std::vector — Dynamic Arrays

Regular arrays have one major weakness: their size is fixed. If you create int scores[30], you’ve reserved exactly 30 slots — no more, no less.

Imagine you’re building a registration app — you don’t know how many participants there will be. Should you allocate 1000 slots? Wasteful! 10 slots? Too few!

The solution: std::vector — an array that can grow and shrink as needed!

Analogy: An Elastic Bag

A regular array is like a fixed-size suitcase — once full, nothing else fits. std::vector is like an elastic bag — you can keep adding things, and it stretches to accommodate!

Setup: Include Header

#include <vector>  // required for std::vector
#include <iostream>

Declaring Vectors

// Empty vector
std::vector<int> numbers;

// Vector with initial size (all filled with 0)
std::vector<int> scores(5);         // {0, 0, 0, 0, 0}

// Vector with size and default value
std::vector<int> allTens(5, 10);    // {10, 10, 10, 10, 10}

// Vector with direct initialization
std::vector<int> data = {3, 1, 4, 1, 5, 9};

// Vector of string
std::vector<std::string> names = {"Andi", "Budi", "Citra"};

Adding Elements: push_back

push_back() adds an element at the end of the vector:

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers;

    std::cout << "Initial size: " << numbers.size() << std::endl;

    numbers.push_back(10);
    numbers.push_back(20);
    numbers.push_back(30);

    std::cout << "Size after push_back: " << numbers.size() << std::endl;

    for (int i = 0; i < numbers.size(); i++) {
        std::cout << numbers[i] << " ";
    }
    std::cout << std::endl;

    return 0;
}

Output:

Initial size: 0
Size after push_back: 3
10 20 30

Accessing Elements

Same as regular arrays — use the index []:

#include <iostream>
#include <vector>

int main() {
    std::vector<std::string> fruits = {"Apple", "Mango", "Banana", "Durian"};

    // Access with index
    std::cout << fruits[0] << std::endl;  // Apple
    std::cout << fruits[2] << std::endl;  // Banana

    // Modify an element
    fruits[1] = "Orange";
    std::cout << fruits[1] << std::endl;  // Orange

    // front() and back()
    std::cout << "First: " << fruits.front() << std::endl;  // Apple
    std::cout << "Last : " << fruits.back() << std::endl;   // Durian

    return 0;
}

Removing Elements

#include <iostream>
#include <vector>

int main() {
    std::vector<int> v = {10, 20, 30, 40, 50};

    // pop_back — remove the last element
    v.pop_back();
    // v is now: {10, 20, 30, 40}

    std::cout << "After pop_back: ";
    for (int x : v) std::cout << x << " ";
    std::cout << std::endl;

    // erase — remove element at a specific position
    // v.begin() + 1 = position at index 1 (element 20)
    v.erase(v.begin() + 1);
    // v is now: {10, 30, 40}

    std::cout << "After erase index 1: ";
    for (int x : v) std::cout << x << " ";
    std::cout << std::endl;

    // clear — remove all elements
    v.clear();
    std::cout << "After clear, size: " << v.size() << std::endl;

    return 0;
}

Output:

After pop_back: 10 20 30 40
After erase index 1: 10 30 40
After clear, size: 0

Important Methods

#include <iostream>
#include <vector>

int main() {
    std::vector<int> v = {5, 3, 8, 1, 9, 2};

    std::cout << "size()   : " << v.size() << std::endl;    // 6
    std::cout << "empty()  : " << v.empty() << std::endl;   // 0 (false)
    std::cout << "front()  : " << v.front() << std::endl;   // 5
    std::cout << "back()   : " << v.back() << std::endl;    // 2

    // Check if empty before accessing
    if (!v.empty()) {
        std::cout << "Vector is not empty, first element: " << v.front() << std::endl;
    }

    return 0;
}

Output:

size()   : 6
empty()  : 0
front()  : 5
back()   : 2
Vector is not empty, first element: 5

Example 1: Dynamic Data Input

#include <iostream>
#include <vector>
#include <string>

int main() {
    std::vector<std::string> nameList;
    std::string input;

    std::cout << "Enter names (type 'done' to stop):" << std::endl;

    while (true) {
        std::cout << "> ";
        std::getline(std::cin, input);

        if (input == "done") break;
        if (!input.empty()) {
            nameList.push_back(input);
        }
    }

    std::cout << "\n=== Participant List ===" << std::endl;
    for (int i = 0; i < nameList.size(); i++) {
        std::cout << (i + 1) << ". " << nameList[i] << std::endl;
    }
    std::cout << "Total: " << nameList.size() << " people" << std::endl;

    return 0;
}

Example interaction:

Enter names (type 'done' to stop):
> Andi
> Budi
> Citra
> done

=== Participant List ===
1. Andi
2. Budi
3. Citra
Total: 3 people

Example 2: Score Statistics

#include <iostream>
#include <vector>

int main() {
    std::vector<int> scores;
    int n, x;

    std::cout << "How many scores? ";
    std::cin >> n;

    for (int i = 0; i < n; i++) {
        std::cout << "Score #" << (i + 1) << ": ";
        std::cin >> x;
        scores.push_back(x);
    }

    // Calculate statistics
    int total = 0;
    int maxVal = scores[0];
    int minVal = scores[0];

    for (int i = 0; i < scores.size(); i++) {
        total += scores[i];
        if (scores[i] > maxVal) maxVal = scores[i];
        if (scores[i] < minVal) minVal = scores[i];
    }

    double average = (double)total / scores.size();

    std::cout << "\n=== Statistics ===" << std::endl;
    std::cout << "Data count : " << scores.size() << std::endl;
    std::cout << "Total      : " << total << std::endl;
    std::cout << "Average    : " << average << std::endl;
    std::cout << "Highest    : " << maxVal << std::endl;
    std::cout << "Lowest     : " << minVal << std::endl;

    return 0;
}

Comparison: Array vs Vector

Range-based for loop

There’s a cleaner way to iterate over vectors:

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};

    // Range-based for loop (C++11 and above)
    for (int x : numbers) {
        std::cout << x * x << " ";  // square each element
    }
    std::cout << std::endl;

    // With strings
    std::vector<std::string> cities = {"Jakarta", "Bandung", "Surabaya"};
    for (const std::string& c : cities) {
        std::cout << "- " << c << std::endl;
    }

    return 0;
}

Output:

1 4 9 16 25
- Jakarta
- Bandung
- Surabaya

Exercises

Exercise 1: Create a program that asks the user to enter numbers until they enter 0. Store all numbers (except 0) in a vector, then display all numbers and their sum.

Exercise 2: Create a shopping list program. The user can add items (push_back), remove the last item (pop_back), or view all items. Use a simple menu system.

Exercise 3: Create a program that receives n numbers from the user, stores them in a vector, then removes all negative numbers using a loop and erase. Display the vector before and after removal.

Summary

std::vector is one of the most important data structures in C++. Master this, and you’ll be ready to tackle almost any programming problem involving data collections!