# Shifting a dolfin.Function by a vector (with UserExpression) **URL:** \ **Category:** Uncategorized\ **Created:** [November 30, 2021, 4:14pm UTC](https://fenicsproject.discourse.group/t/shifting-a-dolfin-function-by-a-vector-with-userexpression/7190 "2021-11-30T16:14:02Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![dstrelnikov](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@dstrelnikov](https://fenicsproject.discourse.group/u/dstrelnikov)\ **Post date:** [November 30, 2021, 4:14pm UTC](https://fenicsproject.discourse.group/t/shifting-a-dolfin-function-by-a-vector-with-userexpression/7190/1 "2021-11-30T16:14:02Z")
I’m trying to define a new function on a finite element space by shifting another function by a vector: ```auto theta_shifted(x) := theta(x + shift) ``` In case the point `x + shift` is outside of the domain, these values should be taken from another function. My idea was to define an expression and interpolate it in the function space. Here is the relevant piece of the code: ```auto class ShiftedExpression(UserExpression): def __init__ ( self, func_interpolate, func_extrapolate, shift, bounding_box_tree, **kwargs ): self.func_interpolate = func_interpolate self.func_extrapolate = func_extrapolate self.shift = shift self.bounding_box_tree = bounding_box_tree super(). __init__ (**kwargs) def eval(self, values, x): def is_inside(point): collisions = self.bounding_box_tree.compute_collisions(point) return len(collisions) > 0 point = Point(*(x[i] + self.shift[i] for i in range(3))) values[0] = self.func_interpolate(point) if is_inside(point) else self.func_extrapolate(point) mesh = ... V = FunctionSpace(mesh, "CG", 1) theta = Function(V) theta.vector().set_local(...) theta_shifted_expression = ShiftedExpression( func_interpolate=(lambda point: theta(point)), func_extrapolate=(lambda _: temp_amb), shift=(d, 0, 0), bounding_box_tree=mesh.bounding_box_tree(), ) theta_shifted_function = interpolate(theta_shifted_expression, V) ``` The expression `theta_shifted_expression` works as expected, but I’m not able to get a function on V out of it: ```auto >>> theta_init_new = dolfin.interpolate(theta_shifted, problem.V) Traceback (most recent call last): File "", line 1, in File "/usr/local/lib/python3.6/dist-packages/dolfin/fem/interpolation.py", line 71, in interpolate Pv.interpolate(v._cpp_object) File "/usr/local/lib/python3.6/dist-packages/dolfin/function/function.py", line 365, in interpolate self._cpp_object.interpolate(u) File "/usr/local/lib/python3.6/dist-packages/dolfin/function/expression.py", line 53, in wrapped_eval self.user_expression.eval(values, x) File "run.py", line 163, in eval values[0] = self.func_interpolate(point) if is_inside(point) else self.func_extrapolate(point) File "run.py", line 178, in func_interpolate=(lambda point: theta_final(point)), File "/usr/local/lib/python3.6/dist-packages/dolfin/function/function.py", line 337, in __call__ self._cpp_object.eval(values, x) RuntimeError: *** ------------------------------------------------------------------------- *** DOLFIN encountered an error. If you are not able to resolve this issue *** using the information listed below, you can ask for help at *** *** fenics-support@googlegroups.com *** *** Remember to include the error message listed below and, if possible, *** include a *minimal* running example to reproduce the error. *** *** ------------------------------------------------------------------------- *** Error: Unable to evaluate function at point. *** Reason: The point is not inside the domain. Consider calling "Function::set_allow_extrapolation(true)" on this Function to allow extrapolation. *** Where: This error was encountered inside Function.cpp. *** Process: 0 *** *** DOLFIN version: 2019.1.0 *** Git changeset: 74d7efe1e84d65e9433fd96c50f1d278fa3e3f3f *** ------------------------------------------------------------------------- ``` I cannot understand at which line and why I’m getting out of the domain. ---
