Take a vector v of indices,
then simplify the indexing system
and compact it from 1 to length(unique(v)).
compact_index(v)a vector of integers, used as indices.
Return a vector of integer,
that can be browsed by the code seq_len(length(unique(v))).
The indexing vector v is simplified
and compacts from 1 to length(unique(v)).
The index order is not changed, only the labels are changed.
The function is useful
for debugging computations on values sorted by a program.