# VecISAXPY
Adds a reduced vector to the appropriate elements of a full-space vector. vfull[is[i]] += alpha*vreduced[i] 
## Synopsis
```
#include "petscvec.h"  
PetscErrorCode VecISAXPY(Vec vfull, IS is, PetscScalar alpha, Vec vreduced)
```

## Input Parameters

- ***vfull    -*** the full-space vector
- ***is       -*** the index set for the reduced space
- ***alpha    -*** the scalar coefficient
- ***vreduced -*** the reduced-space vector



## Output Parameters

- ***vfull    -*** the sum of the full-space vector and reduced-space vector



## Notes
The index set identifies entries in the global vector.
Negative indices are skipped; indices outside the ownership range of vfull will raise an error.




## See Also
 `VecISCopy()`, `VecISSet()`, `VecAXPY()`

## Level
advanced

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/vec/vec/utils/projection.c.html#VecISAXPY">src/vec/vec/utils/projection.c</A>


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


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