data.table not printed when it should

[jangorecki]

Following code will not print DT in the first attempt.

library(data.table)
library(jsonlite)
lot.dict.new <- setkeyv(setDT(fromJSON("https://api.hitbtc.com/api/1/public/symbols")[["symbols"]]),"symbol")[,c("step","lot","takeLiquidityRate","provideLiquidityRate") := lapply(.SD, as.numeric), .SDcols=c("step","lot","takeLiquidityRate","provideLiquidityRate")]
lot.dict.new # there is no print here
lot.dict.new # it works when trying for a second time

it is reproducible on my machine:

R version 3.1.2 (2014-10-31)
Platform: x86_64-pc-linux-gnu (64-bit)
data.table 1.9.5 as of 2014-11-02

[arunsrinivasan]

@jangorecki this is explained in the first bug fix under README.

if (TRUE) DT[,LHS:=RHS] no longer prints, #869. 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 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][]. Thanks to Jureiss for reporting.

[jangorecki]

ok, thanks, might be worth to note somewhere in the FAQ

[arunsrinivasan]

@jangorecki, good point. Will keep this open until then.

[gsee]

For me, having to wrap invisible around DT[, LHS:=RHS] in an if statement was less inconenient than having to add brackets in 300 functions in one of my packages so that they'll print like they have in the past. Just my 2 cents, and I realize other users may prefer the new behavior.

[arunsrinivasan]

Moved to 1.9.8

[franknarf1]

Same with merge:

merge(data.table(a=1,b=2),data.table(a=1,d=2), by="a")
# prints nothing

(I'm assuming this is the same issue based on the title and its still being open.)