What does multiple %in% in a row test?

Question

Consider:

a <- c("a", "b", "c")
b <- c("a", "e", "f")
c <- c("a", "h", "i")

> a %in% b %in% c
[1] FALSE FALSE FALSE

I would have expected this to evaluate to TRUE FALSE FALSE, since the first element in each vector is "a". Why is this not the case?

Colonel Beauvel · Accepted Answer · 2015-06-30T09:41:38.693

5

What you are doing is first this operation: a %in% b resulting in TRUE FALSE FALSE. Then you basically do the chain c(TRUE,FALSE,FALSE) %in% c, which of course, will result in all False.

You can try this to get the wanted vector of boolean (taking into account the position):

a == Reduce(function(u,v) ifelse(u==v, u, F), list(a,b,c))
#[1]  TRUE FALSE FALSE

If you want common elements, not depending on the position, you can do:

Reduce(intersect, list(a,b,c))

edited Jun 30 '15 at 09:41

answered Jun 30 '15 at 09:26

Colonel Beauvel

30,423
11
47
87

That is helpful, but it still doesn't explain what `a %in% b %in% c` does. What does it do? – histelheim Jun 30 '15 at 09:29
1

It is explained in the first two lines ;) – Colonel Beauvel Jun 30 '15 at 09:40

What does multiple %in% in a row test?

1 Answers1