Arrays and Vectors in C++: Description & Usage With Examples

1Why use arrays and vectors?

When you need to store multiple values of the same type, like a list of test scores, student names, or temperatures, you need a way to group them together. That is exactly what arrays and vectors do.

Without an array:

int m1, m2, m3, m4, m5;

With an array:

int marks[5];

Arrays group values together, making your code cleaner, easier to manage, and more scalable. Vectors take this a step further by allowing dynamic resizing.

2Arrays in C++

An array is a fixed-size collection of elements of the same data type. The size must be known at compile time.

Declaration

int numbers[5]; // creates space for 5 integers

Initialization

int numbers[5] = {10, 20, 30, 40, 50};

You can also omit the size if you are initializing:

int numbers[] = {10, 20, 30}; // compiler counts 3 elements

Accessing elements

Array indexing starts at 0. The first element is at index 0, the second at index 1, and so on.

cout << numbers[0]; // prints 10
numbers[2] = 99;    // changes the 3rd element to 99

3Looping through arrays

Use a for loop to access each element in an array. The loop counter serves as the index.

int scores[5] = {88, 76, 92, 85, 69};

for (int i = 0; i < 5; i++) {
    cout << "Score " << i << ": " << scores[i] << endl;
}

4Inputting into an array

You can read values from the user directly into an array using cin inside a loop.

int marks[5];

cout << "Enter marks of 5 students: ";
for (int i = 0; i < 5; i++) {
    cin >> marks[i];
}

5Common mistakes with arrays

6When to use vectors

Arrays are fixed in size. If you need a collection that can grow or shrink, use a vector. Vectors are part of the C++ Standard Template Library (STL).

To use vectors, include:

#include <vector>
using namespace std;

Declaring vectors

vector<int> numbers; // empty vector of integers

Initialization

vector<int> scores = {85, 90, 78};

Add elements

scores.push_back(95); // adds 95 to the end

Access elements

cout << scores[0]; // prints 85
scores[1] = 100;   // changes the second element to 100

7Looping through vectors

You can loop through a vector using a traditional for loop with size(), or use a range-based for loop.

Traditional loop

vector<int> values = {10, 20, 30, 40};

for (int i = 0; i < values.size(); i++) {
    cout << values[i] << endl;
}

Range-based for loop (C++11 and later)

for (int val : values) {
    cout << val << endl;
}

8Vector functions you should know

9Example: Average of N numbers using a vector

This example demonstrates reading N values from the user, storing them in a vector, and calculating the average.

#include <iostream>
#include <vector>
using namespace std;

int main() {
    int n;
    cout << "Enter number of elements: ";
    cin >> n;

    vector<int> nums;
    int input;
    int sum = 0;

    for (int i = 0; i < n; i++) {
        cin >> input;
        nums.push_back(input);
        sum += input;
    }

    double average = (double) sum / nums.size();
    cout << "Average: " << average << endl;

    return 0;
}

10Mini exercises

11Quiz yourself