[R] which element is duplicated?

[Duncan Murdoch]

On 13/11/2018 12:35 AM, Pages, Herve wrote:
> Hi,
> 
> On 11/12/18 17:08, Duncan Murdoch wrote:
>> The duplicated() function gives TRUE if an item in a vector (or row in
>> a matrix, etc.) is a duplicate of an earlier item.  But what I would
>> like to know is which item does it duplicate?
>>
>> For example,
>>
>> v <- c("a", "b", "b", "a")
>> duplicated(v)
>>
>> returns
>>
>> [1] FALSE FALSE  TRUE  TRUE
>>
>> What I want is a fast way to calculate
>>
>>   [1] NA NA 2 1
>>
>> or (equally useful to me)
>>
>>   [1] 1 2 2 1
>>
>> The result should have the property that if result[i] == j, then v[i]
>> == v[j], at least for i != j.
>>
>> Does this already exist somewhere, or is it easy to write?
> 
> I generally use match() for that:
> 
>   > v <- c("a", "b", "b", "a")
> 
>   > match(v, v)
> 
> [1] 1 2 2 1

Yes, this is perfect.  Thanks to you (and the private answer I received 
that suggested the same).

Duncan Murdoch