage
• Thinking compiler catches out-of-bounds

4. Identify the error in the following code snippet:

int arr[4] = {1, 2, 3, 4};
for (int i = 0; i <= 4; i++) {
    std::cout << arr[i] << " ";
}

medium

A. Array size should be 5

B. Initialization of array is incorrect

C. Loop condition should be i < 4, not i <= 4

D. No error, code runs fine

Solution

1. Step 1: Analyze loop bounds

   The loop runs from i = 0 to i = 4 inclusive, so it tries to access arr[4].

2. Step 2: Check array size and valid indexes

   Array size is 4, valid indexes are 0 to 3. Accessing arr[4] is out of bounds and causes undefined behavior.

3. Final Answer:

   Loop condition should be i < 4, not i <= 4 -> Option C

4. Quick Check:

   Loop must use i < size to avoid out-of-bounds [OK]

Hint: Use < size, not <= size, in loops over arrays [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Increasing array size unnecessarily
• Ignoring out-of-bounds access

5. You want to create a function that sums all elements of an integer array passed along with its size. Which of the following function signatures correctly allows safe access to the array elements without causing out-of-bounds errors?

hard

A. int sumArray(int arr[10]);

B. int sumArray(int arr[], int size);

C. int sumArray(int* arr);

D. int sumArray(int arr[]);

Solution

1. Step 1: Understand function parameter requirements

   To safely access array elements, the function must know the array size to avoid out-of-bounds access.

2. Step 2: Evaluate each signature

   int sumArray(int arr[], int size); passes both array and size, allowing safe iteration. int sumArray(int arr[10]); fixes size to 10, which is unsafe if actual size differs. int sumArray(int* arr); passes pointer without size, unsafe. int sumArray(int arr[]); passes array without size, unsafe.

3. Final Answer:

   int sumArray(int arr[], int size); -> Option B

4. Quick Check:

   Pass array and size to avoid out-of-bounds [OK]

Hint: Always pass array size with array to functions [OK]

Common Mistakes:

• Assuming fixed size in function parameter
• Not passing size leads to unsafe access
• Using pointer without size information