Exclude specific value from mutate w/ ifelse

Question

I am sure this is a simple question that has been asked before but the way I am phrasing it may be preventing me from getting the answers. I'm very new at programming and don't know the language very well (or how to code at all).

I am gradually editing Dataset A by matching values of one of its columns to columns of other datasets. On a match, I am writing "E" in column "Existing" of Dataset A. I am using the dplyr package with mutate and ifelse, knowledge of which I got from more experienced users in this community.

This is an example of the code I am using:

DatasetA <- DatasetA %>% mutate(Existing = ifelse(ID %in% DatasetB$ID, 'E',NA))

Since I am doing this more than once, my first thought was to just copy-paste the same code and change the tables and columns I need. Of course, eventually I realised this overwrites the previous "E"s and "NA"s.

I need to change the code so that if there is an "E" in the column already, it is not touched by mutate. It just adds new "E"s.

Answer 1

This will preserve the E of ID 3, even if its not part of DatasetB$ID, but because it was E in the first place:

library(tidyverse)

DatasetB <- tibble(ID = seq(2))
DatasetB
#> # A tibble: 2 x 1
#>      ID
#>   <int>
#> 1     1
#> 2     2
DatasetA <- tibble(ID = c(2, 3, 4), Existing = c(NA, "E", "A"))
DatasetA
#> # A tibble: 3 x 2
#>      ID Existing
#>   <dbl> <chr>   
#> 1     2 <NA>    
#> 2     3 E       
#> 3     4 A

DatasetA %>%
  mutate(
    Existing = case_when(
      # preserve just E
      Existing == "E" ~ "E",
      ID %in% DatasetB$ID ~ "E"
    )
  )
#> # A tibble: 3 x 2
#>      ID Existing
#>   <dbl> <chr>   
#> 1     2 E       
#> 2     3 E       
#> 3     4 <NA>


DatasetA %>%
  mutate(
    Existing = case_when(
      # preserve any non NA character
      !is.na(Existing) ~ Existing,
      ID %in% DatasetB$ID ~ "E"
    )
  )
#> # A tibble: 3 x 2
#>      ID Existing
#>   <dbl> <chr>   
#> 1     2 E       
#> 2     3 E       
#> 3     4 A

case_when is a more flexible way than using ifelse which can handle multiple cases / conditions.