Can I get example opengl code to continously decrease the size of a sphere using animation

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OpenGL

transformation

I tried to continously decrease the sphere radius using a loop, but it does not seem to work.The animation for increasing sphere size is working fine.
Not able to understand why decreasing radius is not rendered.

I request anyone to share any sample code that can help to smoothly animate the same.

Thank You.

What I have tried:

#include<windows.h>
 #include<GL/glut.h>

int r=1,flag=0;

void drawSphere(int r1)
{
    glutSolidSphere(r1,30,30);
    glFlush();
}

 void displaySolid()
 {
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glColor3d(1,0,0);
    drawSphere(100);

    glFlush();
 }

 void animate(int btn,int state,int x,int y)
 {
     if(btn==GLUT_LEFT_BUTTON && state==GLUT_DOWN)
     {
                 for(r=200;r<400;r+=3)
                 {
                     drawSphere(r);
                     //glutPostRedisplay();
                     Sleep(100);
                     flag=1;
                 }
                 r=100;

         }




     if(btn==GLUT_RIGHT_BUTTON && state==GLUT_DOWN)
     {

         for(r=60;r>=30;r--)
         {
             
             drawSphere(r);
             Sleep(100);
         }
         
     }
 }


int main(int argc,char **argv)

{
    glutInit(&argc,argv);
    glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB|GLUT_DEPTH);
    glutInitWindowSize(700,700);
    glutInitWindowPosition(0,0);
    glutCreateWindow("sphere test");
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(-500,500,-500,500,-500,500);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glutDisplayFunc(displaySolid);
    glutMouseFunc(animate);
    glEnable(GL_DEPTH_TEST);
    glutMainLoop();
    return 0;
}