[R] blockwise sums

[Liaw, Andy]

> From: Barry Rowlingson 
> 
> Liaw, Andy wrote:
> > If you insist, here's one way:
> > 
> > my.blockwisesum <- function(x, n, ...) {
> >     tapply(x, seq(1, length(x), by=n), sum, ...)
> > }
> > 
> 
>   Did you test that? I get:

Of course not (slap on wrist)!!  My apologies...

Andy
 
>  > my.blockwisesum(1:10, 3)
> Error in tapply(x, seq(1, length(x), by = n), sum, ...) :
>          arguments must have same length
> 
> 
>   Here's my solution with tapply and rep() to generate a vector like 
> c(1,1,1,2,2,2,3,3,3,4):
> 
> baz.blockwisesum=
>   
> function(v,n){tapply(v,rep(1:(1+length(v)/n),each=n)[1:length(
> v)],sum)}
> 
>  > baz.blockwisesum(1:10,3)
>   1  2  3  4
>   6 15 24 10
> 
>   - just ignore the 1 to 4 names, they cant hurt you.
> 
> Baz
> 
>