How do I mark duplicates in a new column

Question

I would like to mark my duplicated values with respect to one column

Example i have a df

X    Y    Z 
1    4    5
2    5    7
1    3    6
7    2    7

then I want a new data frame df2 creating a new column dup which indicates whether X is duplicated or not as

X    Y    Z   dup
1    4    5   TRUE
2    5    7   FALSE
1    3    6   TRUE
7    2    7   FALSE

Any who could tell me how to do it?

Answer 1

You can do that with data.table, grouping by your common field and checking you have more than one row for each group:

library(data.table)
dt <- fread("X    Y    Z 
1    4    5
2    5    7
1    3    6
7    2    7")

dt[, dup := .N > 1, by = X]

   X Y Z   dup
1: 1 4 5  TRUE
2: 2 5 7 FALSE
3: 1 3 6  TRUE
4: 7 2 7 FALSE

Answer 2

Here's a method using ave():

df$dup <- ave(df$X, df$X, FUN = length) > 1L;
df;
##   X Y Z   dup
## 1 1 4 5  TRUE
## 2 2 5 7 FALSE
## 3 1 3 6  TRUE
## 4 7 2 7 FALSE

Answer 3

Using duplicated from base R:

df2 <- df
df2$dup <- c(duplicated(df2$X, fromLast = TRUE)  | duplicated(df2$X))