One-dimensional arrays in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of array sum calculation

What is the output of this C++ code that sums elements of an array?

C++

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

A 14

B Error: array index out of bounds

C 15

D 10

💡 Hint

Add all numbers from 1 to 5.

The loop adds all elements: 1+2+3+4+5 = 15.

❓ Predict Output

intermediate

Value of array element after modification

What is the value of arr[2] after running this code?

C++

#include <iostream>
int main() {
    int arr[4] = {10, 20, 30, 40};
    arr[2] = arr[1] + arr[3];
    std::cout << arr[2] << std::endl;
    return 0;
}

A 60

B 50

C 30

D Compilation error

💡 Hint

arr[2] is set to arr[1] + arr[3].

arr[1] is 20 and arr[3] is 40, so arr[2] becomes 60.

🔧 Debug

advanced

Identify the error in array initialization

What error does this code produce?

C++

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

A Runtime error: array index out of bounds

B No error, outputs 4

C Output: 1

D Compilation error: too many initializers for 'int [3]'

💡 Hint

Array size is 3 but 4 values are given.

The array is declared with size 3 but initialized with 4 values, causing a compilation error.

📝 Syntax

advanced

Correct syntax to declare and initialize an array

Which option correctly declares and initializes an array of 4 integers with values 5, 10, 15, 20?

A int arr[4] = {5, 10, 15, 20};

B int arr[4] = (5, 10, 15, 20);

C int arr[] = {5, 10, 15, 20};

D int arr = {5, 10, 15, 20};

💡 Hint

Use curly braces and specify size correctly.

Option A correctly declares an array of size 4 and initializes it with 4 values using curly braces.

🚀 Application

expert

Find the number of elements greater than 10

What is the output of this program that counts elements greater than 10 in an array?

C++

#include <iostream>
int main() {
    int arr[6] = {5, 12, 7, 18, 10, 20};
    int count = 0;
    for(int i = 0; i < 6; i++) {
        if(arr[i] > 10) {
            count++;
        }
    }
    std::cout << count << std::endl;
    return 0;
}

A 2

B 3

C 5

D 4

💡 Hint

Count how many numbers are strictly greater than 10.

The numbers greater than 10 are 12, 18, and 20, so count is 3.

Practice

1. What is the starting index of a one-dimensional array in C++?

easy

A. -1

B. 1

C. 0

D. Depends on the array size

Solution

1. Step 1: Understand array indexing in C++

   Arrays in C++ always start indexing from 0, meaning the first element is at index 0.

2. Step 2: Confirm the index range

   Indexes go from 0 up to size-1, so the first index is always 0 regardless of array size.

3. Final Answer:

   0 -> Option C

4. Quick Check:

   Array indexing starts at 0 [OK]

Hint: Remember: C++ arrays always start at zero index [OK]

Common Mistakes:

• Thinking arrays start at 1
• Assuming negative indexes are valid
• Confusing with other languages

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

easy

A. array<int> arr(5);

B. int arr(5);

C. int arr = new int[5];

D. int arr[5];

Solution

1. Step 1: Recall C++ array declaration syntax

   In C++, a fixed-size array is declared using square brackets with size: int arr[5];

2. Step 2: Check other options for correctness

   int arr(5); uses parentheses which is invalid for arrays. array<int> arr(5); uses incorrect syntax (not standard array). int arr = new int[5]; uses dynamic allocation syntax but is incorrect without pointer.

3. Final Answer:

   int arr[5]; -> Option D

4. Quick Check:

   Fixed-size arrays use square brackets [OK]

Hint: Use square brackets [] to declare fixed-size arrays [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Confusing with vector syntax
• Incorrect dynamic allocation syntax

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

int arr[4] = {10, 20, 30, 40};
std::cout << arr[2];

medium

A. 30

B. 20

C. 40

D. Compilation error

Solution

1. Step 1: Identify the array elements and indexes

   The array arr has elements: arr[0]=10, arr[1]=20, arr[2]=30, arr[3]=40.

2. Step 2: Access the element at index 2

   arr[2] is 30, so std::cout will print 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   arr[2] = 30 [OK]

Hint: Remember array index starts at 0, so arr[2] is third element [OK]

Common Mistakes:

• Confusing index 2 with 1 or 3
• Expecting output 20 or 40
• Thinking code causes error

4. Identify the error in the following code snippet:

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

medium

A. Missing semicolon after array declaration

B. Loop runs one time too many, causing out-of-bounds access

C. Syntax error in for loop declaration

D. Array size is incorrect, should be 4

Solution

1. Step 1: Analyze the loop range and array size

   The array has size 3 with valid indexes 0,1,2. The loop runs from i=0 to i=3 inclusive, which means 4 iterations.

2. Step 2: Identify out-of-bounds access

   When i=3, arr[3] is accessed, which is outside the array bounds and causes undefined behavior.

3. Final Answer:

   Loop runs one time too many, causing out-of-bounds access -> Option B

4. Quick Check:

   Loop index must be less than array size [OK]

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

Common Mistakes:

• Using <= instead of <
• Assuming array size is 4
• Ignoring out-of-bounds errors

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

hard

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

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

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

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

Solution

1. Step 1: Understand correct loop bounds for array traversal

   Array indexes go from 0 to n-1, so loop must run i from 0 to less than n.

2. Step 2: Check each option for correctness

   int sum = 0; for(int i = 1; i < n; i++) sum += arr[i]; starts from 1, missing arr[0]. int sum = 0; for(int i = 0; i <= n; i++) sum += arr[i]; uses <= n, causing out-of-bounds access. int sum = 0; for(int i = 0; i < n; i++) sum += arr[i]; correctly sums all elements from 0 to n-1. int sum = arr[0]; for(int i = 1; i < n; i++) sum = arr[i]; overwrites sum instead of adding.

3. Final Answer:

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

4. Quick Check:

   Loop from 0 to n-1 sums all elements [OK]

Hint: Loop from 0 to less than n to cover all elements [OK]

Common Mistakes:

• Starting loop at 1
• Using <= n causing errors
• Overwriting sum instead of adding