Array traversal in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of array traversal with pointer arithmetic

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

C++

#include <iostream>
int main() {
    int arr[] = {10, 20, 30, 40, 50};
    int *p = arr;
    for (int i = 0; i < 5; ++i) {
        std::cout << *(p + i) << " ";
    }
    return 0;
}

A 50 40 30 20 10

B 10 20 30 40 50

C 10 20 30 40

D Compilation error

💡 Hint

Pointer arithmetic moves through the array elements in order.

The pointer p points to the first element of the array. Adding i moves the pointer to the next elements. Dereferencing *(p + i) prints each element in order.

❓ Predict Output

intermediate

Output of array traversal with range-based for loop

What will this C++ program print?

C++

#include <iostream>
int main() {
    int arr[] = {1, 2, 3, 4, 5};
    for (int x : arr) {
        std::cout << x * 2 << " ";
    }
    return 0;
}

A 0 2 4 6 8

B 1 2 3 4 5

C 2 4 6 8 10

D Compilation error

💡 Hint

The loop multiplies each element by 2 before printing.

The range-based for loop goes through each element in arr. Each element is multiplied by 2 and printed with a space.

❓ Predict Output

advanced

Output of nested array traversal

What is the output of this nested loop traversing a 2D array?

C++

#include <iostream>
int main() {
    int matrix[2][3] = {{1, 2, 3}, {4, 5, 6}};
    for (int i = 0; i < 2; ++i) {
        for (int j = 0; j < 3; ++j) {
            std::cout << matrix[i][j] << " ";
        }
    }
    return 0;
}

A 1 2 3 4 5 6

B Compilation error

C 1 4 2 5 3 6

D 1 2 3 1 2 3

💡 Hint

The outer loop goes through rows, inner loop through columns.

The code prints all elements row by row: first row elements then second row elements.

❓ Predict Output

advanced

Output of array traversal with incorrect loop condition

What happens when this code runs?

C++

#include <iostream>
int main() {
    int arr[] = {5, 10, 15};
    for (int i = 0; i <= 3; ++i) {
        std::cout << arr[i] << " ";
    }
    return 0;
}

A 5 10 15 garbage_value or runtime error

B 5 10 15 0

C Compilation error

D 5 10 15

💡 Hint

The loop goes one step beyond the array size.

The loop accesses arr[3], which is out of bounds. This causes undefined behavior: it may print garbage or cause a runtime error.

🧠 Conceptual

expert

Identifying the number of elements traversed

Given the code below, how many elements will be printed?

C++

#include <iostream>
int main() {
    int arr[4] = {7, 14, 21, 28};
    int *ptr = arr + 1;
    for (int i = 0; i < 3; ++i) {
        std::cout << *(ptr + i) << " ";
    }
    return 0;
}

A 4

B 2

C 1

D 3

💡 Hint

Pointer starts at second element, loop runs 3 times.

Pointer ptr points to arr[1]. Loop runs 3 times printing arr[1], arr[2], arr[3]. So 3 elements printed.

Practice

1. What does array traversal mean in C++ programming?

easy

A. Visiting each element of the array one by one

B. Creating a new array with the same elements

C. Sorting the array elements in order

D. Deleting elements from the array

Solution

1. Step 1: Understand the term 'array traversal'

   Array traversal means going through each element in the array in sequence.

2. Step 2: Match the definition to the options

   Visiting each element of the array one by one correctly describes visiting each element one by one.

3. Final Answer:

   Visiting each element of the array one by one -> Option A

4. Quick Check:

   Array traversal = visiting elements in order [OK]

Hint: Traversal means visiting all elements one by one [OK]

Common Mistakes:

• Confusing traversal with sorting
• Thinking traversal means deleting elements
• Mixing traversal with copying arrays

2. Which of the following is the correct syntax to traverse an array arr of size n in C++?

easy

A. for(int i = 1; i <= n; i++) { /* access arr[i] */ }

B. for(int i = n; i > 0; i--) { /* access arr[i] */ }

C. for(int i = 0; i <= n; i++) { /* access arr[i] */ }

D. for(int i = 0; i < n; i++) { /* access arr[i] */ }

Solution

1. Step 1: Identify valid index range for array traversal

   Array indices in C++ start at 0 and go up to n-1, so loop should run from 0 to less than n.

