Let’s say I have a data as below:

test_df_4 <- data.frame(inner_obj= c(1, 3, 2, 6, 5),
                        outer_obj = c(3, 4, 1, 7, 6))

I see there two separate objects, one consist of: 1, 2, 3, 4 and second one is: 5, 6, 7. My understanding is that 2 goes into 1, then 1 goes into 3 and finally 3 goes into 4. For the second object it is: 5 goes into 6 and 6 goes into 7. In my case these are code blocks nested in each other, so to compute 4, I need to start from 2, then 1, then 3, then 4. For the second object: I need to start from 5, then 6, then 7.

I would like to get a list with this order – for the first object it should be 2, 1, 3, 4 and for the second: 5, 6, 7.

I know how I could find a starting points (roots):

root <- igraph::degree(igraph::graph_from_data_frame(test_df_4), mode = "in")
  root <- as.integer(names(root)[which(root == 0)])

But I still have a problem what function should I use to get the desired order. I would prefer to use igraph package.