如何将列中的唯一值从 data.table 子集到新的 data.table

Question

I have a data.table that I am wanting to subset and create another data.table based on the Counter variable.

The pseudo code would be create a new data.table of only unique Subject from old data.table where Counter is = 5

new_data <- old_data[Counter == 5, ]

but will this give me every time the counter reaches 5, I need it to only give it to me the first time

Old data.table:
+---------------+------------+--------------+
| Date          | Subject    | Counter      | 
+---------------+------------+--------------+
| 10-21-20      |   a        |   0          |  
+---------------+------------+--------------+
| 10-22-20      |   a        |   1          |   
+---------------+------------+--------------+
| 10-23-20      |   a        |   2          |  
+---------------+------------+--------------+
| 10-21-20      |   b        |   0          |   
+---------------+------------+--------------+
| 10-26-20      |   b        |   5          |   
+---------------+------------+--------------+
| 11-2-20       |   b        |   5          | 
+---------------+------------+--------------+  
| 11-7-20       |   b        |   5          | 
+---------------+------------+--------------+ 

New data.table
+---------------+------------+--------------+
| 10-26-20      |   b        |   5          |   
+---------------+------------+--------------+

Answer 1

We can wrap with unique to get the first row by the 'Subject'

library(data.table)
unique(old_data[Counter == 5, ], by = c('Subject'))

---

Or another option is match where match returns the index of the first row that matches and use that to subset

old_data[match(5, Counter)]