Morning everyone
In data.table I found that with a left join, when mentioning a column name implicitly i.e. without mentioning the table (in which the column resides in) induces unexpected results despite unique column names.
dummy data
x <- data.table(a = 1:2); x
# a
# 1: 1
# 2: 2
y <- data.table(c = 1
,d = 2); y
# c d
# 1: 1 2
left join without mentioning table name in retrieve of column c
z <- y[x, on=.(c=a), .(a,c,d)]; z
# a c d
# 1: 1 1 2
# 2: 2 2 NA
Problem arises when looking at results above. Row 2 of column c is supposed to be NA. However, it shows 2
This is only rectified when the user explicitly mentions the table:
z <- y[x, on=.(c=a), .(a,x.c,d)]; z
# a x.c d
# 1: 1 1 2
# 2: 2 NA NA
It is perhaps worth mentioning the x in x.c is referring to the position of syntax x[i], in this case, table y
My question is why is the explicit mention of table necessary for a task seemingly basic. Or am I missing something? Thank you.
