I have a data frame:

set.seed(100)
A <- floor(runif(5, min=0, max=10))
B <- floor(runif(5, min=0, max=10))
C <- floor(runif(5, min=0, max=10))
D <- floor(runif(5, min=0, max=10))
df <- data.frame(A,B,C,D)
df$ms <- rowMeans(df)
df
  A B C D   ms
1 3 4 6 6 4.75
2 2 8 8 2 5.00
3 5 3 2 3 3.25
4 0 5 3 3 2.75
5 4 1 7 6 4.50

Now I’d like to add columns (lower and greater) with column names when the value in particular row is lower in columns A and B than mean and greater in columns C and D also than mean. Desired result:

  A B C D   ms  lower greater
1 3 4 6 6 4.75  A,B   C,D
2 2 8 8 2 5.00  A     C
3 5 3 2 3 3.25  B     NA
4 0 5 3 3 2.75  A     NA
5 4 1 7 6 4.50  A,B   C,D

I was trying to do this with which() however I stuck, could you please give me a hint?

lapply(apply(df,1, function(x) which(df$ms)),names)