how to select first 50 value of each group after group by?

[luohui20001]

hi thereI have a DF with 3 columns: id , pv, location.(the rows are already 
grouped by location and sort by pv in des)  I wanna get the first 50 id values 
grouped by location. I checked the API of dataframe,groupeddata,pairRDD, and 
found no match.      is there a way to do this naturally?      any info will be 
appreciated.


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Thanks&Best regards!
San.Luo