dcast with empty data.table

[dbetebenner]

I see dcast throws an error when one submits an empty data.table. This appear to be consistent with an error that reshape2:::dcast throws with an empty data.frame, but inconsistent with how reshape deals with empty data.frames (returns an empty data.frame).

Any possibility for changing the dcast behavior to return an empty data.table?

Here's an example:

> require(data.table)
Loading required package: data.table
data.table 1.9.5  For help type ?data.table or https://github.com/Rdatatable/data.table/wiki
> test.dt <- data.table(ID=rep(1:2, each=2), X=rnorm(4), TIME=rep(1:2, 2))
> test.dt


   ID         X TIME
1:  1 0.5567229    1
2:  1 0.8330417    2
3:  2 0.2159679    1
4:  2 0.3763645    2

Non-empty

> reshape(test.dt, idvar="ID", timevar="TIME", direction="wide")
   ID       X.1       X.2
1:  1 0.5567229 0.8330417
2:  2 0.2159679 0.3763645

> dcast(test.dt, ID ~ TIME, value.var="X")
   ID         1         2
1:  1 0.5567229 0.8330417
2:  2 0.2159679 0.3763645

Empty

> reshape(subset(test.dt, ID==3), idvar="ID", timevar="TIME", direction="wide")
Empty data.table (0 rows) of 1 col: ID```

> dcast(subset(test.dt, ID==3), ID ~ TIME, value.var="X")
Error in dcast.data.table(subset(test.dt, ID == 3), ID ~ TIME, value.var = "X") : 
  Can not cast an empty data.table

[arunsrinivasan]

I agree with base::reshape's behaviour. However, data.table::dcast has to be consistent with reshape2::dcast, not base::reshape. If rehape2::dcast, changes its behaviour, then yes. But I doubt it's possible anymore.

[dbetebenner]

Thanks for responding Arun,

That makes sense. I've created a wrapper function to accomplish what I needed.

Best

[arunsrinivasan]

👍

[jangorecki]

@arunsrinivasan do we need to be consistent to reshape2 here? we already shifting away from reshape2 recently, see #3549
This SO https://stackoverflow.com/questions/57722355/r-shiny-is-breaking-down-by-using-dcast-from-data-table
is another example that it would be useful.
Feel free to close if you don't agree.