IntermediatePractice in the editor

C++ Vectors

std::vector, from the vector header, is the most used container in C++. It works like an array that grows and shrinks as needed, knows its size and frees its memory automatically.

Creating and adding

ExampleTry it in the editor

#include <iostream>
#include <vector>

int main() {
    std::vector<int> scores = {78, 85, 62};
    scores.push_back(90);            // add at the end
    scores.push_back(74);
    std::cout << "size " << scores.size() << "\n";
    std::cout << "first " << scores.front() << ", last " << scores.back() << "\n";
    scores.pop_back();               // remove the last
    std::cout << "after pop_back: " << scores.size() << "\n";
    std::vector<double> zeros(4);    // four zeros
    std::vector<char> stars(3, '*'); // three stars
    std::cout << zeros.size() << " " << stars[0] << stars[1] << stars[2] << "\n";
    return 0;
}

Output

size 5
first 78, last 74
after pop_back: 4
4 ***

Reaching elements

v[i] reads element i without checking; v.at(i) checks and throws std::out_of_range when i is outside the vector. Use at while learning, or turn on Safe mode, which stops a v[i] outside the vector.

Looping

ExampleTry it in the editor

#include <iostream>
#include <vector>

int main() {
    std::vector<int> marks = {78, 85, 62, 90, 74};
    int total = 0;
    for (int m : marks) total += m;
    std::cout << "average " << static_cast<double>(total) / marks.size() << "\n";
    for (int& m : marks) m += 5;           // change every element
    for (std::size_t i = 0; i < marks.size(); i++) {
        std::cout << i << ":" << marks[i] << " ";
    }
    std::cout << "\n";
    return 0;
}

Output

average 77.8
0:83 1:90 2:67 3:95 4:79

Inserting and erasing

insert and erase take a position given as an iterator: v.begin() + i is the position of element i (see Iterators).

ExampleTry it in the editor

#include <iostream>
#include <vector>

int main() {
    std::vector<int> v = {10, 20, 30, 40};
    v.insert(v.begin() + 1, 15);     // before element 1
    v.erase(v.begin() + 3);          // remove element 3 (the 30)
    for (int x : v) std::cout << x << " ";
    std::cout << "\n";
    v.clear();
    std::cout << std::boolalpha << v.empty() << "\n";
    return 0;
}

Output

10 15 20 40 
true

Inserting or erasing in the middle moves all later elements, so it is slow for long vectors; adding and removing at the end is fast.

Two dimensions

A vector of vectors makes a table whose rows can even have different lengths:

ExampleTry it in the editor

#include <iostream>
#include <vector>

int main() {
    std::vector<std::vector<int>> grid(3, std::vector<int>(4, 0));   // 3 rows of 4 zeros
    grid[1][2] = 7;
    for (const auto& row : grid) {
        for (int x : row) std::cout << x << " ";
        std::cout << "\n";
    }
    return 0;
}

Output

0 0 0 0 
0 0 7 0 
0 0 0 0

Performance tip

A vector grows by reserving a bigger block and moving its elements. When you know roughly how many elements will come, v.reserve(n) reserves the room once. Pass vectors to functions by const& to avoid copying them.

Common mistakes

v[i] does not check the index; reading past the end is undefined behaviour. Use v.at(i) while learning, it throws std::out_of_range instead.
size() is unsigned, so v.size() - 1 on an empty vector becomes a huge number. Check v.empty() first.
Adding elements can move the whole vector to new memory, which makes pointers, references and iterators to its elements invalid.

Practice exercises

Read numbers until 0 into a vector and print them in reverse.
Remove all negative numbers from a vector (hint: build a new vector, or see std::erase_if in C++20).
Store a 3 by 3 table in a vector of vectors and print each row's sum.