[R] Sampling from a Matrix

[Liaw, Andy]

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

 
> See ?apply and ?t
> 
> HTH,
> 
> Marc Schwartz