Re: [R] Sampling from a Matrix

From: Marc Schwartz (via MN) <mschwartz_at_mn.rr.com>
Date: Thu 10 Aug 2006 - 00:22:38 EST

On Tue, 2006-08-08 at 14:10 -0400, Liaw, Andy wrote:
> From: Marc Schwartz
>
> > On Fri, 2006-08-04 at 12:46 -0400, Daniel Gerlanc wrote:
> > > Hello all,
> > >
> > > Consider the following problem:
> > >
> > > 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
> > >
> > > I want to sample 3 values from each row.
> > >
> > > One way to do this follows:
> > >
> > > index <- 1:ncol(probs)
> > >
> > > for(i in 1:nrow(probs)){
> > >
> > > ## gets the indexes of the values chosen
> > >
> > > sample(index, size = 3, replace = TRUE, prob = probs[i, ])
> > >
> > > }
> > >
> > > Is there a another way to do this?
> > >
> > > Thanks!
> >
> > > t(apply(probs, 1, function(x) sample(x, 3)))
> > [,1] [,2] [,3]
> > 1 0.210 0.5000 0.0148
> > 2 0.258 0.0053 0.1300
> > 3 0.025 0.2800 0.3115
> > 4 0.190 0.5300 0.2710
> > 5 0.196 0.1000 0.1100
>
> 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



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