[R] vector indexing

[Duncan Murdoch]

On 12-10-23 5:39 AM, Rui Barradas wrote:
> Hello,
>
> Try the following.
>
> y=c(2,3,5,2,4,6,8,3,6,2,5)
>
> first <- function(x) min(which(x))
> prefix <- function(x, v) x[seq_len(v)]
> suffix <- function(x, v) x[-seq_len(v)]
>
> first(y > 4)
> prefix(y, first(y > 4))
> suffix(y, first(y > 4))
>

Be careful with this:  it fails if the condition is FALSE for every 
element, e.g.

 > first(y > 10)
[1] Inf
Warning message:
In min(which(x)) : no non-missing arguments to min; returning Inf

I don't know if this is possible in the original context, or what the 
desired result would be if it happens:  but it's something to look out for.

Duncan Murdoch