Error when using “diff” function inside of dplyr mutate

Question

I try to mutate new column to data.frame. When V column order changes from decreasing to increasing order, I use diff function inside

Answer 1

We need to concatenate with one more value to make the length equal as diff returns with a length one less than the length of the group. i.e.

length(df$V)
#[1] 30
length(diff(df$V))
#[1] 29

So, we concatenate with a dummy number at the beginning to make the length equal.

 df %>%
   group_by(gr) %>%
   mutate(H=ifelse(c(0,diff(V))<0,"back","forward"))

If we need the first value to be 'back', change the condition to <=0