On Aug 10, 2012, at 12:57 PM, Bert Gunter wrote:
... 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
I agree that it is more elegant, but I do not get an error or an
unexpected result with my method.
> N=10
> k=15
> w <- tapply( v ,rep(1:(N/k +1), each=k, len=N ) , sum)
> w
1
55
A different label but the same result. I'm protected from the typical
1:0 problem that seq_along solves by including +1 in the second
argument to ":"/seq(). Unless, of course, you set N to a negative
number, but that wouldn't make much sense would it, and you get an
error from rep() anyway.
Best;
David.
-- Bert
On Fri, Aug 10, 2012 at 12:37 PM, David Winsemius
<dwinsem...@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
Internal Contact Info:
Phone: 467-7374
Website:
http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm
David Winsemius, MD
Alameda, CA, USA
______________________________________________
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and provide commented, minimal, self-contained, reproducible code.
