Sum by categorical variable

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

 

---------------------------------------------------------------
Fabrizio De Amicis

IT Department
Generali Information Technologies - (GIT)

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CH - 6928 Manno - Switzerland
Tel  +41 91 806 6220
Fax +41 91 806 6298
E-mail: fabrizio.deamicis at git.generali.ch  

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