R: For loop- For each subset, add character values that incorporate the subsetted factor level and unique iD numbers

Question

Apologies for the terrible title. However, I had trouble making this particular question a concise title.

I have a data frame like so (Note: it is over 50000 rows long

Answer 1

If you can use dplyr and don't need consecutive (though unique) numbering:

df %>%
  group_by(lepfam) %>% 
  mutate(lep_species=ifelse(!is.na(lep_species), lep_species, 
         paste0(lepfam, "_morphosp", rank(lep_notes, ties.method ="min"))))

       lepfam            lep_notes         lep_species
        <chr>                <chr>               <chr>
1 Geometridae                 <NA>             Eois sp
2    Erebidae         black/orange  Erebidae_morphosp2
3    Erebidae         black spikes  Erebidae_morphosp1
4    Erebidae   redthorax/red legs  Erebidae_morphosp3
5   Noctuidae          fuzzy/green Noctuidae_morphosp2
6   Noctuidae black hair/greenbody Noctuidae_morphosp1
7   Noctuidae          fuzzy/green Noctuidae_morphosp2
8  Saturnidae                 <NA>      Polyphemous sp

Or with consecutive numbers:

df %>%
  group_by(lepfam) %>% 
  mutate(lep_species=ifelse(!is.na(lep_species), lep_species,
         paste0(lepfam, "_morphosp", match(lep_notes,unique(lep_notes)))))

       lepfam            lep_notes         lep_species
        <chr>                <chr>               <chr>
1 Geometridae                 <NA>             Eois sp
2    Erebidae         black/orange  Erebidae_morphosp1
3    Erebidae         black spikes  Erebidae_morphosp2
4    Erebidae   redthorax/red legs  Erebidae_morphosp3
5   Noctuidae          fuzzy/green Noctuidae_morphosp1
6   Noctuidae black hair/greenbody Noctuidae_morphosp2
7   Noctuidae          fuzzy/green Noctuidae_morphosp1
8  Saturnidae                 <NA>      Polyphemous sp