R help - Feb 2019 - data.table: reference column in "i"-part by column name in string object

I am converting data.frame-based code to data.table, for performance 
reasons.

Is there a way to refer to columns using expressions in the "i"-part?

Here is an example:

a <- data.table(x=rep(LETTERS[1:10],each=2), y=1:20)
v <- "x"

For the j-part, I can access the column whose name is stored in v as

a[,..v]

However, for the i-part I did not find a good way to achieve the same. 
For a simple case, the following works

xx <- "B"
a[xx, on=v]

However, I can't see how this is easily expanded to more complex logical 
expressions.

Pascal

Hello,

I don't understand the question.
Like this?

xx <- "B"
yy <- "D"
a[xx, on = v]
a[c(xx, yy), on = v]


Note that .(xx, yy) doesn't work. It outputs something else.

Could you give an example of more complicated expressions you have 
doubts with?

Hope this helps,

Rui Barradas

?s 14:10 de 21/02/2019, Pascal A. Niklaus escreveu:
> I am converting data.frame-based code to data.table, for performance 
> reasons.
> 
> Is there a way to refer to columns using expressions in the
"i"-part?
> 
> Here is an example:
> 
> a <- data.table(x=rep(LETTERS[1:10],each=2), y=1:20)
> v <- "x"
> 
> For the j-part, I can access the column whose name is stored in v as
> 
> a[,..v]
> 
> However, for the i-part I did not find a good way to achieve the same. 
> For a simple case, the following works
> 
> xx <- "B"
> a[xx, on=v]
> 
> However, I can't see how this is easily expanded to more complex
logical
> expressions.
> 
> Pascal
> 
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On 21.02.19 20:43, Rui Barradas wrote:
> Hello,
>
> I don't understand the question.
> Like this?
>
> xx <- "B"
> yy <- "D"
> a[xx, on = v]
> a[c(xx, yy), on = v]
>
> Note that .(xx, yy) doesn't work. It outputs something else.
>
> Could you give an example of more complicated expressions you have 
> doubts with?
Let's assume I'd like to create a subset like this, in data.frame
syntax:

d[ (d[[v]] == a | d$b == "c") & d$c>2, ]

Here I have a column referenced by its name (stored in v), and compared 
to a value stored in a, and the whole thing is part of a more complex 
logical expression. I could write it in this conventional way, but I 
wondered whether there is a better way that would leverage some of 
data.tables syntax.

Pascal