Common array operations in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of array sum using std::accumulate

What is the output of the following C++ code?

C++

#include <iostream>
#include <numeric>
#include <array>

int main() {
    std::array<int, 5> arr = {1, 2, 3, 4, 5};
    int sum = std::accumulate(arr.begin(), arr.end(), 0);
    std::cout << sum << std::endl;
    return 0;
}

A 15

B 10

C 0

D Compilation error

💡 Hint

std::accumulate adds all elements starting from the initial value.

The std::accumulate function sums all elements in the array starting from 0, so 1+2+3+4+5 = 15.

❓ Predict Output

intermediate

Output of array element access and modification

What is the output of this C++ code snippet?

C++

#include <iostream>
#include <array>

int main() {
    std::array<int, 3> arr = {10, 20, 30};
    arr[1] = arr[0] + arr[2];
    std::cout << arr[1] << std::endl;
    return 0;
}

A 40

B Runtime error

C 30

D 20

💡 Hint

arr[1] is assigned the sum of arr[0] and arr[2].

arr[0] is 10 and arr[2] is 30, so arr[1] becomes 40. The output is 40.

❓ Predict Output

advanced

Output of array reverse using std::reverse

What is the output of this C++ program?

C++

#include <iostream>
#include <array>
#include <algorithm>

int main() {
    std::array<int, 4> arr = {1, 2, 3, 4};
    std::reverse(arr.begin(), arr.end());
    for (int x : arr) {
        std::cout << x << " ";
    }
    return 0;
}

A 1 2 3 4

B Runtime error

C Compilation error

D 4 3 2 1

💡 Hint

std::reverse reverses the order of elements in the array.

The std::reverse function reverses the array elements, so the output is '4 3 2 1 '.

❓ Predict Output

advanced

Output of array fill and print

What does this C++ code print?

C++

#include <iostream>
#include <array>

int main() {
    std::array<int, 5> arr;
    arr.fill(7);
    for (auto val : arr) {
        std::cout << val << " ";
    }
    return 0;
}

A Compilation error

B 0 0 0 0 0

C 7 7 7 7 7

D Random values

💡 Hint

The fill() method sets all elements to the given value.

arr.fill(7) sets every element in the array to 7, so the output is '7 7 7 7 7 '.

🧠 Conceptual

expert

Behavior of std::array size and data pointers

Consider the following C++ code snippet. What is the output?

C++

#include <iostream>
#include <array>

int main() {
    std::array<int, 3> arr = {5, 10, 15};
    std::cout << arr.size() << " " << *(arr.data() + 1) << std::endl;
    return 0;
}

A 3 5

B 3 10

C Compilation error

D Runtime error

💡 Hint

arr.size() returns the number of elements; arr.data() returns pointer to first element.

arr.size() returns 3 because the array has 3 elements. arr.data() points to the first element (5), so *(arr.data() + 1) is the second element (10).

Practice

1. What does the following C++ code do?
int arr[5] = {1, 2, 3, 4, 5};

easy

A. Creates an array of 5 integers with values 1 to 5

B. Creates a vector of 5 integers with values 1 to 5

C. Creates a pointer to an integer array

D. Declares an uninitialized array of size 5

Solution

1. Step 1: Understand array declaration

   The code declares an integer array named arr with size 5.

2. Step 2: Check initialization

   The array is initialized with values 1, 2, 3, 4, 5 in order.

3. Final Answer:

   Creates an array of 5 integers with values 1 to 5 -> Option A

4. Quick Check:

   Array declaration with initialization = Creates array with values [OK]

Hint: Array size and values in braces means initialized array [OK]

Common Mistakes:

• Confusing array with vector
• Thinking it's a pointer declaration
• Assuming array is uninitialized

2. Which of the following is the correct syntax to access the third element of an array named arr in C++?

easy

A. arr[3];

B. arr(3);

C. arr{3};

D. arr[2];

Solution

1. Step 1: Recall zero-based indexing

   Arrays in C++ start at index 0, so the third element is at index 2.

2. Step 2: Identify correct syntax for access

   Array elements are accessed using square brackets, so arr[2] is correct.

3. Final Answer:

   arr[2]; -> Option D

4. Quick Check:

   Third element index = 2, use square brackets [OK]

Hint: Remember arrays start at 0, so third element is index 2 [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using 1-based indexing
• Using curly braces for access

3. What is the output of this C++ code?

int arr[4] = {10, 20, 30, 40};
int sum = 0;
for(int i = 0; i < 4; i++) {
    sum += arr[i];
}
std::cout << sum;

medium

A. 100

B. 90

C. 40

D. 10

Solution

1. Step 1: Identify array elements and loop

   The array has elements 10, 20, 30, 40 and the loop runs from i=0 to 3.

2. Step 2: Calculate sum of elements

   sum = 10 + 20 + 30 + 40 = 100

3. Final Answer:

   100 -> Option A

4. Quick Check:

   Sum of all elements = 100 [OK]

Hint: Add all elements in loop to get sum [OK]

Common Mistakes:

• Stopping loop early
• Using wrong indices
• Printing sum before loop

4. Find the error in this C++ code that tries to print all elements of an array:

int arr[3] = {5, 10, 15};
for(int i = 1; i <= 3; i++) {
    std::cout << arr[i] << " ";
}

medium

A. Loop index starts at 1 instead of 0

B. Array size is incorrect

C. Using <= causes out-of-bounds access

D. Missing semicolon after cout statement

Solution

1. Step 1: Analyze loop indices

   The loop runs i from 1 to 3 inclusive, so i=3 is included.

2. Step 2: Check array bounds

   Array indices are 0,1,2. Accessing arr[3] is out-of-bounds and causes error.

3. Final Answer:

   Using <= causes out-of-bounds access -> Option C

4. Quick Check:

   Loop must use i < size, not <= [OK]

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

Common Mistakes:

• Starting loop at 1 instead of 0 (not error but unusual)
• Ignoring out-of-bounds access
• Assuming array size is wrong

5. You have an array int arr[6] = {2, 4, 6, 8, 10, 12};. How do you update the element with value 8 to 9 using a loop?

hard

A. arr[3] = 9;

B. for(int i=0; i<6; i++) { if(arr[i] == 8) arr[i] = 9; }

C. for(int i=1; i<=6; i++) { if(arr[i] == 8) arr[i] = 9; }

D. arr[4] = 9;

Solution

1. Step 1: Understand the problem

   We want to find the element with value 8 and update it to 9 using a loop.

2. Step 2: Check loop and condition

   Loop from i=0 to 5 checks each element; if arr[i] == 8, assign arr[i] = 9.

3. Final Answer:

   for(int i=0; i<6; i++) { if(arr[i] == 8) arr[i] = 9; } -> Option B

4. Quick Check:

   Loop with condition to find and update element [OK]

Hint: Loop through array, check value, then update [OK]

Common Mistakes:

• Using wrong indices in loop
• Assigning wrong array index
• Not using loop to find value