OpenGL Line Drawings using algoritgms

#include<windows.h>
#include <GL/glut.h>
#include <GL/gl.h>
#include <bits/stdc++.h>

using namespace std;

double x1,y11,x2,y2,delx,dely,delm,x,y,pk;

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1.0,0.0,0.0);

//    glBegin(GL_POLYGON);
//        glVertex2i(10,10);
//        glVertex2i(30,10);
//        glVertex2i(30,60);
//        glVertex2i(10,60);
//    glEnd();

    /*x = x1, y= y11;
        delx = x2-x1 ,dely = y2-y11;
        delm = dely/delx;

        if( abs( delm ) <= 1 ){
            while( x < x2 ){
                x++, y += delm;
                glBegin(GL_POINTS);
                glVertex2i(x,round(y));
                glEnd();
            }
        }
        else{
            while( y < y2 ){
                y++; x += ( 1/delm );
                glBegin(GL_POINTS);
                glVertex2i(round(x),y);
                glEnd();
            }
        }
    */
    x = x1, y= y11;
    delx = x2-x1 ,dely = y2-y11;
    delm = dely/delx;
    pk =( 2* dely) - delx;




    if( delm == 0.0000 ) {
        cout << delm << endl;
        while( x < x2  ){
            x++;
            glBegin(GL_POINTS);
            glVertex2i(x,y);
            glEnd();
        }
    }
    else if( delm < 1  ){
            while( x < x2 ){
                x++;
                if( pk > 0 ){
                    pk +=( 2 * ( dely - delx) );
                    y++;
                }
                else{
                    pk += (dely*2);
                    y++;
                }
                glBegin(GL_POINTS);
                glVertex2i(x,y);
                glEnd();
            }
        }
    else {
        while( y < y2  ){
            y++;
            glBegin(GL_POINTS);
            glVertex2i(x,y);
            glEnd();
        }
    }

    glFlush();

}

void init(void)
{
    glClearColor(1,1,1,5.0);
    glMatrixMode(GL_PROJECTION);
    // glClear(GL_COLOR_BUFFER_BIT);
    glLoadIdentity();
    glPointSize(5.0);
    glOrtho(0.0,100.0,0.0,100.0,-100.0,100.0);

}
int main(int a,char **ar)
{
    glutInit(&a,ar);
    glutInitDisplayMode(GLUT_RED | GLUT_SINGLE);
    glutInitWindowPosition(100,100);
    glutInitWindowSize(800,500);
    glutCreateWindow("Polygon");
    cin >> x1 >> y11 >> x2 >> y2 ;

    init();
    glutDisplayFunc(display);
    glutMainLoop();

    return 0;

}