gistfile1.txt

> X[Y, z := i.z]
> X
    a b c g         z
 1: j j j 1 0.2486045
 2: d d d 2 0.5365723
 3: f f f 2 0.5365723
 4: g g g 2 0.5365723
 5: p p p 2 0.5365723
 6: r r r 2 0.5365723
 7: i i i 3 0.7218104
 8: k k k 3 0.7218104
 9: n n n 3 0.7218104
10: v v v 3 0.7218104
11: x x x 3 0.7218104
12: y y y 3 0.7218104
13: c c c 4 0.8301997
14: l l l 4 0.8301997
15: o o o 4 0.8301997
16: q q q 4 0.8301997
17: s s s 4 0.8301997
18: a a a 5 0.6240269
19: e e e 5 0.6240269
20: w w w 5 0.6240269
21: b b b 7        NA
22: h h h 7        NA
23: m m m 7        NA
24: t t t 7        NA
25: u u u 7        NA
26: z z z 7        NA
    a b c g         z
> sort(unique(X$a))
 [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" "y" "z"
> ?data.table
> X[Y]
    g  a  b  c         z       z.1
 1: 1  j  j  j 0.2486045 0.2486045
 2: 2  d  d  d 0.5365723 0.5365723
 3: 2  f  f  f 0.5365723 0.5365723
 4: 2  g  g  g 0.5365723 0.5365723
 5: 2  p  p  p 0.5365723 0.5365723
 6: 2  r  r  r 0.5365723 0.5365723
 7: 3  i  i  i 0.7218104 0.7218104
 8: 3  k  k  k 0.7218104 0.7218104
 9: 3  n  n  n 0.7218104 0.7218104
10: 3  v  v  v 0.7218104 0.7218104
11: 3  x  x  x 0.7218104 0.7218104
12: 3  y  y  y 0.7218104 0.7218104
13: 4  c  c  c 0.8301997 0.8301997
14: 4  l  l  l 0.8301997 0.8301997
15: 4  o  o  o 0.8301997 0.8301997
16: 4  q  q  q 0.8301997 0.8301997
17: 4  s  s  s 0.8301997 0.8301997
18: 5  a  a  a 0.6240269 0.6240269
19: 5  e  e  e 0.6240269 0.6240269
20: 5  w  w  w 0.6240269 0.6240269
21: 6 NA NA NA        NA 0.4634932
    g  a  b  c         z       z.1