summarize a vector
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