2. Step 2: Check each option's loop condition

   for(int i = 0; i < n; i++) { /* access arr[i] */ } uses i from 0 to i < n, which is correct. Others either start at 1 or go out of bounds.

3. Final Answer:

   for(int i = 0; i < n; i++) { /* access arr[i] */ } -> Option D

4. Quick Check:

   Loop from 0 to n-1 = correct traversal [OK]

Hint: Array indices start at 0, loop till less than size [OK]

Common Mistakes:

• Starting loop at 1 instead of 0
• Using <= n causing out-of-bounds access
• Decrementing loop without correct index

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

int arr[] = {2, 4, 6};
for(int i = 0; i < 3; i++) {
    std::cout << arr[i] << " ";
}

medium

A. 0 2 4 6

B. 2 4 6 0

C. 2 4 6

D. 6 4 2

Solution

1. Step 1: Understand the loop and array elements

   The array has elements 2, 4, 6 at indices 0, 1, 2. The loop runs from i=0 to i=2.

2. Step 2: Print elements in order

   Each iteration prints arr[i] followed by a space: 2, then 4, then 6.

3. Final Answer:

   2 4 6 -> Option C

4. Quick Check:

   Print elements in order = 2 4 6 [OK]

Hint: Loop prints elements in order from index 0 to 2 [OK]

Common Mistakes:

• Expecting extra zero at end
• Printing elements in reverse order
• Confusing array indices with values

4. Identify the error in this C++ code snippet for array traversal:

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

medium

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

B. Array size should be 5 instead of 4

C. Loop variable i should start from 1

D. No error, code is correct

Solution

1. Step 1: Check loop condition against array size

   The array has size 4, valid indices are 0 to 3. Loop runs while i <= 4, which includes i=4 (out of bounds).

2. Step 2: Correct the loop condition

   Change condition to i < 4 to avoid accessing arr[4] which is invalid.

3. Final Answer:

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

4. Quick Check:

   Loop must stay within 0 to size-1 [OK]

Hint: Loop index must be less than array size, not less or equal [OK]

Common Mistakes:

• Using <= instead of < causing out-of-bounds
• Starting loop at 1 instead of 0
• Increasing array size unnecessarily

5. Given an integer array arr of size n, which code snippet correctly calculates the sum of all elements using array traversal?

hard

A. int sum = arr[0];
for(int i = 1; i <= n; i++) {
sum += arr[i];
}

B. int sum = 0;
for(int i = 0; i < n; i++) {
sum += arr[i];
}

C. int sum = 0;
for(int i = 1; i < n; i++) {
sum += arr[i];
}

D. int sum = 0;
for(int i = 0; i <= n; i++) {
sum += arr[i];
}

Solution

1. Step 1: Initialize sum and loop correctly

   Sum should start at 0. Loop must run from i=0 to i < n to include all elements.

2. Step 2: Check each option for correctness

   int sum = 0; for(int i = 0; i < n; i++) { sum += arr[i]; } initializes sum to 0 and loops correctly. Others either start sum wrong or loop out of bounds.

3. Final Answer:

   int sum = 0; for(int i = 0; i < n; i++) { sum += arr[i]; } -> Option B

4. Quick Check:

   Sum all elements from index 0 to n-1 [OK]

Hint: Start sum at 0 and loop from 0 to less than n [OK]

Common Mistakes:

• Starting sum with arr[0] and looping incorrectly
• Looping with i <= n causing out-of-bounds
• Skipping first element by starting at i=1