Affect a value without replacing if the value is not empty

[laplanca]

Hi! I have a column to class my data in different categories like 'AB', 'KD', 'other'.
But in my column "Class", some rows were already classified before as 'classb' for example and I don't want the values to be replaced. I don't know how to do. I do my classification with

mutate(class = case_when(cond1 ~ "AB",...))

To illustrate I have this :
Id / result / difftime / class
12 / 2 / 12 /
45 / 1 / 24 / classb
33 / 1 / 12 /

and I want this :
Id / result / difftime / class
12 / 2 / 12 / KD
45 / 1 / 24 / classb
33 / 1 / 12 / AB

instead of this :
Id / result / difftime / class
12 / 2 / 12 / KD
45 / 1 / 24 / AB
33 / 1 / 12 / AB

I would like to specify that I try to add as a condition class == 'class2' ~ 'class2' to keep the same value but it doesn't worked.
I hope I was clear. Thank you for your time.

[pieterjanvc]

Hello,

Here is a way of doing this

library(dplyr)

df = data.frame(
  id = 1:5,
  class = NA
)

df$class[3] = "classb"
df
#>   id  class
#> 1  1   <NA>
#> 2  2   <NA>
#> 3  3 classb
#> 4  4   <NA>
#> 5  5   <NA>

df %>% mutate(class = case_when(
  class == "classb" ~ class,
  id > 4 ~ "X",
  TRUE ~ 'Y'
))
#>   id  class
#> 1  1      Y
#> 2  2      Y
#> 3  3 classb
#> 4  4      Y
#> 5  5      X

^{Created on 2022-03-31 by the reprex package (v2.0.1)}

Basically, when you don't want the result to change, you put the same variable as output

Hope this helps,
PJ