First time execute a data.table it doesn't show anything.

[skanskan]

Since the a long time ago all versions of data.table I've been using have a small bug.
Sometimes when I create a data.table using a function with many steps (reading a file and manipulating the result) and I try to show it's contents it doesn't show any result nor errors.

I need to execute its name twice in order to show anything.

It happens in different computers with different R versions and different data.table versions.

Has anybody noticed the same issue?

It's not serious but it could indicate something is wrong in memory.
I'm not able to give a small reproducible example now.

PD:OK, I've got to create a reproducible example.

Create a csv file named prueba.csv with this content.

numb, first,second
1,aaa,2009-12-17
1,bbb,2009-01-28
1,www,2019-12-02
1,ddd,1999-07-01
2,aaa,1999-08-28
2,bbb,2009-05-30
3,bbb,2009-06-14
3,ccc,2009-05-30
3,aaa,1999-05-19
4,ddd,2009-03-24
4,ddd,2009-07-17
5,www,2009-02-02
6,ccc,1999-06-28
6,aaa,1979-07-01
7,bbb,2009-09-09
7,ddd,2009-07-18
8,aaa,2009-03-28
8,ccc,2009-06-08
8,ddd,2009-03-01
9,www,1999-07-17
9,ddd,2009-02-14

Now execute this lines:

lectura <- function() {
     todo <- fread("prueba.csv")  
     todo[,other:=ifelse(any(first=="aaa"),.SD[first=="aaa",first],"2020-10-31"),by=numb]
     return(todo)
}
ttt <- lectura()

Now run
ttt

You will notice it doesn't show its content.
Why?

[franknarf1]

I think this covers it: https://stackoverflow.com/questions/32988099/data-table-objects-not-printed-after-returned-from-function

[HughParsonage]

This is addressed in the FAQ: https://github.com/Rdatatable/data.table/blob/master/vignettes/datatable-faq.Rmd

Why do I have to type DT sometimes twice after using := to print the result to console?

This is an unfortunate downside to get #869 to work. If a := is used inside a function with no DT[] before the end of the function, then the next time DT is typed at the prompt, nothing will be printed. A repeated DT will print. To avoid this: include a DT[] after the last := in your function. If that is not possible (e.g., it's not a function you can change) then print(DT) and DT[] at the prompt are guaranteed to print. As before, adding an extra [] on the end of := query is a recommended idiom to update and then print; e.g.> DT[,foo:=3L][].