Sample random rows within each group in a data.table
data.table, r
Solution
Maybe something like this?
> DT[,.SD[sample(.N, min(3,.N))],by = a]
a b
1: 1 744
2: 1 497
3: 1 167
4: 2 888
5: 2 950
6: 2 343
(Thanks to Josh for the correction, below.)
Problem
How would you use data.table to efficiently take a sample of rows within each group in a data frame? ``` DT = data.table(a = sample(1:2), b = sample(1:1000,20)) DT a b 1: 2 562 2: 1 183 3: 2 180 4: 1 874 5: 2 533 6: 1 21 7: 2 57 8: 1 20 9: 2 39 10: 1 948 11: 2 799 12: 1 893 13: 2 993 14: 1 69 15: 2 906 16: 1 347 17: 2 969 18: 1 130 19: 2 118 20: 1 732 ``` I was thinking of something like: `DT[ , sample(??, 3), by = a]` that would return a sample of three rows for each "a" (the order of the returned rows isn't significant): ``` a b 1: 2 180 2: 2 57 3: 2 799 4: 1 69 5: 1 347 6: 1 732 ```