How do I get summary statistics in R after negative selection of a data frame

Question

I would like to negatively select (all but the given row value for each level of a factor variable) and summarize the data that remains. For a simple example, I have a data frame, DF, with two columns.

>DF
Category      Value  
A               5  
B               2  
C               3  
A               1  
C               1

It would look something like this if dplyr could negative select (can it?).

> DF %>% group_by(!Category) %>% summarise(avg = mean(Value))
!Category    avg
A            2.00               #average of all rows where category isn't A
B            2.50
C            2.67

Answer 1

Here's a way you could do it in base R:

edit: thanks for suggesting an extensible change @Ryan

> sapply(levels(DF$Category), FUN = function(x) mean(subset(DF, Category != x)$Value))

       A        B        C 
2.000000 2.500000 2.666667 

Answer 2

Using data.table we can try:

library(data.table)
setDT(DF)[, DF[!Category %in% .BY[[1]], mean(Value)], by = Category]
#   Category       V1
#1:        A 2.000000
#2:        B 2.500000
#3:        C 2.666667