[R] vector manipulations -- differences

[Bert Gunter]

Use ?mappy and ?rep.int

> x[unlist(mapply(":",2:4,4))] - x[rep.int(1:3,3:1)]
[1]  3  7 20  4 17 13

Cheers,

Bert


Bert Gunter

"Data is not information. Information is not knowledge. And knowledge
is certainly not wisdom."
   -- Clifford Stoll


On Mon, Sep 21, 2015 at 2:17 PM, Dan D <ddalthorp at usgs.gov> wrote:
> I need an efficient way to build a new n x (n-1)/2 vector from an n-vector x
> as:
>
> c(x[-1]-x[1], x[-(1:2)]-x[2], ... , x[-(1:(n-1)] - x[n-1])
>
> x is increasing with x[1] = 0.
>
> The following works but is not the greatest:
> junk<-outer(x, x, '-')
> junk[junk>0]
>
> e.g.,
> given
> x<-c(0, 3, 7, 20)
> junk<-outer(x, x, '-')
> junk[junk>0] # yields: c(3, 7, 20, 4, 17, 13) as needed, but it has to go
> through
> junk
> #     [,1] [,2] [,3] [,4]
> #[1,]    0   -3   -7  -20
> #[2,]    3    0   -4  -17
> #[3,]    7    4    0  -13
> #[4,]   20   17   13    0
>
> Anyone have a better idea?
>
> -Dan
>
>
>
> --
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