Is this what you want:
> x <- data.frame(Name=c("A","A","C"), Category=c("a","a","b"),
> Quantity=c(1,2,3))
> aggregate(x$Quantity, list(x$Name, x$Category), FUN=sum)
Group.1 Group.2 x
1 A a 3
2 C b 3
>
'by' is "a list of grouping elements, each as long as the variables
>From what I read this should work. So please help my misunderstanding:
> x <- data.frame(Name=c("A","A","C"), Category=c("a","a","b"),
> Quantity=c(1,2,3))
> x
Name Category Quantity
1Aa1
2Aa2
3Cb3
> aggregate(x, by=as.list(setdiff(n
2 matches
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