Subsetting data.table using variables with same name as column

Question

I want to subset a data.table using a variable which has the same name as the column which leeds to some problems:

dt <- data.table(a=sample(c('a', 'b', 'c'), 20, replace=TRUE),
                 b=sample(c('a', 'b', 'c'), 20, replace=TRUE),
                 c=sample(20), key=c('a', 'b'))

evn <- environment()
a <- 'b'
dt[a == a]

#Expected Result
dt[a == 'b']

I came across this possible solution:

env <- environment()
dt[a == get('a',env)]

But it is as unhandy as:

this.a = a
dt[a == this.a]

So is there another elegant solution?

We're aware of this scoping issue. This is very much a priority and will be fixed ASAP. Thanks for reporting. For now, using a different variable name would be the way to go. — Arun
I'm confused - why would you think that a == a should work or is good syntax? R-forge seems to be down for me atm, so I can't see the link from @Arun and what exactly it's about, but making a == a work (in the way OP wants it to work) seems like a bad idea to me and I think your last solution is the correct one. — eddi
Separately from my above comment, since your data.table is keyed by a, you can do dt[a] — eddi

xb. xb. · Accepted Answer · 2014-02-09T19:49:29

For now, a temporary solution could be,

`..` <- function (..., .env = globalenv())
{
  get(deparse(substitute(...)), env = .env)
}

..(a)
## [1] "b"

dt[a==..(a)]
##    a b  c
## 1: b a 15
## 2: b a 11
## 3: b b  8
## 4: b b  4
## 5: b c  5
## 6: b c 12

Though this looks elegant, I am still waiting for a more robust solution to such scope issues.

Edited according to @mnel's suggestion,

`..` <- function (..., .env = sys.parent(2))
{
  get(deparse(substitute(...)), env = .env)
}

Subsetting data.table using variables with same name as column

2 Answers