# PCDiagonalScaleLeft
Scales a vector by the left scaling as needed by certain time-stepping codes. 
## Synopsis
```
#include "petscksp.h" 
PetscErrorCode PCDiagonalScaleLeft(PC pc, Vec in, Vec out)
```
Logically Collective on pc


## Input Parameters

- ***pc -*** the preconditioner context
- ***in -*** input vector
- ***out -*** scaled vector (maybe the same as in)





## Notes
The system solved via the Krylov method is
```none
        D M A D^{-1} y = D M b  for left preconditioning or
        D A M D^{-1} z = D b for right preconditioning
```


`PCDiagonalScaleLeft()` scales a vector by D. `PCDiagonalScaleRight()` scales a vector by D^{-1}.

If diagonal scaling is turned off and in is not out then in is copied to out


## See Also
 `PCCreate()`, `PCSetUp()`, `PCDiagonalScaleSet()`, `PCDiagonalScaleRight()`, `PCDiagonalScale()`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/interface/precon.c.html#PCDiagonalScaleLeft">src/ksp/pc/interface/precon.c</A>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/ksp/pc/interface/precon.c)


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