[R] how to count unique observations by variables

[Chuck Cleland]

On 10/16/2008 1:27 AM, Lijiang Guo wrote:
> Dear R-helpers,
> 
> I have a data frame with 3 variables, each record is a unique combination of
> the three variables. I would like to count the number of unique values of v3
> in each v1, and save it as a new variable v4 in the same data frame.
> e.g.
> df1
>      [v1] [v2] [v3]
> [1,] "a"  "C"  "1"
> [2,] "b"  "C"  "2"
> [3,] "c"  "B"  "3"
> [4,] "a"  "B"  "3"
> [5,] "b"  "A"  "2"
> [6,] "c"  "A"  "1"
> 
> In this case, the 4th column would become (2, 1, 2, 2, 1, 2).
> 
> Could someone tell me how to do this?

df1 <- data.frame(V1=rep(c('a','b','c'),2),
                  V2=rep(c('C','B','A'),each=2),
                  V3=c(1,2,3,3,2,1))

df1$V4 <- with(df1, ave(V3, V1, FUN = function(x){length(unique(x))}))

df1
  V1 V2 V3 V4
1  a  C  1  2
2  b  C  2  1
3  c  B  3  2
4  a  B  3  2
5  b  A  2  1
6  c  A  1  2

?ave
?unique
?length

> regards,
> Lijiang
> 
> --
> 
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-- 
Chuck Cleland, Ph.D.
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