[R] vectorized approach to cumulative sampling

[Daniel E. Bunker]

Hi All,

I need to sample a vector ("old"), with replacement, up to the point 
where my vector of samples ("new") sums to a predefined value 
("target"), shortening the last sample if necessary so that the total 
sum ("newsum") of the samples matches the predefined value.

While I can easily do this with a "while" loop (see below for example 
code), because the length of both "old" and "new" may be > 20,000, a 
vectorized approach will save me lots of CPU time.

Any suggestions would be greatly appreciated.

Thanks, Dan

# loop approach
old=c(1:10)
p=runif(1:10)
target=20

newsum=0
new=NULL
while (newsum<target) {
    i=sample(old, size=1, prob=p);
    new[length(new)+1]=i;
    newsum=sum(new)
    }
new
newsum
target
if(newsum>target){new[length(new)]=target-sum(new[-length(new)])}
new
newsum=sum(new); newsum
target

-- 

Daniel E. Bunker
Associate Coordinator - BioMERGE
Post-Doctoral Research Scientist
Columbia University
Department of Ecology, Evolution and Environmental Biology
1020 Schermerhorn Extension
1200 Amsterdam Avenue
New York, NY 10027-5557

212-854-9881
212-854-8188 fax
deb37ATcolumbiaDOTedu