**Author:** ![dokken](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/dokken/32/1560_2.png) [@dokken](https://fenicsproject.discourse.group/u/dokken)\ **Post date:** [December 1, 2021, 9:36am UTC](https://fenicsproject.discourse.group/t/shifting-a-dolfin-function-by-a-vector-with-userexpression/7190/2 "2021-12-01T09:36:26Z")
Please make sure that your minimal code example can reproduce your error message for anyone reading the post. See: [Read before posting: How do I get my question answered? - #2](https://fenicsproject.discourse.group/t/read-before-posting-how-do-i-get-my-question-answered/21/2) for details. Please note that the bounds of a boundingboxtree is not the same as the bounds of an actual mesh entitiy. ---
**Author:** ![dstrelnikov](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@dstrelnikov](https://fenicsproject.discourse.group/u/dstrelnikov)\ **Post date:** [December 1, 2021, 11:31am UTC](https://fenicsproject.discourse.group/t/shifting-a-dolfin-function-by-a-vector-with-userexpression/7190/3 "2021-12-01T11:31:35Z")
Thank you for the answer! You are right, it’s my fault I didn’t provide a reproducible minimal example. However, my question is rather “what is the right way to define a shifted function?” instead of “how to fix this error?”. > Please note that the bounds of a boundingboxtree is not the same as the bounds of an actual mesh entitiy. Could you please give me a hint on a more correct way to check if a point lies within the boundaries of the mesh? ---
**Author:** ![dstrelnikov](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@dstrelnikov](https://fenicsproject.discourse.group/u/dstrelnikov)\ **Post date:** [December 1, 2021, 2:06pm UTC](https://fenicsproject.discourse.group/t/shifting-a-dolfin-function-by-a-vector-with-userexpression/7190/4 "2021-12-01T14:06:57Z")
I managed to fix the error and get the result I expected. The function `theta` needed the `set_allow_extrapolation=True` parameter (as the error message suggests), which makes sense with @dokken’s remark concerning the `bounding_box_tree`. It is interesting that ```auto theta = Function(V, set_allow_extrapolation=True) ``` does not work, but ```auto theta = Function(V) theta.set_allow_extrapolation(True) ``` works as expected. ---
**Author:** ![conpierce8](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/conpierce8/32/2464_2.png) [@conpierce8](https://fenicsproject.discourse.group/u/conpierce8)\ **Post date:** [December 1, 2021, 2:54pm UTC](https://fenicsproject.discourse.group/t/shifting-a-dolfin-function-by-a-vector-with-userexpression/7190/5 "2021-12-01T14:54:26Z")
> [@dstrelnikov](#): > > It is interesting that > > ```auto > theta = Function(V, set_allow_extrapolation=True) > > ``` > > does not work, but > > ```auto > theta = Function(V) > theta.set_allow_extrapolation(True) > > ``` > > works as expected. Per [the source code for `Function`](https://bitbucket.org/fenics-project/dolfin/src/master/python/dolfin/function/function.py), the only `kwarg` used by `Function. __init__ ` is `name`, so it is unsurprising that `Function(V, set_allow_extrapolation=True)` does not work. ---
**Author:** ![dstrelnikov](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@dstrelnikov](https://fenicsproject.discourse.group/u/dstrelnikov)\ **Post date:** [December 1, 2021, 2:59pm UTC](https://fenicsproject.discourse.group/t/shifting-a-dolfin-function-by-a-vector-with-userexpression/7190/6 "2021-12-01T14:59:46Z")
Yes, you are right, I just realized it and was about to write this remark. I don’t know from where I got the idea to use the keyword parameter, probably just a confusion. Anyway, thanks! ---
**Author:** ![dokken](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/dokken/32/1560_2.png) [@dokken](https://fenicsproject.discourse.group/u/dokken)\ **Post date:** [December 1, 2021, 5:05pm UTC](https://fenicsproject.discourse.group/t/shifting-a-dolfin-function-by-a-vector-with-userexpression/7190/7 "2021-12-01T17:05:27Z")
As a side-note, you could use `compute_entity_collisions` as shown in: [Bitbucket](https://bitbucket.org/fenics-project/dolfin/src/3eacdb46ed4e6dcdcbfb3b2f5eefa73f32e1b8a8/python/test/unit/geometry/test_bounding_box_tree.py#lines-200), which would check for actual collision with the cell ---
**Author:** ![dstrelnikov](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@dstrelnikov](https://fenicsproject.discourse.group/u/dstrelnikov)\ **Post date:** [December 1, 2021, 6:41pm UTC](https://fenicsproject.discourse.group/t/shifting-a-dolfin-function-by-a-vector-with-userexpression/7190/8 "2021-12-01T18:41:48Z")
Thanks for this hint!