R dplyr replace "missing" column data with first non-"missing" value

Question

This is a tricky one to describe concisely in a headline (or to google). I have a taxonomy table where some columns may be listed as "dropped' based on a confidence level. I'd like to replace any column that says "dropped" with "Unidentified" followed by the value from the first column that doesn't say "dropped", in a row-wise fashion. So, the input would look like this:

#> # A tibble: 21 x 4
#>    domain    class       order           species
#>    <chr>     <chr>       <chr>           <chr>  
#>  1 Eukaryota dropped     dropped         dropped
#>  2 Eukaryota dropped     dropped         dropped
#>  3 Eukaryota dropped     dropped         dropped
#>  4 Eukaryota dropped     dropped         dropped
#>  5 Eukaryota dropped     dropped         dropped
#>  6 Eukaryota dropped     dropped         dropped
#>  7 Eukaryota Hexanauplia Calanoida       dropped
#>  8 Eukaryota dropped     dropped         dropped
#>  9 Eukaryota Dinophyceae Syndiniales     dropped
#> 10 Animals   Polychaeta  Terebellida     dropped
#> 11 Eukaryota Acantharia  Chaunacanthida  dropped
#> 12 Eukaryota dropped     dropped         dropped
#> 13 Animals   Ascidiacea  Stolidobranchia dropped
#> 14 Eukaryota Haptophyta  dropped         dropped
#> 15 Eukaryota dropped     dropped         dropped
#> 16 Eukaryota dropped     dropped         dropped
#> 17 Eukaryota dropped     dropped         dropped
#> 18 Animals   Ascidiacea  Stolidobranchia dropped
#> 19 Eukaryota dropped     dropped         dropped
#> 20 Eukaryota dropped     dropped         dropped

And the output should look like this:

#> # A tibble: 21 x 4
#>    domain    class                order                species                  
#>    <chr>     <chr>                <chr>                <chr>                    
#>  1 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  2 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  3 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  4 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  5 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  6 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  7 Eukaryota Hexanauplia          Calanoida            Unidentified Calanoida   
#>  8 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  9 Eukaryota Dinophyceae          Syndiniales          Unidentified Syndiniales 
#> 10 Animals   Polychaeta           Terebellida          Unidentified Terebellida 
#> 11 Eukaryota Acantharia           Chaunacanthida       Unidentified Chaunacanth…
#> 12 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#> 13 Animals   Ascidiacea           Stolidobranchia      Unidentified Stolidobran…
#> 14 Eukaryota Haptophyta           Unidentified Haptop… Unidentified Haptophyta  
#> 15 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#> 16 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#> 17 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#> 18 Animals   Ascidiacea           Stolidobranchia      Unidentified Stolidobran…
#> 19 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#> 20 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   

I've come up with a fine solution using purrr::pmap_dfr but I'm curious to know if there's a more "pure" dplyr way to do it? The one flaw in my method is that it doesn't work for columns where the first non-"dropped" column comes after one or more "dropped" columns (see row 21 in the output below). Here's my current solution:

library(tidyverse)
otu_table <- structure(list(domain = c("Eukaryota", "Eukaryota", "Eukaryota", 
"Eukaryota", "Eukaryota", "Eukaryota", "Eukaryota", "Eukaryota", 
"Eukaryota", "Animals", "Eukaryota", "Eukaryota", "Animals", 
"Eukaryota", "Eukaryota", "Eukaryota", "Eukaryota", "Animals", 
"Eukaryota", "Eukaryota", "dropped"), class = c("dropped", "dropped", 
"dropped", "dropped", "dropped", "dropped", "Hexanauplia", "dropped", 
"Dinophyceae", "Polychaeta", "Acantharia", "dropped", "Ascidiacea", 
"Haptophyta", "dropped", "dropped", "dropped", "Ascidiacea", 
"dropped", "dropped", "not dropped"), order = c("dropped", "dropped", 
"dropped", "dropped", "dropped", "dropped", "Calanoida", "dropped", 
"Syndiniales", "Terebellida", "Chaunacanthida", "dropped", "Stolidobranchia", 
"dropped", "dropped", "dropped", "dropped", "Stolidobranchia", 
"dropped", "dropped", "dropped"), species = c("dropped", "dropped", 
"dropped", "dropped", "dropped", "dropped", "dropped", "dropped", 
"dropped", "dropped", "dropped", "dropped", "dropped", "dropped", 
"dropped", "dropped", "dropped", "dropped", "dropped", "dropped", 
"dropped")), row.names = c(NA, -21L), class = c("tbl_df", "tbl", 
"data.frame"))

