R which() function with vectors

Question

I have three vectors

evens <- c(2,4,6,8,10,12,14,16,18,20)
r <- c(4,5,8,9,12)
t <- c(10,12)

which( evens > r)# returns to 6,7,8,9,10
which( evens > t)# returns to 7,8,9,10

Why are the results different?

Could you please explain how which function works?

Answer 1

As others have already pointed out this is due to the recycling rules of R. You can see how it works in detail by doing this:

> data.frame(evens, r, t, evens > r, evens > t)
   evens  r  t evens...r evens...t
1      2  4 10     FALSE     FALSE
2      4  5 12     FALSE     FALSE
3      6  8 10     FALSE     FALSE
4      8  9 12     FALSE     FALSE
5     10 12 10     FALSE     FALSE
6     12  4 12      TRUE     FALSE
7     14  5 10      TRUE      TRUE
8     16  8 12      TRUE      TRUE
9     18  9 10      TRUE      TRUE
10    20 12 12      TRUE      TRUE

So when you do which(evens > r) you get the indices of all TRUE s in the column event...r above.