Creating a second VAO and drawing two shapes in OpenGL 4 - opengl

Question

I am trying to solve the experiments in the book Anton's OpenGL 4 Tutorials. In the first chapter's experiments it ask to create a second VAO to draw 2 shapes instead of one but I have no clue how to do this, how can a second be displayed simultaneously?
#include <GL/glew.h>
#include <GLFW/glfw3.h>
#include <iostream>
int main () {
// start GL context and O/S window using the GLFW helper library
if (!glfwInit ()) {
fprintf (stderr, "ERROR: could not start GLFW3\n");
return 1;
}
GLFWwindow* window = glfwCreateWindow (640, 480, "Hello Triangle", NULL, NULL);
if (!window) {
fprintf (stderr, "ERROR: could not open window with GLFW3\n");
glfwTerminate();
return 1;
}
glfwMakeContextCurrent (window);
// start GLEW extension handler
glewExperimental = GL_TRUE;
glewInit ();
// get version info
const GLubyte* renderer = glGetString (GL_RENDERER); // get renderer string
const GLubyte* version = glGetString (GL_VERSION); // version as a string
printf ("Renderer: %s\n", renderer);
printf ("OpenGL version supported %s\n", version);
// tell GL to only draw onto a pixel if the shape is closer to the viewer
glEnable (GL_DEPTH_TEST); // enable depth-testing
glDepthFunc (GL_LESS); // depth-testing interprets a smaller value as "closer"
GLfloat points[] = {
0.0f, 0.5f, 0.0f,
0.5f, -0.5f, 0.0f,
-0.5f, -0.5f, 0.0f,
0.0f, 0.5f, 0.0f,
0.5f, 0.5f, 0.0f,
0.5f, -0.5f, 0.0f
};
GLuint vbo = 0;
glGenBuffers (1, &vbo);
glBindBuffer (GL_ARRAY_BUFFER, vbo);
glBufferData (GL_ARRAY_BUFFER, 18 * sizeof (float), points, GL_STATIC_DRAW);
GLuint vao = 0;
glGenVertexArrays (1, &vao);
glBindVertexArray (vao);
glEnableVertexAttribArray (0);
glBindBuffer (GL_ARRAY_BUFFER, vbo);
glVertexAttribPointer (0, 3, GL_FLOAT, GL_FALSE, 0, NULL);
//second vao
GLuint vao_two = 0;
glGenVertexArrays(1, &vao_two);
glBindVertexArray(vao_two);
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, NULL);
const char* vertex_shader =
"#version 400\n"
"in vec3 vp;"
"in vec3 vp2;"
"void main() {"
" gl_Position = vec4(vp.x, vp.y + 0.2, vp.z, 1.0);"
" gl_Position2 = vec4(vp2.x, vp2.y - 0.9, vp2.z, 1.0);"
"}";
const char* fragment_shader =
"#version 400\n"
"out vec4 frag_colour;"
"void main () {"
" frag_colour = vec4 (0.0, 1.0, 0.0, 1.0);"
"}";
GLuint vs = glCreateShader (GL_VERTEX_SHADER);
glShaderSource (vs, 1, &vertex_shader, NULL);
glCompileShader (vs);
GLuint fs = glCreateShader (GL_FRAGMENT_SHADER);
glShaderSource (fs, 1, &fragment_shader, NULL);
glCompileShader (fs);
GLuint shader_programme = glCreateProgram();
glAttachShader (shader_programme, fs);
glAttachShader (shader_programme, vs);
glLinkProgram(shader_programme);
glClearColor(0.6f, 0.6f, 0.6f, 0.0f);
while (!glfwWindowShouldClose (window)) {
// wipe the drawing surface clear
glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glUseProgram (shader_programme);
glBindVertexArray (vao);
// draw points 0-3 from the currently bound VAO with current in-use shader
glDrawArrays (GL_TRIANGLES, 0, 6);
glBindVertexArray (vao_two);
glDrawArrays (GL_TRIANGLES, 0, 6);
// update other events like input handling
glfwPollEvents ();
// put the stuff we've been drawing onto the display
glfwSwapBuffers (window);
}
// close GL context and any other GLFW resources
glfwTerminate();
return 0;
}

Answer 1

Actually, you are drawing two different shapes, however, because you use the same vertex buffer object in both vertex attribute objects (vao and vao_two) they have the same coordinates and they overlap.
Try to make 2 GLfloat arrays of points with different coordinates and attribute each one to a different vertex buffer object and then each vertex buffer object to a different vertex attribute object, then your shapes will be distinct.

Answer 2

First bind the buffers and then draw them all:
glBindVertexArray(vao);
glBindVertexArray(vao2);
glDrawArrays(GL_TRIANGLES, 0, 6);
Alternatively you can draw first the first shape and then the second:
glBindVertexArray(vao);
glDrawArrays(GL_TRIANGLES, 0, 3);
glBindVertexArray(vao2);
glDrawArrays(GL_TRIANGLES, 3, 3);