Once again, thanks for your help. I did not state the problem correctly, though the code is correct for what I want to do.
A better description of the problem would be that there is a matrix of probabilities: > set.seed(1) > probs <- array(abs(rnorm(25, sd = 0.33)), dim = c(5,5), dimnames = list(1:5, > letters[1:5])) > probs a b c d e 1 0.21 0.27 0.50 0.0148 0.303 2 0.06 0.16 0.13 0.0053 0.258 3 0.28 0.24 0.21 0.3115 0.025 4 0.53 0.19 0.73 0.2710 0.656 5 0.11 0.10 0.37 0.1960 0.205 The column names, dimnames(probs)[[2]], are the names of units to be sampled. Each row is a trial. For each row (trial), I want to sample 3 of the units such that for each row I get a vector like the following: [1] "a", "b", "a" The samples are to be done with replacement, and these vectors could be combined to form a matrix of the samples. The purpose of the "probs" matrix is to give each unit a probability that it will be sampled. One way to do this follows: index <- 1:ncol(probs) res <- matrix(0, nrow = dim(probs)[1], ncol = 3 ) for(i in 1:nrow(probs)){ ## gets the indexes of the values chosen res[, i] <- sample(index, size = 3, replace = TRUE, prob = probs[i, ]) } Using "apply" as Andy described would accomplish the intended result. -- Dan > > Hmm... If I read Daniel's code (which is different from his description) > > correctly, that doesn't seem to be what he wanted. Perhaps something like > > this: > > > > apply(probs, 1, function(p) sample(1:ncol(probs), 3, replace=TRUE, prob=p)) > > > > Andy > > Andy, > > You are of course correct. I had focused on the description of the > problem, rather than the code provided, presuming that the code was not > correct, including the use of 'replace' and 'prob' in sample(). > > I suppose it would be up to Daniel for clarification. > > Regards, > > Marc > > > -- Daniel Gerlanc Williams College '07 ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.