R: Compare a row to all the rows that come before it-CodePudding

I have a dataframe that looks something like the following. For each row within a group, I would like to compare the ID to the ID of all rows before it. In a new column, the value would be "TRUE" if the ID of the row is the same to the ID of any row that comes before it, and "FALSE" if it is not the same to any row before it.

group    id      order
1 g1    27391    1   
2 g1    29381    2   
3 g2    27391    3   
4 g2    48401    1   
5 g2    27391    2   
6 g2    27391    3   
7 g2    48401    4   
8 g3    27391    1  
8 g3    48401    2

So the output would look something like:

group    id      order  same?
1 g1    27391    1      NA
2 g1    29381    2      FALSE
3 g2    27391    3      TRUE
4 g2    48401    1      NA
5 g2    27391    2      FALSE
6 g2    27391    3      TRUE
7 g2    48401    4      TRUE
8 g3    27391    1      NA
8 g3    48401    2      FALSE

CodePudding user response：

We may group by 'group', use duplicated on the 'id', replace the first element with NA and ungroup

library(dplyr)
dat %>%
   group_by(group) %>% 
   mutate(same = replace(duplicated(id), 1, NA)) %>%
   ungroup