[R] FW: Sum by categorical variable

[De Amicis Fabrizio (G.I.T.)]

Another easy(/stupid) question:

with the following command
 j2<-xf1[1:10,"V4"]
I have 

> j2
 [1] CHROMOLI             LINEAGERMAI          RINALDI              GIUNTIMA
AUTOSTELLA          
 [6] CAIZZONE             CENTRO B PEL E C SNC CONSORZI             MAN NORD
PDM                 
1304 Levels:   MACHIAVELLI Snc  MENARINI MANUFACTURERS LOGISTICS SERVICES
... ZUCCHERIFICIO DEL MOLISE

  
Why j2 has 1304 Levels (I know that xf has 1304 rows)? How can I obtain a
vector of 10 elements?

Thank you 


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