Opengl Weird Rotation - c++

Question

I'm trying to rotate a simple triangle. The problem is that while it rotates correctly, it decrease its size until it disappears.
Some pieces of my code so far:
// Vertices
GLfloat vertexArray[] =
{
1.0f, -1.0f, 0.0f,
-1.0f, -1.0f, 0.0f,
0.0f, 1.0f, 0.0f,
};
// Render funcion (called every frame)
void render()
{
glClearColor(0.0f, 0.0f, 0.4f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
glBindBuffer(GL_ARRAY_BUFFER, vertexBufferObj);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertexArray), vertexArray, GL_DYNAMIC_DRAW);
glUseProgram(programID); // simple vertex/frag shader
glDrawArrays(GL_TRIANGLES, 0, 3);
// Swap buffers
glfwSwapBuffers();
}
// Update funcion (called every frame before render function)
void update(float elapsedTime)
{
printf("elapsedTime: %f \r", elapsedTime);
float static theta = elapsedTime * 0.2f;
for(int i = 0; i < 9; i+=3)
{
vertexArray[i] = (vertexArray[i] * cosf(theta)) - (vertexArray[i+1] * sinf(theta));
vertexArray[i+1] = (vertexArray[i] * sinf(theta)) + (vertexArray[i+1] * cosf(theta));
vertexArray[i+2] = 0;
}
}
As you can see, I'm rotating every vertex on update function with a for loop. Maybe the best way to do this is using the shader (correct me if I'm wrong), but I wanted to keep things simple here just to illustrate the problem.

Answer 1

I believe the problem is, that when you compute vertexArray[i+1] = (vertexArray[i] * sinf(theta)) + (vertexArray[i+1] * cosf(theta)); you are not using the value of vertexArray[i] from the previous iteration, but rather the new vertexArray[i] computed in the first assignment of the for loop.
Try this:
for(int i = 0; i < 9; i+=3)
{
double tmp = vertexArray[i];
vertexArray[i] = (tmp * cosf(theta)) - (vertexArray[i+1] * sinf(theta));
vertexArray[i+1] = (tmp * sinf(theta)) + (vertexArray[i+1] * cosf(theta));
vertexArray[i+2] = 0;
}