Moving part of model on circle

[Hitokage]

Hi guys.
Need help with something.

I have imported model and I need to rotate part of it.
Here is what I mean.

I have this in the beginning:


And need to transform it to this (like when I press button I want this part of model to smoothly rotate until right angle):



So this is the code I used

//somewhere in the beginning
float angle = 0.0;
 
 
case GLUT_KEY_RIGHT:                                                                                                                          //when this button is pressed
 
int i, xo = 20;                                                                                                     // i is for loop and xo is x coordinate of the circle center 
 
 
	for (i=0; i<=object.vertices_qty; i++)                                                                     //go through all the vertices
	{
		if(object.vertex[i].x > xo)                                                                         //if the x coordinate is bigger than xo - choose the part to rotate
		{
 
		     if(object.vertex[i].r == 0)                                                             //calculate the circle radius if it hasn't been already done
		     object.vertex[i].r = object.vertex[i].x-xo;
 
		angle += 0.001;                                                                                      //increment the angle
 
                       //now I want to calculate new vertex coordinates (after rotation)
 
                       object.vertex[i].y = object.vertex[i].w + object.vertex[i].r*cos(angle);             //object.vertex[i].w is original y coordinate (before rotation) which is also coordinate of the circle center
		object.vertex[i].x = xo + object.vertex[i].r*sin(angle);
 
		}
	}		
 
break;

What is the problem here? The code is not working properly. The result is trash .

Sry if there are some serious mistakes. I am just a beginner in OpenGL and geometry is not my big friend .

Have a nice day