CODEWITHSUNDEEP
rdata.table
user123276
user123276

Reputation: 179

How can I choose the first 3 rows of a data table in data.table by group?

I currently have a dataset like:

ID    RESULTS
1     M
1     A
1     M
1     C
1     B
2     Q
2     E
2     S
2     G
2     Z
......

From this, I would like to keep the first 3 rows, by group. Meaning, I'd like:

ID    RESULTS
1     M
1     A
1     M
2     Q
2     E
2     S

I dug around in data.table, the closest I found was using something like mult or .I. Does anyone have a simple workaround? Thanks!

Upvotes: 1

Views: 71

Answers (1)

DJJ
DJJ

Reputation: 2539

I would suggest a more concise way. You can have more detail with ?data.table or with example(data.table)

DT = data.table(ID=rep(c(1,2),each=5),RESULTS= 
 c("M","A","M","C","B","Q","E","S","G","Z"))

> DT[,.SD[1:3],by=ID]
##    ID RESULTS
## 1:  1       M
## 2:  1       A
## 3:  1       M
## 4:  2       Q
## 5:  2       E
## 6:  2       S

Upvotes: 2

Related Questions