What is the reason that vector indices in R start with 1, instead of the usual 0?
Example:
> arr<-c(10,20)
> arr[0]
numeric(0)
> arr[1]
[1] 10
> arr[2]
[1] 20
Is it just that they want to store extra information about the vector and didn't know where to store it except as the vector's first element?
0 is only "usual" because that's what C did, and a lot of later languages slavishly copied C syntax. By default in Fortran arrays are 1-based.
In Ada there is no default and you have to pick the beginnning and end ranges. Interestingly, it seems that most code I've come across picks '1' for the lower bound. I think that's a pretty good indication of where folks would have gone given a free choice.