# Using expressions to define subdomains, RuntimeError: Unable to compile C++ code with dijitso

**URL:** <https://fenicsproject.discourse.group/t/using-expressions-to-define-subdomains-runtimeerror-unable-to-compile-c-code-with-dijitso/7568>\
**Category:** Errors\
**Created:** [January 31, 2022, 3:07am UTC](https://fenicsproject.discourse.group/t/using-expressions-to-define-subdomains-runtimeerror-unable-to-compile-c-code-with-dijitso/7568 "2022-01-31T03:07:05Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![mary\_h](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/mary_h/32/1943_2.png) [@mary\_h](https://fenicsproject.discourse.group/u/mary_h)\
**Post date:** [January 31, 2022, 3:07am UTC](https://fenicsproject.discourse.group/t/using-expressions-to-define-subdomains-runtimeerror-unable-to-compile-c-code-with-dijitso/7568/1 "2022-01-31T03:07:05Z")

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Hello,  
I am trying to follow this link ([https://fenicsproject.org/pub/tutorial/html/.\_ftut1013.html](https://fenicsproject.org/pub/tutorial/html/._ftut1013.html)) in order to assign different values for the different subdomains. I want to create a hollow circle as below:

```auto
from dolfin import *
from mshr import *
domain = Circle(Point(0, 0), 1)
mesh = generate_mesh(domain, 64)
mu_0 = 0
mu_1 = 100
mu = Expression("(x[1])**2 + (x[0])**2 < 0.64 ? mu_0 : mu_1", mu_0 = mu_0, mu_1 = mu_1, degree=0)

```

However, I get the error: `RuntimeError: Unable to compile C++ code with dijitso`  
Any help is much appreciated.

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<div class="post-metadata">

**Author:** ![nate](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/nate/32/17_2.png) [@nate](https://fenicsproject.discourse.group/u/nate)\
**Post date:** [January 31, 2022, 3:40am UTC](https://fenicsproject.discourse.group/t/using-expressions-to-define-subdomains-runtimeerror-unable-to-compile-c-code-with-dijitso/7568/2 "2022-01-31T03:40:21Z")

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C++ does not implement the operator `**`. See for example [pow - C++ Reference](https://www.cplusplus.com/reference/cmath/pow/).

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<div class="post-metadata">

**Author:** ![mary\_h](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/mary_h/32/1943_2.png) [@mary\_h](https://fenicsproject.discourse.group/u/mary_h)\
**Post date:** [January 31, 2022, 3:51am UTC](https://fenicsproject.discourse.group/t/using-expressions-to-define-subdomains-runtimeerror-unable-to-compile-c-code-with-dijitso/7568/3 "2022-01-31T03:51:09Z")

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Thank you so much for the help!