tax_data <- otu_table %>%
  pmap_dfr(~{
    items <- list(...)
    first_dropped = match("dropped",items)
    if (first_dropped > 1) {
      dropped_name <- str_c("Unidentified ",items[first_dropped-1])
    } else {
      dropped_name <- "Unidentified"
    }
    items[-c(1:first_dropped-1)] <- dropped_name
    items
  })
print(tax_data,n=30)
#> # A tibble: 21 x 4
#>    domain    class                order                species                  
#>    <chr>     <chr>                <chr>                <chr>                    
#>  1 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  2 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  3 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  4 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  5 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  6 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  7 Eukaryota Hexanauplia          Calanoida            Unidentified Calanoida   
#>  8 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#>  9 Eukaryota Dinophyceae          Syndiniales          Unidentified Syndiniales 
#> 10 Animals   Polychaeta           Terebellida          Unidentified Terebellida 
#> 11 Eukaryota Acantharia           Chaunacanthida       Unidentified Chaunacanth…
#> 12 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#> 13 Animals   Ascidiacea           Stolidobranchia      Unidentified Stolidobran…
#> 14 Eukaryota Haptophyta           Unidentified Haptop… Unidentified Haptophyta  
#> 15 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#> 16 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#> 17 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#> 18 Animals   Ascidiacea           Stolidobranchia      Unidentified Stolidobran…
#> 19 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#> 20 Eukaryota Unidentified Eukary… Unidentified Eukary… Unidentified Eukaryota   
#> 21 dropped   not dropped          dropped              dropped

Update:

Some good answers below. I've accepted the one with the most upvotes, but it turns out that after running all the suggestions through microbenchmark, the purrr solution is the fastest by almost an order of magnitude.

Answer 1

Here's another approach, using rowwise() in combination with across().

• We are using rowwise because it helps in using a row as a single vector through cur_data()
• across(everything(), ~) helps us in mutating all columns at once
• max.col(cur_data() != 'dropped', ties.method = 'last') will retrieve last column index where the value != 'dropped'
• we store its column name in a temp variable say x
• lastly we use if()..else from base R to mutate only those columns where value is dropped

Hope the answer is clear enough

library(tidyverse)

otu_table %>% rowwise() %>%
  mutate(across(everything(), ~ {x<- names(cur_data())[max.col(cur_data() != 'dropped', ties.method = 'last')]; 
  if (. == 'dropped') paste0('unidentified ', get(x)) else . }))

#> # A tibble: 21 x 4
#> # Rowwise: 
#>    domain    class                 order                 species                
#>    <chr>     <chr>                 <chr>                 <chr>                  
#>  1 Eukaryota unidentified Eukaryo~ unidentified Eukaryo~ unidentified Eukaryota 
#>  2 Eukaryota unidentified Eukaryo~ unidentified Eukaryo~ unidentified Eukaryota 
#>  3 Eukaryota unidentified Eukaryo~ unidentified Eukaryo~ unidentified Eukaryota 
#>  4 Eukaryota unidentified Eukaryo~ unidentified Eukaryo~ unidentified Eukaryota 
#>  5 Eukaryota unidentified Eukaryo~ unidentified Eukaryo~ unidentified Eukaryota 
#>  6 Eukaryota unidentified Eukaryo~ unidentified Eukaryo~ unidentified Eukaryota 
#>  7 Eukaryota Hexanauplia           Calanoida             unidentified Calanoida 
#>  8 Eukaryota unidentified Eukaryo~ unidentified Eukaryo~ unidentified Eukaryota 
#>  9 Eukaryota Dinophyceae           Syndiniales           unidentified Syndinial~
#> 10 Animals   Polychaeta            Terebellida           unidentified Terebelli~
#> # ... with 11 more rows

^{Created on 2021-06-19 by the reprex package (v2.0.0)}