OpenGL Line Drawing - opengl

Question

I am practicing the exercises from my textbook but I could not get the outputs that I should.
Here is what I have :
#include <math.h>
#include <GLUT/glut.h>
#include <OpenGL/OpenGL.h>
//Initialize OpenGL
void init(void) {
glClearColor(0.0,0.0,0.0,0.0);
glMatrixMode(GL_PROJECTION);
gluOrtho2D(0.0,300.0,0.0,300.0);
}
void drawLines(void) {
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(0.0,0.4,0.2);
glPointSize(3.0);
glBegin(GL_LINES);
glVertex2d(180, 15);
glVertex2d(10, 145);
glEnd();
}
int main(int argc, char**argv) {
glutInit(&argc, argv);
glutInitWindowPosition(10,10);
glutInitWindowSize(500,500);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutCreateWindow("Example");
init();
glutDisplayFunc(drawLines);
glutMainLoop();
}
When I run this piece of code, I get completely blank white screen.

Answer 1

i'm also not an expert on OpenGL but the problem is that you haven't set a viewport to where your scene should be projected. Your init should look somewhat like this:
glClearColor(0, 0, 0, 0);
glViewport(0, 0, 500, 500);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, 500, 0, 500, 1, -1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
You also need to put a glFlush(); after your drawing.
void drawLines(void) {
...
glFlush();
}

Answer 2

Determine and draw the output of OpenGL sub function below:
1.
glColor3f(0.0, 0.0, 0.0, 0.0);
glBegin(Gl_LINES);
glVertex2i (200, 200);
glVertex2i (70, 20);
glVertex2i (120, 150);
glVertex2i (200, 20);
glVertex2i (60, 100);
glEnd();
int p1[]={200,100};
int p2[]={70,20};
int p3[]={120,150};
int p4[]={200,20};
int p5[]={60,100};
glBegin(GL_LINE_STRIP);
glVertex2iv (p1);
glVertex2iv (p2);
glVertex2iv (p3);
glVertex2iv (p4);
glVertex2iv (p5);
glEnd();