parsing column names when joining data.tables - r

Question

I need to join the information of column h of dataframe Y into dataframe X. The code below shows the desired output.
library(data.table)
X <- data.table(
a1 = rep("A", 6),
b1 = rep(1,6),
c1 = rep(c(0,1), 1, each = 3),
d = letters[1:6]
)
Y <- data.table(
a2 = rep(c("A","B", "C"), 1, each = 2),
b2 = rep(c(1, 2, 3), 1, each = 2),
c2 = rep(c(0,1), 3),
h = letters[7:12]
)
# final result
X[Y,
on = .(a1 = a2,
b1 = b2,
c1 = c2),
h := i.h
][]
#> a1 b1 c1 d h
#> 1: A 1 0 a g
#> 2: A 1 0 b g
#> 3: A 1 0 c g
#> 4: A 1 1 d h
#> 5: A 1 1 e h
#> 6: A 1 1 f h
Created on 2020-08-03 by the reprex package (v0.3.0)
The problem, however, is that the names of the columns that I use for making the join vary depending on the information stored somewhere else. So, let's assume that the name of the column c1 in X is stored in var, say var <- "c2". Now, when I tried to do the join, nothing seems to work.
# None the attempts below works
var <- "c1"
# attempt 1
X[Y,
on = .(a1 = a2,
b1 = b2,
eval(var) = c2),
h := i.h
][]
# attempt 2
X[Y,
on = .(a1 = a2,
b1 = b2,
get(var) = c2),
h := i.h
][]
# attempt 3
cond <- paste0(deparse(var), " = c2")
parcond <- parse(text = cond)
X[Y,
on = .(a1 = a2,
b1 = b2,
eval(parcond)),
h := i.h
][]
At the end, the only way I found to solve it is very inelegant, but it seems to be working.
var <- "c1"
setnames(X, var, "c2")
X[Y,
on = c("a1" = "a2",
"b1" = "b2",
"c2"),
h := i.h
][]
setnames(X, "c2", var)
However, I wonder if there is a better way to do this programmatically.
I checked all these links, but I could not find a solution that works for me.
Thank you so much for your help.

Answer 1

Thanks to #chinsoon12 for his/her comment, the solution to the problem would be as follows,
library(data.table)
X <- data.table(
a1 = rep("A", 6),
b1 = rep(1,6),
c1 = rep(c(0,1), 1, each = 3),
d = letters[1:6]
)
Y <- data.table(
a2 = rep(c("A","B", "C"), 1, each = 2),
b2 = rep(c(1, 2, 3), 1, each = 2),
c2 = rep(c(0,1), 3),
h = letters[7:12]
)
var <- "c1"
onkey <- c("a1==a2", "b1==b2", paste0(var,"==c2"))
X[Y,
on=onkey,
h := i.h
][]
#> a1 b1 c1 d h
#> 1: A 1 0 a g
#> 2: A 1 0 b g
#> 3: A 1 0 c g
#> 4: A 1 1 d h
#> 5: A 1 1 e h
#> 6: A 1 1 f h
Created on 2020-08-11 by the reprex package (v0.3.0)