r data.table 以列名作为变量的行子集

Question

Let's first create a simple data.table in r.

dt=data.table(x1=1:5,
           x2=11:21)

If we want to subset the data.table with conditions for rows, we can simple do, for example

dt[x1==1]

Now my question is: what if the column name is a variable? I tried with:

var="x1"
dt[eval(var)==1,]

But this code doesn't work.

eval works with the following example: if we want to get the some columns by name (that is a variable).

dt[x1==1,eval(var),with=F]

Answer 1

I guess you are looking for get :

library(data.table)

DT <- data.table(x1=1:11, x2=11:21)
var <- "x1"
DT[get(var)==1,]

Answer 2

An option with .SD

 DT[DT[, .SD[[var]] ==1]]
   x1 x2
1:  1 11

which gives the same output as the accepted answer

 DT[get(var)==1,]
   x1 x2
1:  1 11