Subsetting data.table with a condition

Question

How to sample a subsample of large data.table (`data.table` package)? Is there more elegant way to perform the following ``` DT<- data.table(cbind(site = rep(letters[1:2], 1000), value = runif(2000))) DT[site=="a"][sample(1:nrow(DT[site=="a"]), 100)] ``` Guess there is a simple solution, but can't choose the right wording to search for. UPDATE: More generally, how can I access a row number in data.table's `i` argument without creating temporary column for row number?

Accepted Answer

One of the biggest benefits of using `data.table` is that you can set a key for your data. Using the `key` and then `.I` (a built in vairable. see `?data.table` for more info) you can use:

```
setkey(DT, site)
DT[DT["a", sample(.I, 100)]] 
```

As for your second question `"how can I access a row number in data.table's i argument"`

```
# Just use the number directly:
DT[17]
```

Subsetting data.table with a condition

Solution

Problem