使用与列同名的 object 过滤 data.table 中的行

Question

I have a data.table dt with column names x and y . I also have object y that I need to use to filter some rows in dt using the values in dt$y . Unfortunately, since one of the columns has the same name as the object, I can't make it work. What am I doing wrong? Thank you so much.

library(data.table)
dt <- data.table( x = c(1:4), 
                  y = letters[1:4])
y <- "c"

dt[y == y]
#>    x y
#> 1: 1 a
#> 2: 2 b
#> 3: 3 c
#> 4: 4 d
dt[y == (y)]
#>    x y
#> 1: 1 a
#> 2: 2 b
#> 3: 3 c
#> 4: 4 d
dt[y == ..y]
#> Error in eval(stub[[3L]], x, enclos): object '..y' not found
eval(quote(y))
#> [1] "c"
dt[y == eval(quote(y))]
#>    x y
#> 1: 1 a
#> 2: 2 b
#> 3: 3 c
#> 4: 4 d

^{Created on 2021-04-12 by the reprex package (v2.0.0)}

PS: I apologize if this question has been asked somewhere else. I searched for an answer, but I could not find it.

Answer 1

You could use get() and the env argument:

dt[y == get('y', env = -2)]
#    x y
# 1: 3 c

Beware this makes certain assumptions about your setup. If your y is defined in the global environment as in your example, then a safer solution (as suggested by Ian Campell) would be:

dt[y == get('y', env = globalenv())]