对于两个data.tables的特定设置，连接不会提供我期望的结果。我在代码中犯了错误，或者这是一个data.table问题吗？
请看下面的示例。
library(data.table)
# In the code below the join does not deliver the result I would expect
DT1 <- data.table(colname=c("test1","test2","test2","test3"), colname_with_suffix=c("other","test","includes test within","other"))
DT2 <- data.table(lookup=c("test1","test2","test3"), lookup_result=c(1,2,3))
DT1[colname_with_suffix == "not found", ] # automatically creates index on colname_with_suffix
DT1[DT2, lookup_result := i.lookup_result, on=c("colname"="lookup")][]
# PLEASE NOTE: same result with slightly different syntax: DT1[DT2, lookup_result := i.lookup_result, on=c(colname="lookup")][]
# colname colname_with_suffix lookup_result
# 1: test1 other NA
# 2: test2 test NA
# 3: test2 includes test within NA
# 4: test3 other 3
# Expected result:
# colname colname_with_suffix lookup_result
# 1: test1 other 1
# 2: test2 test 2
# 3: test2 includes test within 2
# 4: test3 other 3
对于以下变体，连接按预期工作。如果列的名称是连接列名称的前缀并且两者都具有相似的文本内容，则上面的意外行为似乎只会发生。
# For all following alternatives the join delivers the correct result
# (a) Same data tables as above, but no index
DT1 <- data.table(colname=c("test1","test2","test2","test3"), colname_with_suffix=c("other","test","includes test within","other"))
DT2 <- data.table(lookup=c("test1","test2","test3"), lookup_result=c(1,2,3))
DT1[DT2, lookup_result := i.lookup_result, on=c("colname"="lookup")][]
# (b) Index on DT2, but completely different values in indexed column than in join column
DT1 <- data.table(colname=c("test1","test2","test2","test3"), colname_with_suffix=c("other","other","other","other"))
DT2 <- data.table(lookup=c("test1","test2","test3"), lookup_result=c(1,2,3))
DT1[colname_with_suffix == "not found", ] # automatically creates index on colname_with_suffix
DT1[DT2, lookup_result := i.lookup_result, on=c("colname"="lookup")][]
# (c) Index on DT2, similar values in indexed column, but indexed column name is not a prefix of join column name
DT1 <- data.table(colname=c("test1","test2","test2","test3"), x.colname_with_suffix=c("other","test","includes test within","other"))
DT2 <- data.table(lookup=c("test1","test2","test3"), lookup_result=c(1,2,3))
DT1[x.colname_with_suffix == "not found", ] # automatically creates index on x.colname_with_suffix
DT1[DT2, lookup_result := i.lookup_result, on=c("colname"="lookup")][]
SessionInfo：
# R version 3.3.2 (2016-10-31)
# Platform: x86_64-w64-mingw32/x64 (64-bit)
# Running under: Windows 7 x64 (build 7601) Service Pack 1
#
# locale:
# [1] LC_COLLATE=German_Germany.1252 LC_CTYPE=German_Germany.1252 LC_MONETARY=German_Germany.1252 LC_NUMERIC=C LC_TIME=German_Germany.1252
#
# attached base packages:
# [1] stats graphics grDevices utils datasets methods base
#
# other attached packages:
# [1] data.table_1.10.0
#
# loaded via a namespace (and not attached):
# [1] tools_3.3.2
请注意，Windows下的data.table 1.10.4和R.Version 3.4.2以及Ubuntu Linux 14.04也会出现相同的行为。