The last code block in this function also draws a square using a line loop, but this time uses the data structure defined at the beginning of the function. Notice that this time glVertex2fv() is used since its parameters are two-element vectors.
glColor4fv( blue );
glBegin( GL_LINE_LOOP );
for ( i = 0; i < 4; i++ )
{
glVertex2fv( p[i] );
}
glEnd();