Vectors in C++ STL

A vector is a dynamic array — unlike regular arrays, it can grow or shrink automatically as elements are added or removed. It is the most commonly used STL container and offers the performance of arrays combined with the convenience of automatic memory management.

Including and Declaring a Vector

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

int main() {
    vector<int> nums;             // empty vector of ints
    vector<string> names;         // empty vector of strings
    vector<double> prices(5);     // vector with 5 zeros
    vector<int> scores(4, 100);   // vector with 4 elements, all 100
    return 0;
}

Adding Elements

vector<int> v;
v.push_back(10);
v.push_back(20);
v.push_back(30);

Accessing Elements

cout << v[0]     << endl;   // 10 (no bounds check)
cout << v.at(1)  << endl;   // 20 (bounds checked — safer)
cout << v.front()<< endl;   // 10 (first element)
cout << v.back() << endl;   // 30 (last element)

Iterating Over a Vector

vector<int> v = {10, 20, 30, 40, 50};

// Range-based for loop
for (int x : v) {
    cout << x << " ";
}
cout << endl;

Output:

10 20 30 40 50

Common Vector Functions

Full Example — Student Scores

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

int main() {
    vector<int> scores = {78, 92, 55, 88, 70};

    // Sort
    sort(scores.begin(), scores.end());
    cout << "Sorted: ";
    for (int s : scores) cout << s << " ";
    cout << endl;

    // Min and Max
    cout << "Min: " << *min_element(scores.begin(), scores.end()) << endl;
    cout << "Max: " << *max_element(scores.begin(), scores.end()) << endl;

    // Sum
    int total = accumulate(scores.begin(), scores.end(), 0);
    cout << "Total: " << total << endl;
    cout << "Average: " << total / (double)scores.size() << endl;

    return 0;
}

Output:

Sorted: 55 70 78 88 92
Min: 55
Max: 92
Total: 383
Average: 76.6

Inserting and Erasing

vector<int> v = {1, 2, 4, 5};

v.insert(v.begin() + 2, 3);    // insert 3 at index 2
for (int x : v) cout << x << " ";
cout << endl;   // 1 2 3 4 5

v.erase(v.begin() + 1);        // remove element at index 1
for (int x : v) cout << x << " ";
cout << endl;   // 1 3 4 5

2D Vector — Vector of Vectors

vector<vector<int>> matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

for (auto &row : matrix) {
    for (int val : row) {
        cout << val << " ";
    }
    cout << endl;
}

Output:

1 2 3
4 5 6
7 8 9

Vector vs Array

Key Takeaways

• Vectors are dynamic arrays that resize automatically.
• push_back() adds elements; pop_back() removes the last one.
• Use .at() for safe access with bounds checking.
• STL algorithms like sort() and find() work directly with vectors.
• 2D vectors (vector of vectors) replace 2D arrays with more flexibility.