Strange behavior on dt[J(d2), a := b, nomatch = 0]

[riabusan]

Hi,

I am using a magnificent data.table package version 1.9.4. Thanks a lot for such useful software masterpiece.

In example below, data.table complains that columns o and n are of different length while I don't see a reason why they should be different.

library(data.table)

tr = data.table(k = c(0, 2, 3, 7), o = "b", key = "k")
en = data.table(k = c(1, 2, 3, 4, 5), n = c("n1", "n2", "n3", "n4", "n5"), key = "k")

tr_copy = copy(tr)

## Gives a warning about recycling and produces wrong result, 
## why `o` and `n` of a different length, aren't they supposed to be equal after join?
tr_copy[en, o := n, nomatch = 0]

## Warning message: In `[.data.table`(tr_copy, en, `:=`(o, n), nomatch = 0) :  Supplied 2 items to be assigned to 5 items of column 'o' (recycled leaving remainder of 1 items).

#######################################################
#######################################################

tr_copy = copy(tr)

## No error and gives correct result, seems counterintuitive
tr[en, o := n, nomatch = NA]

[jangorecki]

Good MRE! same on 1.9.5.

[cguill95]

Hi,

I am also extensively using this wonderful package and I encoutered the same kind of strange behaviour, but in addition to what riabusan said I would add two things :
1) First, sometimes you don't even get a warning which makes it really tricky. For instance if I slightly modify riabusan's example by adding 0, 6, 7 and the corresponding n0, n6, n7 into his "en" variable it gives :

library(data.table)
tr = data.table(k = c(0, 2, 3, 7), o = "b", key = "k")
en = data.table(k = c(0, 1, 2, 3, 4, 5, 6, 7), n = c("n0", "n1", "n2", "n3", "n4", "n5", "n6", "n7"), key = "k")
tr[en, "o":=n, nomatch=0L]

> tr
   k  o
1: 0 n0
2: 2 n3
3: 3 n7
4: 7 n7

2. Second, for some weird reason when you add have a duplicate key in the "tr" variable it works just fine (I duplicated the 2 here)

library(data.table)
tr = data.table(k = c(0, 2, 2, 3, 7), o = "b", key = "k")
en = data.table(k = c(0, 1, 2, 3, 4, 5, 6, 7), n = c("n0", "n1", "n2", "n3", "n4", "n5", "n6", "n7"), key = "k")
tr[en, "o":=n, nomatch=0L]

> tr
   k  o
1: 0 n0
2: 2 n2
3: 2 n2
4: 3 n3
5: 7 n7

[arunsrinivasan]

Thanks for the reports. Now fixed, although (the current behaviour of) nomatch=0L has no effect with :=.