From: David Winsemius <dwinsemius_at_comcast.net>

Date: Wed, 19 Nov 2008 11:41:12 -0500

Date: Wed, 19 Nov 2008 11:41:12 -0500

Using rbinom might be more clear than my sample() solution. May I speculate without actual testing that:

inv <- 2*rbinom(length(vec),1,0.5) - 1

might be more efficient than a 2-step rbinom / ifelse strategy?

-- David Winsemius Heritage Labs On Nov 18, 2008, at 5:21 PM, joris meys wrote:Received on Wed 19 Nov 2008 - 16:43:42 GMT

> The function rbinom might be a solution.

>> Try following simple program :> vec <- c(-1,1,-1,1,1,-1,-1,1,1)>> inv <-rbinom(length(vec),1,0.5)> inv <-ifelse(inv==0,-1,1)>> vec2 <- vec*inv #switches sign with p=0.5>> In this, inv is a random binomial vector, where the probability for> being 1 is 0.5 in all positions. Changing the values of 0 in this> vector to -1, gives you a vector you can multiply with the original> one to change the sign with a probability of 0.5 for all positions>> Kind regards> Joris> On Tue, Nov 18, 2008 at 6:11 PM, Salas, Andria Kay> <aks2515_at_uncw.edu> wrote:>> To clear up a question regarding my earlier posting regarding>> random changes in a vector:>>>> Say you have a vector (1, -1, -1, 1, 1, -1). I want each value in>> the vector to have a probability that it will change signs when>> this vector is regenerated. For example, probability = 50%. When>> the vector is regenerated, the first value in the vector (1) has a>> 50% chance of switching to -1. If I regenerated this vector 10>> times, 5 of the times it would switch to -1. Similarly, I need>> each value in the vector to have this same probability of switching>> signs when the vector is regenerated, and each value's chances of>> doing so is independent of the other values. So the second value>> (-1) also has a 50% chance of switching to 1, and whether or not it>> does so is independent of if the first value changes from 1 to -1>> (also a 50% probability).>>>> I hope this clears up the confusion, and I would appreciate any>> help anyone can provide!>> ______________________________________________>> R-help_at_r-project.org 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.>>>> ______________________________________________> R-help_at_r-project.org 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.

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