Finding strings not %in% other vector of strings

Question

If you have a vector of strings and you want to know which match. It's a simple matter of using %in%.

x <- c("red","blue","green")
y <- c("yellow","blue","orange")

which(x %in% y) # Literally, which X are in Y.

But what about the opposite, where you would like to find which X are not in Y?

@RomanLuštrik I was using `which(is.na(match(x,y))` it just seemed too ugly. — Brandon Bertelsen, Apr 05 '13 at 22:48
You know what they say, there's a 100 ways to skin a cat. :) — Roman Luštrik, Apr 05 '13 at 22:49
Another "ugly" way is: `as.vector(na.action(na.omit(match(x,y))))` — Arun, Apr 05 '13 at 22:51

score 8 · Accepted Answer · answered Apr 05 '13 at 22:49

8

A neat way that I like (that I learnt from @joran, iirc) is:

`%nin%` <- Negate(`%in%`)
which(x %nin% y)
[1] 1 3

answered Apr 05 '13 at 22:49

Arun

116,683
26
284
387

2

`%nin%` is also provided by the `Hmisc` package, among others. – krlmlr Apr 05 '13 at 22:54
1

+1 to Arun for going the extra mile/kilometer. – Roman Luštrik Apr 06 '13 at 07:31
@BrandonBertelsen I suggest you accept this is the correct answer, it's more general. – Roman Luštrik Apr 06 '13 at 07:41
@RomanLuštrik as per your recommendation. – Brandon Bertelsen Apr 06 '13 at 07:51

score 5 · Answer 2 · answered Apr 05 '13 at 22:48

5

Doing %in% returns a vector of trues and falses. Using an exclamation mark will turn those Ts and Fs around, and by wrapping everything in which will give you indices.

> which(!x %in% y)
[1] 1 3
> which(x %in% y)
[1] 2

answered Apr 05 '13 at 22:48

Roman Luštrik

69,533
24
154
197

1

Also, if you are going to do `x[which(!x %in% y)]`, then you might prefer `setdiff(x, y)`. The latter is also applying `unique`. – flodel Apr 05 '13 at 23:16
2

I may be paranoid, but I'd humbly suggest `which(!(x %in% y))`, to improve readability. – Ferdinand.kraft Apr 06 '13 at 01:09
@Ferdinand.kraft I agree with you and I like the parenthesis too. However, that's one more reason why the `%ni%` suggestion below is so great! – Ricardo Saporta Apr 06 '13 at 04:06

Finding strings not %in% other vector of strings

2 Answers2