:orphan:
# ISExpand
Computes the union of two index sets, by concatenating 2 lists and removing duplicates. 
## Synopsis
```
#include "petscis.h"  
PetscErrorCode ISExpand(IS is1, IS is2, IS *isout)
```
Collective


## Input Parameters

- ***is1 -*** first index set
- ***is2 -*** index values to be added



## Output Parameter

- ***isout -*** `is1` + `is2` The index set `is2` is appended to `is1` removing duplicates





## Notes
Negative values are removed from the lists. This requires O(imax-imin)
memory and O(imax-imin) work, where imin and imax are the bounds on the
indices in `is1` and `is2`.

`is1` and `is2` do not need to be sorted.

The operations are performed separately on each MPI rank


## See Also
 [](sec_scatter), `IS`, `ISDestroy()`, `ISView()`, `ISDifference()`, `ISSum()`, `ISIntersect()`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/vec/is/is/utils/isdiff.c.html#ISExpand">src/vec/is/is/utils/isdiff.c</A>

## Examples
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex81.c.html">src/ksp/ksp/tutorials/ex81.c</A><BR>


---
[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/vec/is/is/utils/isdiff.c)


[Index of all IS routines](index.md)  
[Table of Contents for all manual pages](/manualpages/index.md)  
[Index of all manual pages](/manualpages/singleindex.md)