get the index of deleted row

Question

I have to delete a row in a dataframe and get the index of the deleted row.

In practice I click on a row of a table in a shiny app and delete it with an action button:

From this row I need the index:

Here is an example to demonstrate what I would like to do:

library(dplyr)

df1 <- diamonds %>% 
  select(1:2) %>% 
  slice(1:5)

  carat cut    
  <dbl> <ord>  
1  0.23 Ideal  
2  0.21 Premium
3  0.23 Good   
4  0.29 Premium
5  0.31 Good

From df1 I delete row 3 and get df2. But I need the index of the deleted row as vector.

df2 <- df1 %>% 
  slice(-3)

  carat cut    
  <dbl> <ord>  
1  0.23 Ideal  
2  0.21 Premium
3  0.29 Premium
4  0.31 Good 

I have tried:

anti_join(df1, df2) %>% 
  rownames_to_column("id") %>% 
  pull(id)

which gives:

[1] "1"

Expected Output

[1] "3"

Answer 1

We could create a sequence column in 'df1'

df1 <- diamonds %>% 
       select(1:2) %>% 
       slice(1:5) %>%
       mutate(rn = row_number())
df2 <- df1 %>% 
  slice(-3)

and then just pull the rn

anti_join(df1, df2) %>% 
    pull('rn')
#[1] 3

---

rownames_to_column doesn't give the correct row number because tibble doesn't allow for row names and it resets the row number on each subset, thus we get only the changed row number instead of the original one. In addition to the tibble case, here the slice does reset i.e. if we check the functions involved in it, there is a call to dplyr_new_data_frame, which may be resetting the row number

> methods("slice")
#[1] slice.data.frame* slice.index    
> getAnywhere("slice.data.frame")
function (.data, ..., .preserve = FALSE) 
{
    loc <- slice_rows(.data, ...)
    dplyr_row_slice(.data, loc, preserve = .preserve)
}
> dplyr:: dplyr_row_slice
function (data, i, ...) 
{
    if (!is.numeric(i) && !is.logical(i)) {
        abort("`i` must be an numeric or logical vector.")
    }
    UseMethod("dplyr_row_slice")
}
> methods("dplyr_row_slice")
[1] dplyr_row_slice.data.frame* dplyr_row_slice.grouped_df* dplyr_row_slice.rowwise_df*

> getAnywhere("dplyr_row_slice.data.frame")
function (data, i, ...) 
{
    dplyr_reconstruct(vec_slice(data, i), data)
}
> dplyr_reconstruct
function (data, template) 
{
    data <- dplyr_new_data_frame(data)
    return(dplyr_reconstruct_dispatch(data, template))
    UseMethod("dplyr_reconstruct", template)
}
> dplyr:::dplyr_new_data_frame
function (x = data.frame(), n = NULL, ..., row.names = NULL, 
    class = NULL) 
{
    row.names <- row.names %||% .row_names_info(x, type = 0L)
    new_data_frame(x, n = n, ..., row.names = row.names, class = class)
}