Common array operations in C++

Introduction

Arrays help us store many values in one place. Common operations let us add, find, or change these values easily.

You want to keep a list of scores in a game.

You need to find if a number exists in a list.

You want to change a value at a certain position.

You want to add a new value to the end of a list.

You want to print all values to see what is stored.

Syntax

C++

#include <iostream>
using namespace std;

class Array {
public:
    int* data;
    int size;
    int capacity;

    Array(int capacity) {
        this->capacity = capacity;
        size = 0;
        data = new int[capacity];
    }

    ~Array() {
        delete[] data;
    }

    void add(int value) {
        if (size < capacity) {
            data[size] = value;
            size++;
        } else {
            cout << "Array is full!" << endl;
        }
    }

    int find(int value) {
        for (int i = 0; i < size; i++) {
            if (data[i] == value) {
                return i;
            }
        }
        return -1; // not found
    }

    void update(int index, int newValue) {
        if (index >= 0 && index < size) {
            data[index] = newValue;
        } else {
            cout << "Index out of range!" << endl;
        }
    }

    void print() {
        for (int i = 0; i < size; i++) {
            cout << data[i] << " ";
        }
        cout << endl;
    }
};

This class creates a simple array with fixed capacity.

Operations include adding, finding, updating, and printing values.

Examples

Add two numbers and print: output will be '10 20 '

C++

Array numbers(5);
numbers.add(10);
numbers.add(20);
numbers.print();

Print an empty array: output will be empty line

C++

Array emptyArray(3);
emptyArray.print();

Update the only element from 5 to 15 and print

C++

Array singleElement(2);
singleElement.add(5);
singleElement.update(0, 15);
singleElement.print();

Try to add 4 elements to capacity 3 array; last add prints 'Array is full!'

C++

Array numbers(3);
numbers.add(1);
numbers.add(2);
numbers.add(3);
numbers.add(4);

Sample Program

This program creates an array with capacity 5. It adds three numbers, finds one, updates one, tries to add more than capacity, and prints the array at each step.

C++

#include <iostream>
using namespace std;

class Array {
public:
    int* data;
    int size;
    int capacity;

    Array(int capacity) {
        this->capacity = capacity;
        size = 0;
        data = new int[capacity];
    }

    ~Array() {
        delete[] data;
    }

    void add(int value) {
        if (size < capacity) {
            data[size] = value;
            size++;
        } else {
            cout << "Array is full!" << endl;
        }
    }

    int find(int value) {
        for (int i = 0; i < size; i++) {
            if (data[i] == value) {
                return i;
            }
        }
        return -1;
    }

    void update(int index, int newValue) {
        if (index >= 0 && index < size) {
            data[index] = newValue;
        } else {
            cout << "Index out of range!" << endl;
        }
    }

    void print() {
        for (int i = 0; i < size; i++) {
            cout << data[i] << " ";
        }
        cout << endl;
    }
};

int main() {
    Array numbers(5);
    cout << "Initial array: ";
    numbers.print();

    numbers.add(10);
    numbers.add(20);
    numbers.add(30);
    cout << "After adding 3 elements: ";
    numbers.print();

    int index = numbers.find(20);
    cout << "Index of 20: " << index << endl;

    numbers.update(1, 25);
    cout << "After updating index 1 to 25: ";
    numbers.print();

    numbers.add(40);
    numbers.add(50);
    numbers.add(60); // should print full message

    cout << "Final array: ";
    numbers.print();

    return 0;
}

Important Notes

Adding an element is O(1) if space is available.

Finding an element is O(n) because it checks each item.

Updating an element is O(1) if index is valid.

Common mistake: adding beyond capacity without resizing causes errors.

Use dynamic arrays or vectors if you need flexible size.

Summary

Arrays store multiple values in order.

Common operations: add, find, update, print.

Always check array size before adding new elements.

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