VulnTech Vectors: Lists That Grow | Learn C++

A vector is like an array, but it can grow and shrink while the program runs. In modern C++, vectors are what you'll use most of the time. Add #include <vector>:

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

int main() {
    vector<int> numbers = {10, 20, 30};
    cout << "Size: " << numbers.size() << "\n";

    numbers.push_back(40);         // add to the end
    numbers.push_back(50);
    cout << "Size now: " << numbers.size() << "\n";
    cout << "First: " << numbers[0] << ", last: " << numbers.back() << "\n";

    numbers.pop_back();            // remove the last item
    cout << "After pop_back the last is " << numbers.back() << "\n";
    return 0;
}

Output

Size: 3
Size now: 5
First: 10, last: 50
After pop_back the last is 40

The type in the angle brackets says what the vector holds: vector<int>, vector<double>, vector<string>...

Useful vector tools

Code	What it does
`v.push_back(x)`	add `x` to the end
`v.pop_back()`	remove the last item
`v.size()`	how many items
`v.empty()`	`true` if there are no items
`v[i]`	the item at index `i`
`v.at(i)`	the same, but checks the index is valid
`v.front()` / `v.back()`	first / last item
`v.clear()`	remove everything

Building a vector from input

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

int main() {
    int n;
    cin >> n;
    vector<int> values;
    for (int i = 0; i < n; i++) {
        int x;
        cin >> x;
        values.push_back(x);
    }
    cout << "You entered " << values.size() << " numbers. The last was " << values.back() << ".\n";
    return 0;
}

Input (what the user types)

4
8 3 12 5

Output

You entered 4 numbers. The last was 5.

A vector of a certain size

vector<int> v(5, 0); makes 5 items that all start at 0. Great for counting things:

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

int main() {
    vector<int> votes(3, 0);      // 3 candidates, 0 votes each
    votes[1]++;
    votes[1]++;
    votes[2]++;
    for (int i = 0; i < 3; i++) {
        cout << "Candidate " << i << ": " << votes[i] << " votes\n";
    }
    return 0;
}

Output

Candidate 0: 0 votes
Candidate 1: 2 votes
Candidate 2: 1 votes