SFML drawing primitives from VertexArray

How do I draw a primitive of my choice from a constructed VertexArray? In the example below I am adding two vertices to the 'vertices' array and I am trying to draw it with 'window.draw(vertices, 2, sf::Lines)' but it gives me an error. I know I can just create line object with 'sf::Vertex foo[] = {..}' but I want to be able to keep appending vertices to the array instead of initializing all at once.

#include <SFML/Window.hpp>
#include <SFML/OpenGL.hpp>
#include <SFML/Graphics.hpp>
#include <iostream>

int main()
{
    // Create the main window
    sf::RenderWindow window(sf::VideoMode(640, 480), "SFML");
    sf::Clock clock;

    sf::VertexArray vertices;
    sf::Vertex vertex;

    vertex.position = sf::Vector2f(0, 0);
    vertex.color = sf::Color(100, 0, 200);
    vertices.append(vertex);
    vertex.position = sf::Vector2f(100, 100);
    vertex.color = sf::Color(100, 0, 200);
    vertices.append(vertex);

    // Start the game loop
    bool running = true;
    while (running)
    {
        // Process events
        sf::Event event;
        while (window.pollEvent(event))
        {
            // Close window: exit
            if (event.type == sf::Event::Closed)
                running = false;
        }

        window.clear(sf::Color::Black);
        window.draw(vertices,2 ,sf::Lines);

        window.display();
    }

    return 0;
}

You can call vertices.setPrimitiveType(sf::Lines); after declaring sf::VertexArray vertices;, then draw it: window.draw(vertices);

Or you can set its primitive type in the constructor, along with number of points, then you can access those points with operator[]:

sf::VertexArray line(sf::Lines, 2); //or sf::LineStrip
line[0].position = sf::Vector2f(0, 0);
line[0].color = sf::Color(100, 0, 200);
line[1].position = sf::Vector2f(100, 100);
line[1].color = sf::Color(100, 0, 200);
...
window.draw(line);

Reference: enum sf::PrimitiveType, sf::VertexArray, VertexArray tutorial