Akhil Nair - 2 months ago 19

R Question

Can I filter by a list-column in a

`data.table`

`library(data.table)`

dt = data.table(a = c(1, 2, 3), b = list(c("A", "B"), character(0), c("C", "D", "E")))

> dt

a b

1: 1 1,2

2: 2

3: 3 1,2,3

i.e. the expected result is

`> dt[filter(b)]`

a b

1: 2

The obvious filtering doesn't work

`> dt[length(b) == 0]`

Empty data.table (0 rows) of 2 cols: a,b

> dt[length(b[[1]]) == 0]

Empty data.table (0 rows) of 2 cols: a,b

I thought I might be able to define a function to result in the right boolean value, but as I have to use a group by to make it actually work, it doesn't work in the filter argument

`is_null_list = function(l) is.list(l) & length(l[[1]]) == 0`

> dt[, is_null_list(b), a]

a V1

1: 1 FALSE

2: 2 TRUE

3: 3 FALSE

> dt[is_null_list(b)]

Empty data.table (0 rows) of 2 cols: a,b

I guess the more general question is also, can any filtering be done on

`data.table`

Thanks

Answer

You can filter by the length of each element of a list column with `lengths`

. For example,

```
dt[ lengths(b) == 0L ]
```