Dynamic updating and editing of matrix

Question

I am trying to create a user interface for a demo project that uses matrix as an input. I provide user with a default matrix (as an example) for a start, but user can edit it or add extra row to it and then they could run the calculation on the input matrix by pressing Run button. I am getting the following error (shown in the figure below) whenever I open my document anew, but this error does not come when I run my cell once or twice.

When the cell is run once or twice it should work like this

Here is my code:

mat = ( {
 {21, 72, 3.0044},
 {0, 62.5, 3.5091},
 {1, 200, 3.6835}
} );
Column[{
 Dynamic[Table[
 With[{xi = i, yj = j}, 
  InputField[Dynamic[mat[[xi, yj]]], ImageSize -> 100]],
 {i, Dimensions[mat][[1]]}, {j, 3}] // TableForm],
Row[{Button["ADD ROW (+)", Dynamic[mat = Append[mat, {0, 0, 0}]], 
   ImageSize -> 100],
  Button["DEL. ROW (-)", 
   If[Dimensions[mat][[1]] > 1, mat = Delete[mat, -1]], 
   ImageSize -> 100]
   }], y = "Press Run";
 Button["Run", y = 2*mat // TableForm, ImageSize -> 200],
 Dynamic@y
}]

Note: In reality, operation on matrix is more complicated function than multiplying it twice (y = 2*mat). It is shown here for simplicity.

It would be helpful if one could provide solution to solve this problem concerning: updating or editing a matrix dynamically and then performing some operation on it after pressing 'Run' button.

Thank you in advance.

Answer 1

Here is one solution using Manipulate, but one still needs to use Dynamic inside InputField for proper working.

Manipulate[
 Column[{
   Table[
    With[{xi = i, yj = j}, 
    InputField[Dynamic[mat[[xi, yj]]], ImageSize -> 100]],
   {i, Dimensions[mat][[1]]}, {j, 3}] // TableForm, y
  }],
   {{mat, ( {
    {21, 72, 3.0044},
   {0, 62.5, 3.5091},
   {1, 200, 3.6835}
    } )}, ControlType -> None},
   {{y, "Press Run"}, ControlType -> None},
  Button["ADD ROW (+)", mat = Append[mat, {0, 0, 0}], ImageSize -> 100],
   Button["DEL. ROW (-)", 
  If[Dimensions[mat][[1]] > 1, mat = Delete[mat, -1]], 
  ImageSize -> 100],
 Button["Run", y = 2*mat // TableForm, ImageSize -> 200]
  ]