[datatable-help] possible FR: in x[y], switch to nomatch=0 instead of failing with "Error in vecseq..."

[Frank Erickson]

I don't know if this error shows up in other cases, but I always see it
when I'm about to do

x[y,b:=b]

but first want to check how

x[y]

looks before creating or overwriting x$b. Here's an example:

x <- data.table(a=rep(2:3,2),key='a')
y <- data.table(a=1:4,b=4:1,key='a')

x[y]           # error
x[y,nomatch=0] # ok
x[y,b:=b]      # ok

I'd prefer to see the first attempt mapped to the second (with a suitable
message), instead of erroring out. What do you all think? Is that
reasonable/worthwhile?

Best,

Frank

P.S. One other point, regarding the message itself (reproduced down below):
I don't understand why repeated values in i are mentioned.

-- For x[y] in my example, the problem seems to be coming from x having
repeated rows, not i (y in this case);
-- whereas y[x] works just fine (despite the repeated/duplicated values in
i...which is x here).

Error in vecseq(f__, len__, if (allow.cartesian) NULL else
as.integer(max(nrow(x),  :
  Join results in 6 rows; more than 4 = max(nrow(x),nrow(i)). Check for
duplicate key values in i, each of which join to the same group in x over
and over again. If that's ok, try including `j` and dropping `by`
(by-without-by) so that j runs for each group to avoid the large
allocation. If you are sure you wish to proceed, rerun with
allow.cartesian=TRUE. Otherwise, please search for this error message in
the FAQ, Wiki, Stack Overflow and datatable-help for advice.
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