[R] vector manipulations -- differences
Frank Schwidom
schwidom at gmx.net
Tue Sep 22 23:43:10 CEST 2015
Hi,
xr <- rev( x)
vec <- 1:(length( x) - 1)
rev( xr[ sequence( vec)] - rep.int( xr[ -1], vec))
On 2015-09-21 14:17:40, Dan D 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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>
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