Rotation of an object in opengl - c++

Question

I need to rotate the rectangular object upon pressing the left arrow key.
I am not able to rotate it, or maybe it is. There may be a problem playing with the buffer.
Weather the rotatef() function is placed correctly or where the problem lies.
#include<stdlib.h>
#include<glut.h>
#include<stdio.h>
#include<math.h>
#include<time.h>
void rot()
{ glMatrixMode(GL_MODELVIEW);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glPushMatrix();
glRotatef(30,0.0,0.0,1.0);
glBegin(GL_POLYGON);
glVertex2i(50,50);
glVertex2i(170,50);
glVertex2i(170,100);
glVertex2i(50,100);
glEnd();
glPopMatrix();
}
void shape()
{
glColor3f(1.0,0.0,0.0);
glBegin(GL_POLYGON);
glVertex2i(50,50);
glVertex2i(170,50);
glVertex2i(170,100);
glVertex2i(50,100);
glEnd();
}
void display()
{
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
shape();
glutSwapBuffers();
glFlush();
}
void init()
{
glClearColor(1.0,1.0,1.0,1.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glOrtho(0,700,0,700,-1,1);
glMatrixMode(GL_MODELVIEW);
}
void Keys(int key,int x,int y)
{
if(key==GLUT_KEY_LEFT)
rot();
glutPostRedisplay();
}
void main(int argc,char**argv)
{
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGB|GLUT_DEPTH);
glutInitWindowSize(700,700);
glutCreateWindow("tetris");
glutDisplayFunc(display);
glutSpecialFunc(Keys);
init();
glutMainLoop();
}

Answer 1

You post a redisplay immediately after drawing your rotated vertices.
So right after drawing the rotated vertices, in your display callback function, you clear the window and then draw your vertices without rotation. So that is what you'll see (without rotation), until the next update.
One way to proceed is:
Do all your drawing in your display function
As reaction to a keypress, just update the angle ( I guess you want to rotate incrementally) and post a redisplay. Don't draw anything, your display function will use the updated angle.