Query data.table by key in R

Question

I have followed the data.table introduction. A key is set on the x column of the data.table and then queried. I have tried to set the key on the v column and it does not work has expected. Any ideas of what I am doing wrong?

> set.seed(34)
> DT = data.table(x=c("b","b","b","a","a"),v=rnorm(5))
> DT
   x          v
1: b -0.1388900
2: b  1.1998129
3: b -0.7477224
4: a -0.5752482
5: a -0.2635815
> setkey(DT,v)
> DT[1.1998129,]
   x          v
1: b -0.7477224  

EXPECTED:
   x          v
1: b  1.1998129

Answer 1

When the first argument of [.data.table is a number, it will not do a join, but a simple row number lookup. Since after the setkey your data.table looks like so:

DT
#   x          v
#1: b -0.7477224
#2: a -0.5752482
#3: a -0.2635815
#4: b -0.1388900
#5: b  1.1998129

And since as.integer(1.1998129) is equal to 1 you get the first row.

Now if you intended to do a join instead, you have to use the syntax DT[J(...)] or DT[.(...)] , and that will work as expected, provided you use the correct number (as a convenience, you're not required to use the J when dealing with eg character columns, because there is no default meaning for what DT["a"] would mean):

DT[J(v[5])]
#   x        v
#1: b 1.199813

Note that DT[J(1.1998129)] will not work, because:

DT$v[5] == 1.1998129
#[1] FALSE

You could print out a lot of digits, and that would work:

options(digits = 22)
DT$v[5]
#[1] 1.199812896606383683107

DT$v[5] == 1.199812896606383683107
#[1] TRUE

DT[J(1.199812896606383683107)]
#   x                v
#1: b 1.199812896606383683107

but there is an additional subtlety here, worth noting, in that R and data.table have different precisions for when floating point numbers are equal:

DT$v[5] == 1.19981289660638
#[1] FALSE
DT[J(1.19981289660638)]
#   x                       v
#1: b 1.199812896606379908349

Long story short - be careful when joining floating point numbers.