[R] vector manipulations -- differences

[Dan D]

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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