Manual modification of a mesh

Hi everybody,
I am working on a predictor-corrector algorithm in mechanics and I am stuck on the following technicality.

The problem I am interested in is the following.
Given a mesh (half a circle), I would like to manually move the point on the top of the mesh, and recompute/reshape the mesh after this manual deformation (manual, in the sense that it is not computed by solving an equation.

I attach my attempted solution but I receive a compilation error.
Would you know how to solve this issue?

Thanks :slight_smile:

real r=0.5;
//half circle mesh
border c1(t=0,pi){x=r*cos(t);y=r*sin(t);label=1;}
border c2(t=0,1){x=(t-1)*r+t*r;y=0.0;label=2;}
mesh Th= buildmesh(c1(100)+c2(100));
plot(Th, wait=1);

int NbVertices = Th.nv;
cout << "Number of vertices = " << NbVertices << endl;
int target;

for (int i = 0; i < NbVertices; i++){
    cout << "Th(" << i << ") : " << Th(i).x << " " << Th(i).y << endl;
        target=I; // there is only one point with such feature
    cout<< target<<endl; // the index corresponding to the point on the top is 1563

Th(target).y=0.45; // decrement from 0.5 to 0.45