Keyed lookup on data.table without 'with'

Question

I have a data.table structure like so (except mine is really huge):

dt <- data.table(x=1:5, y=3:7, key=\'x\')

I want to loo

Answer 1

Setting a key is not required and it's faster:

dt[eval(dt[, x %in% ..x])]

   x y
1: 3 5
2: 4 6

Benchmark with the previously posted answers

microbenchmark(dt[eval(dt[, x %in% ..x])],
               dt[J(get('x', parent.frame(3)))],
               dt[eval(list(x))],
               dt[eval(J(x))],
               dt[eval(.(x))],
               merge(dt, data.table(x)),
               times = 100L)

Unit: microseconds
                                  expr    min      lq     mean  median      uq    max neval
      dt[eval(dt[, x %in% ..x])]  486.1  500.60  518.529  503.70  512.65 1238.0   100
dt[J(get("x", parent.frame(3)))]  837.3  853.25  891.424  860.00  868.30 1675.3   100
               dt[eval(list(x))]  831.8  842.70  929.521  851.95  859.85 3878.3   100
                  dt[eval(J(x))]  833.8  845.50  948.535  856.00  870.00 4599.2   100
                  dt[eval(.(x))]  828.6  846.40  871.054  851.75  859.35 1985.6   100
        merge(dt, data.table(x)) 1766.0 1804.70 1907.617 1819.95 1870.95 3123.1   100

Answer 2

Adding some benchmarking results, by request.

dt is a 53080731 x 5 data.table object, keyed by a numeric column with around 100 unique values, fairly evenly distributed. x is a vector containing 5 of those values.

library(microbenchmark)
> mb <- microbenchmark(
+     dt[eval(J(x))],
+     merge(dt, data.table(x)),
+     times=10
+ )
> mb
Unit: milliseconds
                     expr      min       lq    median       uq      max neval
           dt[eval(J(x))]  127.324  127.549  133.5305  154.410  159.433    10
 merge(dt, data.table(x)) 5028.349 5083.792 5129.6590 5170.451 5250.255    10

@Tyler, if you can assist me with how to use qdap::lookup() for this case with multiple columns, I can add that too.

Answer 3

New answer, now that I think I understand what was requested:

> X <- data.table(x=x)
> merge(dt, X)
   x y
1: 3 6
2: 4 7

Answer 4

There is an item in the NEWS for 1.8.2 that suggests a ..() syntax will be added at some point, allowing this

New DT[.(...)] syntax (in the style of package plyr) is identical to DT[list(...)], DT[J(...)] and DT[data.table(...)]. We plan to add ..(), too, so that .() and ..() are analogous to the file system's ./ and ../; i.e., .() evaluates within the frame of DT and ..() in the parent scope.

In the mean time, you can get from the appropriate environment

dt[J(get('x', envir = parent.frame(3)))]
##    x y
## 1: 3 5
## 2: 4 6

or you could eval the whole call to list(x) or J(x)

dt[eval(list(x))]
dt[eval(J(x))]
dt[eval(.(x))]