I have two data.tables:
DT1 <- data.table(A=c('A', 'B', 'C'), idx=c(1,2,3))
DT2 <- data.table(idx=c(1,2,3), A=rep('foo', 3), B=rep('bar', 3), C=rep('baz', 3))
> DT1
A idx
1: A 1
2: B 2
3: C 3
> DT2
idx A B C
1: 1 foo bar baz
2: 2 foo bar baz
3: 3 foo bar baz
And I want to get to this:
> DT3
idx value
1: 1 foo
2: 2 bar
3: 3 baz
Basically I want to merge DT1 and DT2 on idx, but I only want the column from DT2 that corresponds to the value of A in the row of DT1. both DT1 and DT2 have the same number of rows and are in the rows are in same order
Is there a data.table native way to do this?
Updated answer following the old (implicit) by-without-by feature being replaced with by=.EACHI (and also using on= argument instead of setting key:
require(data.table) # v1.9.6+
DT2[DT1, .(value=get(i.A)), on="idx", by=.EACHI]
# idx value
# 1: 1 foo
# 2: 2 bar
# 3: 3 baz
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