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in my function, I modify column(s) of a data.table given as argument. I have noticed that if I then try to display this data.table (I mean, just by its name) right after calling the function, nothing happens, and only subsequent calls display the table:

DT <- data.table(colA=1:4, colB=4:1) # sample table
modcol <- function(dtIn){
 dtIn[, colA:=NULL];
 return(TRUE);
}

DT # display table before any changes:
#   colA colB
#1:    1    4
#2:    2    3
#3:    3    2
#4:    4    1

modcol(DT) # run our function
#[1] TRUE

DT # silent!
DT # only second call display modified table
#   colB
#1:    4
#2:    3
#3:    2
#4:    1

it only happens when my function returns a value (doesn't matter, by return() or by invisible()) and only if table content is modified (if, for example, instead of deleting a column I change columns names - this effect does not occur). This behavior is not really making any problems for me, but I am still curious why does it happen.

Vasily A
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1 Answers1

12

If you are using v1.9.6, see the corresponding Readme (sec. Bugfixes, 1st entry, https://github.com/Rdatatable/data.table):

if (TRUE) DT[,LHS:=RHS] no longer prints, #869 and #1122. Tests added. To get this to work we've had to live with one downside: if a := is used inside a function with no DT[] before the end of the function, then the next time DT or print(DT) is typed at the prompt, nothing will be printed. A repeated DT or print(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 DT[] at the prompt is guaranteed to print. As before, adding an extra [] on the end of a := query is a recommended idiom to update and then print; e.g. > DT[,foo:=3L][]. Thanks to Jureiss and Jan Gorecki for reporting.

Thus: Does calling DT[] after your function call help?

Helix123
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