Given a reference set R of elements (each unique but identical in type), and a list V of elements drawn from the set R, possibly with duplicates, return K such that R(K) == V. The elements of R may be numeric or string.
For example, refIndex(9:-1:5,[7 5 6 5]) should return [3 5 4 5].
Can this be done efficiently using regexp?
so simple and clear
Make a run-length companion vector
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