[R] Error in using nlevels in apply function

[Sébastien]

Dear R users,

I am currently trying to create my first personnal function and use it 
with the apply function. The purpose of this function is to create a 
vector summarizing the number of levels in a given selection of 
data.frame columns.
I tried to transpose the indexation method used by the nlevels function 
but it doesn't seem to work. I did not find anything uesful in the 
archives so could someone point to me where my mistake(s) is (are) ?

Thanks in advance

Sebastien

#-------------------------

mydata<-data.frame(1:6,1:6,1:6,1:6,1:6,1:6,1:6,1:6,1:6,1:6,1:6,1:6,1:6,1:6,1:6,1:6,1:6,1:6)
mycol.index<-c(1,5,3)

nelem<-function(x,col.id) nlevels(factor(x[,col.id]))
my.nlevels.col<-apply(mydata,2,nelem,mycol.index)
my.nlevels.col

#----------------------------

#The error message is the following
#>Error in x[, col.id] : incorrect number of dimensions