[R] summarize a vector

[Bert Gunter]

... or perhaps even simpler:

> sz <- function(x,k)tapply(x,(seq_along(x)-1)%/%k, sum)
> sz(1:10,3)
 0  1  2  3
 6 15 24 10

Note that this works for k>n, where the previous solution does not.
> sz(1:10,15)
 0
55

-- Bert

On Fri, Aug 10, 2012 at 12:37 PM, David Winsemius
<dwinsemius at comcast.net> wrote:
>
> On Aug 10, 2012, at 12:20 PM, Sam Steingold wrote:
>
>> I have a long numeric vector v (length N) and I want create a shorter
>> vector of length N/k consisting of sums of k-subsequences of v:
>>
>> v <- c(1,2,3,4,5,6,7,8,9,10)
>>
>> N=10, k=3
>> ===> [6,15,24,10]
>>
>> I can, of course, iterate:
>>
>>> w <- vector(mode="numeric",length=ceiling(N/k))
>>> for (i in 1:length(w)) w[i] <- sum(v(i*k:(i+1)*k))
>>
>>
>> (modulo boundary conditions)
>> but I wonder if there is a better way.
>
>
> Well, using v with parentheses instead of square-brackets might not be the
> right way, since v is not a function.
>
> Consider this alternate (no need to pre-allocate 'w'):
>
>> w <- tapply( v ,rep(1:(N/k +1), each=k, len=N ) , sum)
>> w
>  1  2  3  4
>  6 15 24 10
>
> --
>
> David Winsemius, MD
> Alameda, CA, USA
>
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-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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