dplyr - Subset dataframe by unique values within a column in R

Question

Hello I have a dataframe such as

Group COL1 Event 
G1 SP1  1
G1 SP2  1
G1 SP3  2
G1 SP3  2 
G2 SP4  3
G2 SP7  3
G2 SP5  6
G3 SP1  1 
G4 SP1  6  

And I want to keep only COL1 if Event is unique (so here for exemple SP3 and SP5 are unique within the column Event).

Then I should get :

Group COL1 Event 
G1 SP3  2
G1 SP3  2 
G2 SP5  6 
G3 SP1  1 
G4 SP1  6 

SP1 and SP2 were 2 in column Event1 so they do not pass

SP4 and SP7 were 2 in column Event3 so they do not pass

question from:https://stackoverflow.com/questions/65902624/subset-dataframe-by-unique-values-within-a-column-in-r

Answer 1

You can use data.table to group by Group and Event and only return the group contents (.SD) if the number of unique COL1 values (uniqueN(COL1)) is 1.

library(data.table)
setDT(df)

df[, if(uniqueN(COL1) == 1) .SD, by = .(Group, Event)]
#    Group Event COL1
# 1:    G1     2  SP3
# 2:    G1     2  SP3
# 3:    G2     6  SP5
# 4:    G3     1  SP1
# 5:    G4     6  SP1

Data used:

df <- fread('
Group COL1 Event 
G1 SP1  1
G1 SP2  1
G1 SP3  2
G1 SP3  2 
G2 SP4  3
G2 SP7  3
G2 SP5  6
G3 SP1  1 
G4 SP1  6  
')