Question

What happens if I want to select all the rows in a data.table that do not contain a particular value in the key variable using binary search? By the way, what is the correct jargon for what I want to do? Is it “nojoin”? Is it “negative selection”?

DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9)
setkey(DT,x)

Lets do a positive selection for all rows where x==”a” but using binary search

DT["a"]

That’s beautiful but I want the opposite of that. I want all the rows that are not “a” in other words where x!=”a”

DT[x!="a"]

That is a vector scanning. The above line works but is uses vector scanning. I want to use binary. I was expecting the following to work, but alas…

DT[!"a"]
DT[-"a"]

The above two do not work and trying to play with nomatch got me nowhere.

Answer 1

The idiom is this:

DT[-DT["a", which=TRUE]]

   x y v
1: b 1 4
2: b 3 5
3: b 6 6
4: c 1 7
5: c 3 8
6: c 6 9

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Inspiration from:

• The mailing list posting Return Select/Join that does NOT match?
• The previous question non-joins with data.tables
• Matthew Dowle’s answer to Porting set operations from R’s data frames to data tables: How to identify duplicated rows?

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Update. New in v1.8.3 is not-join syntax. Farrel’s first expectation (! rather than -) has been implemented :

DT[-DT["a",which=TRUE,nomatch=0],...]   # old idiom
DT[!"a",...]                            # same result, now preferred.

See the NEWS item for more detailed info and example.