I have a dataframe with missing NA values in column X1 and a grouping variable group. I want to replace all NA values with a value sampled from the non-NA values of that group. This should be done for all groups except for one (group==C). For this conditional replacement with resample data I tried if/else and case_when within the mutate command of dplyr, however without success. I guess this is because the TRUE and FALSE are both evaluated before assessing the condition. (The case_when condition works and selects the appropriate cases as shown when calculating X2, however using the sample command causes a problem.)

#Original dataframe
df <- 
      data.frame(
      id = 1:10,
      group = c(rep("A",5),rep("B",4),"C"),
      X1 = c(NA, 2, 1, NA,4, 3, NA, 8, 9, NA))%>%
      group_by(group)%>%
      mutate(X2 = case_when(is.na(X1)&group!="C"~3,
                                             TRUE~2))
    
    # Approach with if else (doesn't work)
    df%>%
      mutate(X3 = if(is.na(X1)&group!="C") sample(X1[!is.na(X1)],size=n(), replace = TRUE) else X1)

    # Approach with case_when  (doesn't work either)
    df%>%
      mutate(X3 = case_when(is.na(X1)&group!="C"~
                              ~sample(X1[!is.na(X1)],size=n(), replace = TRUE),
                            TRUE~X1))