Calculate mean difference per row and per group

Question

I have a data.frame with many rows and columns and I want to calculate the mean difference of each value to each of the other values within a group.
Here an exa

Answer 1

Here's a solution using only base R:

myData <- data.frame(ID=c(1,1,1,2,2,2,2), value=c(4,5,7,8,6,5,6), diff=NA)
for(i in 1:nrow(myData))
    myData$diff[i] <- with(data = myData,
        sum((value[i] - value[ID==ID[i]])**2)/length(value[ID==ID[i]]))

myData

  ID value     diff
1  1     4 3.333333
2  1     5 1.666667
3  1     7 4.333333
4  2     8 4.250000
5  2     6 1.250000
6  2     5 2.750000
7  2     6 1.250000