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# PetscDTSubsetIndex
Convert an ordered subset of k integers from the set [0, ..., n - 1] to its encoding as an integers in [0, n choose k) in lexicographic order. This is the inverse of `PetscDTEnumSubset`. 
## Input Parameters

- ***n -*** a non-negative integer (see note about limits below)
- ***k -*** an integer in [0, n]
- ***subset -*** an ordered subset of the integers [0, ..., n - 1]



## Output Parameter

- ***index -*** the rank of the subset in lexicographic order





## Note
Limited by arguments such that `n` choose `k` can be represented by `PetscInt`


## See Also
 `PetscDTEnumSubset()`, `PetscDTFactorial()`, `PetscDTFactorialInt()`, `PetscDTBinomial()`, `PetscDTBinomialInt()`, `PetscDTEnumPerm()`, `PetscDTPermIndex()`

## Level
beginner

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/include/petscdt.h.html#PetscDTSubsetIndex">include/petscdt.h</A>


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