How do I pass column name as variable to data.table in R?

Question

I would like to pass a variable (that holds the column name as a string) as argument to data.table. How do I do it?

Consider a data.table below:

myvariable <- "a"
myvariable_2 <- "b"

DT = data.table(ID = c("b","b","b","a","a","c"), a = 1:6, b = 7:12, c = 13:18)
DT
#    ID a  b  c
# 1:  b 1  7 13
# 2:  b 2  8 14
# 3:  b 3  9 15
# 4:  a 4 10 16
# 5:  a 5 11 17
# 6:  c 6 12 18

1. I can use subset to extract columns i.e: subset(DT, TRUE, myvariable)but this just outputs the column/s
2. How do I use subset to extract column based on some criteria? e.g: extract myvariable column when myvariable_2 < 10
3. How do I extract summary statistics over groups by passing column names as variables?
4. How do I plot descriptive plots using data.table by passing column names as variables?

I know that this could be easier in data.frame i.e. passing variables as column names. But I read everywhere that data.table is faster/memory efficient hence would like to stick with it.

Does switching between data.table and data.frame have huge memory/performance implications?

I do not want to explicitly code the column names as I want this piece of code to be re-usable.

Answer 1

the comment from @thelatemail is a very good start. Do read that first! Another quick way is below

library(data.table)
df = data.table(a=1:10, b=letters[1:2], c=11:20)

var1="a"
var2="b"

dt1=df[,c(var1,var2), with=F]

Think of "with=F" as making "j" part data.table behave like that of data.frame

Edit 1 : to subset on a condition within a datatable

df[get(var1) > 5, c(var1, var2),with = F]