[R] vector manipulations -- differences

[Frank Schwidom]

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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