Fill down column based on values in another

Question

I have a df:

ID  Tag
999 YES
999 
100
100 NO
500

I need to fill the 'Tag' values to the matching ID variable - so that every instance of that ID has a matching Tag So I'll end up with:

ID  Tag
999 YES
999 YES
100 NO 
100 NO
500 

Answer 1

You can use ave to fill na per group.

df$Tag  <- with(df, ave(Tag, ID, FUN=function(x) {
  i <- is.na(x)
  x[i]  <- x[!i][1]
  x
}))
df
#   ID  Tag
#1 999  YES
#2 999  YES
#3 100   NO
#4 100   NO
#5 500 <NA>

Or using fill

library(dplyr)
library(tidyr)
df %>% group_by(ID) %>% fill(Tag, .direction = "downup")
## A tibble: 5 x 2
## Groups:   ID [3]
#     ID Tag  
#  <int> <fct>
#1   999 YES  
#2   999 YES  
#3   100 NO   
#4   100 NO   
#5   500 NA