# [R] How to delete repeated values in MCMC sampling and get index of unique values?

Duncan Murdoch murdoch.duncan at gmail.com
Fri Jan 18 00:14:41 CET 2013

```On 13-01-17 5:33 PM, C W wrote:
> I was looking for the first answer.
>
> In MCMC, at time t, when the candidate sample is rejected,
>
>  > candidate_sample[t] <- current_sample
>
> say, at time t+1, the sample is rejected AGAIN, we have
>
>  > candidate_sample[t+1] <- current_sample
>
> so, at time t, and t+1, we have the same value.  When I calculate the
> monte carlo mean, I don't want repeated value.

Then you will get the wrong answer, unless you want something very strange.

Duncan Murdoch

>
> Mike
>
> On Thu, Jan 17, 2013 at 5:20 PM, Bert Gunter <gunter.berton at gene.com
> <mailto:gunter.berton at gene.com>> wrote:
>
>     What answer is wanted for
>
>     c(1,1,1,2,3,1)   ?
>
>     Note that Duncan's  two suggestions below give different answers for
>     this.
>
>     -- Bert
>
>     On Thu, Jan 17, 2013 at 1:59 PM, Duncan Murdoch
>     <murdoch.duncan at gmail.com <mailto:murdoch.duncan at gmail.com>> wrote:
>      > On 13-01-17 4:50 PM, C W wrote:
>      >>
>      >> Dear list,
>      >> How do you delete repeated samples?  In MCMC, when your
>     candidate value
>      >> has
>      >> been reject, so you remain on the same point, so you keep that
>     value.
>      >>
>      >> Say I have this toy example,
>      >>
>      >>> c(1,6,6,6,3,5,4,4,2,3,5)
>      >>
>      >>
>      >> The 6 and 4 are repeated, I only want the index of the non-repeated
>      >> values.
>      >>
>      >> I thought of using which() and unique(), but that does not give
>     you the
>      >> index of the unique values.
>      >
>      >
>      > You could use x[!duplicated(x)] or rle(x)\$values, depending on your
>      > definition of "repeated".  I hope you're aware that you can't use
>     either for
>      > things like quantiles and moments of the limiting distribution.
>      >
>      >> x <- c(1,6,6,6,3,5,4,4,2,3,5)
>      >> x[!duplicated(x)]
>      > [1] 1 6 3 5 4 2
>      >> rle(x)\$values
>      > [1] 1 6 3 5 4 2 3 5
>      >
>      > Duncan Murdoch
>      >
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>
>
>
>     --
>
>     Bert Gunter
>     Genentech Nonclinical Biostatistics
>
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