[R] How to implement a "select distinct x, count(distinct y) ... group by x" for a data frame

[Dimitri Shvorob]

... Being an R newbie, I can only think of extracting distinct x values with
unique, looping over them, extracting matching rows from the original data
frame, applying table, and recording the size of table's output alongside
the x value being checked. Is there a more elegant way? 

Thank you.
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