[R] Remove specific rows in a matrix/data.frame

[syrvn]

Hello again,

	
dtf<-read.table(textConnection("Letter Test Number 
							a b 1 
							a b 1 
							b b 1 
							b b 0 
							c b 0 
							c b 1 
							d b 0 
							d b 0"),header=T) 
aggregate(Number ~ Letter,data=dtf,max)

how can I adjust this solution that the results also includes "Test"?

I tried:

aggregate(Number ~ Letter,data=dtf,max,by=list("Letter", "Test", "Number"))

But it breaks with the following error message:

Error in aggregate.data.frame(mf[1L], mf[-1L], FUN = FUN, ...) : 
  arguments must have same length





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