Submitted by: Timothee Carayol; Assigned to: Nobody; R-Forge link
I say it's scary because it returns no error and is potentially quite difficult to detect in existing code..
FYI I tried this on data.table 1.8.2 and R 2.14.1.
A <- data.table(
keyvar=rev(letters[1:10]),
count=1:10
)
A[, c(
'count1',
'count2'
) := count, with=F
]
A
keyvar count count1 count2
1: j 1 1 1
2: i 2 2 2
3: h 3 3 3
4: g 4 4 4
5: f 5 5 5
6: e 6 6 6
7: d 7 7 7
8: c 8 8 8
9: b 9 9 9
10: a 10 10 10
All looks good.
Let's make a copy of A; call it B. We'll use it later on.
B <- copy(A)
Now let's set a key on "keyvar" in A:
setkeyv(A, 'keyvar')
A
keyvar count count1 count2
1: a 10 1 1
2: b 9 2 2
3: c 8 3 3
4: d 7 4 4
5: e 6 5 5
6: f 5 6 6
7: g 4 7 7
8: h 3 8 8
9: i 2 9 9
10: j 1 10 10
That's the scary bit. "keyvar" and "count" got sorted nicely, but "count1" and "count2" are now matched to wrong rows.
Let's try the same thing on the copy of A that we made earlier..
setkeyv(B, 'keyvar')
B
keyvar count count1 count2
1: j 1 1 1
2: i 2 2 2
3: h 3 3 3
4: g 4 4 4
5: f 5 5 5
6: e 6 6 6
7: d 7 7 7
8: c 8 8 8
9: b 9 9 9
10: a 10 10 10
That's fine!
I.e. whatever was wrong with A, is fixed in its copy B.
Submitted by: Timothee Carayol; Assigned to: Nobody; R-Forge link
I say it's scary because it returns no error and is potentially quite difficult to detect in existing code..
FYI I tried this on data.table 1.8.2 and R 2.14.1.
A <- data.table(
keyvar=rev(letters[1:10]),
count=1:10
)
A[, c(
'count1',
'count2'
) := count, with=F
]
A
1: j 1 1 1
2: i 2 2 2
3: h 3 3 3
4: g 4 4 4
5: f 5 5 5
6: e 6 6 6
7: d 7 7 7
8: c 8 8 8
9: b 9 9 9
10: a 10 10 10
All looks good.
Let's make a copy of A; call it B. We'll use it later on.
B <- copy(A)
Now let's set a key on "keyvar" in A:
setkeyv(A, 'keyvar')
A
1: a 10 1 1
2: b 9 2 2
3: c 8 3 3
4: d 7 4 4
5: e 6 5 5
6: f 5 6 6
7: g 4 7 7
8: h 3 8 8
9: i 2 9 9
10: j 1 10 10
That's the scary bit. "keyvar" and "count" got sorted nicely, but "count1" and "count2" are now matched to wrong rows.
Let's try the same thing on the copy of A that we made earlier..
setkeyv(B, 'keyvar')
B
1: j 1 1 1
2: i 2 2 2
3: h 3 3 3
4: g 4 4 4
5: f 5 5 5
6: e 6 6 6
7: d 7 7 7
8: c 8 8 8
9: b 9 9 9
10: a 10 10 10
That's fine!
I.e. whatever was wrong with A, is fixed in its copy B.