# DMFieldEvaluateFV
Evaluate the mean of a field and its finite volume derivatives on a set of points. 
## Synopsis
```
#include "petscdmfield.h" 
#include "petscdmfield.h" 
PetscErrorCode DMFieldEvaluateFV(DMField field, IS cellIS, PetscDataType datatype, void *B, void *D, void *H)
```
Not collective


## Input Parameters

- ***field -*** The `DMField` object
- ***cellIS -*** Index set for cells on which to evaluate the field
- ***datatype -*** The PetscDataType of the output arrays: either `PETSC_REAL` or `PETSC_SCALAR`.
If the field is complex and datatype is `PETSC_REAL`, the real part of the
field is returned.



## Output Parameters

- ***B -*** pointer to data of size c * n * sizeof(datatype), where c is the number of components in the field.
If B is not NULL, the values of the field are written in this array, varying first by component,
then by point.
- ***D -*** pointer to data of size d * c * n * sizeof(datatype).
If D is not NULL, the values of the field's spatial derivatives are written in this array,
varying first by the partial derivative component, then by field component, then by point.
- ***H -*** pointer to data of size d * d * c * n * sizeof(datatype).
If H is not NULL, the values of the field's second spatial derivatives are written in this array,
varying first by the second partial derivative component, then by field component, then by point.





## See Also
 `DMField`, `IS`, `DMFieldGetNumComponents()`, `DMFieldEvaluate()`, `DMFieldEvaluateFE()`, `PetscDataType`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/dm/field/interface/dmfield.c.html#DMFieldEvaluateFV">src/dm/field/interface/dmfield.c</A>

## Examples
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/dm/field/tutorials/ex1.c.html">src/dm/field/tutorials/ex1.c.html</A><BR>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/dm/field/impls/da/dmfieldda.c.html#DMFieldEvaluateFV_DA">DMFieldEvaluateFV_DA in src/dm/field/impls/da/dmfieldda.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/dm/field/impls/ds/dmfieldds.c.html#DMFieldEvaluateFV_DS">DMFieldEvaluateFV_DS in src/dm/field/impls/ds/dmfieldds.c</A><BR>


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