Identify the first instance of a duplicate in a column

Question

I have a column named 'A'

A
Dog
Cat
Dog
Sheep

I want to create a new column, called 'Keep' that has a 1 for the first instance of a duplicate as well as all unique values

A	Keep
Dog	1
Cat	1
Dog	0
Sheep	1

Answer 1

We can use duplicated :

df$Keep <- as.integer(!duplicated(df$A))

If you want to do this using data.table :

library(data.table)
setDT(df)[, Keep := as.integer(!duplicated(A))]
df

#       A Keep
#1:   Dog    1
#2:   Cat    1
#3:   Dog    0
#4: Sheep    1

data

df <- structure(list(A = c("Dog", "Cat", "Dog", "Sheep")), 
      class = "data.frame", row.names = c(NA, -4L))