Question

I am trying to create a solid cylinder using triangle fan.
What I’ve done so far is this:

float base = 0.5;
float height = 20;
float radius = 2.0f;
glBegin(GL_TRIANGLE_FAN);
for(float j=0; j<=height; j+=0.1)
{
    glVertex3f(0,j,0);
    for(int i=0; i<360; i++)
    {
        glVertex3f(radius*sin((float)i),j, radius*cos((float)i));
    }   
}
glEnd();
glPopMatrix();

The problem appears in these 3 screenshots:

As you see in all 3 screenshots there appears to be some space and not a solid cylinder.
Is that ok?

Answer 1

First thing you should pay attention to (Edit: I slightly misread your code. You are doing fine with the triangle fan) is that a triangle fan works like this:

glVertex: Center point
for each outer point p
    glVertex: p

For example:

                  p2__
                 /|   ---___p1
                /  |         /
               /    |       /
            p3/      |     /
              |\      |   /
              |   \    | /
               |     \_O   Center
               | __---
               p4

Second thing is that a cylinder consists of three parts:

               __
              /  \
              \__/        <---- circle on top (facing up)
              |  |
              |  |
              |  |
              |  |        <---- tube in the middle
              |  |
              |  |
              |  |
              \__/        <---- circle on the bottom (facing down)

So the algorithm you need is:

/* top triangle */
glBegin(GL_TRIANGLE_FAN);
    glVertex3f(0, height, 0);  /* center */
    for (i = 0; i <= 2 * PI; i += resolution)
        glVertex3f(radius * cos(i), height, radius * sin(i));
glEnd();

/* bottom triangle: note: for is in reverse order */
glBegin(GL_TRIANGLE_FAN);
    glVertex3f(0, 0, 0);  /* center */
    for (i = 2 * PI; i >= 0; i -= resolution)
        glVertex3f(radius * cos(i), 0, radius * sin(i));
    /* close the loop back to 0 degrees */
    glVertex3f(radius, height, 0);
glEnd();

/* middle tube */
glBegin(GL_QUAD_STRIP);
    for (i = 0; i <= 2 * PI; i += resolution)
    {
        glVertex3f(radius * cos(i), 0, radius * sin(i));
        glVertex3f(radius * cos(i), height, radius * sin(i));
    }
    /* close the loop back to zero degrees */
    glVertex3f(radius, 0, 0);
    glVertex3f(radius, height, 0);
glEnd();

The way you were trying to do is first, not correct, since you are not actually making a cylinder, but stacking many circles, and second is inefficient since you are filling in space that is mostly invisible (the inside of the cylinder).