[R] loop troubles

[David Winsemius]

On Jul 18, 2010, at 9:12 AM, Joe P King wrote:

> I tried that, this is what I tried, but I only get it to do one  
> iteration
> and then it wont cycle back. I am not sure how to tell it how to  
> look at the
> previous precision to add to the current new sample precision to get  
> the new
> precision.

And I'm not exactly sure what you are trying to do since I found your  
problem statement insufficiently precise, but  ....  if you have a  
vector, "p" and an index in a loop is "i", then the "previous p" is  
just p[i-1]. Sometimes you can get constructs like the following to  
accomplish an indexed assignment:

p[2:length(n)] <-a*p[1:(length(n)-1)]


> This is my first try at a loop so I am not sure if I am doing
> anything right, sorry about the elementary question.
>
> n=c(5,10,15)
> ybar=c(12,13,14)
> sd=c(2,3,4)
> mutotals<-matrix(nrow=length(n), ncol=1)
> mu=1;p0=0
> for (i in 1:length(n)){
>  var = sd[i]^2 #sample variance
>  p = n[i]/var #sample precision

At this point you seem to be using "p" as a constant
>  p0[i]= i

Comments might help to figure out why you are doing each of htese steps.

>  pn = p0[i]+p[i]

Wouldn't "pn" need to be indexed as well? And now you seem to be using  
"p" as a vector but haven't extended it with c() or some other  
function, so on the second iteration it should fail.


>  mu.out = ((p0[i])/(pn)*mu[i])+((p)/(pn))*ybar[i]
>  sigma[i] = 1/sqrt(pn[i])
>  mutotals[i,]<-mu.out[i]
> }

I get:
Error in sigma[i] = 1/sqrt(pn[i]) : object 'sigma' not found

You are not reporting the error messages this throws (which should  
have made it apparent that you needed to do some further setup of the  
data structures you are using, not just with "p", "pn", but also  
"sigma". )

>
> Joe King
> 206-913-2912
> jp at joepking.com
> "Never throughout history has a man who lived a life of ease left a  
> name
> worth remembering." --Theodore Roosevelt
>
>
>
> -----Original Message-----
> From: David Winsemius [mailto:dwinsemius at comcast.net]
> Sent: Sunday, July 18, 2010 5:36 AM
> To: Joe P King
> Cc: r-help at r-project.org
> Subject: Re: [R] loop troubles
>
>
> On Jul 17, 2010, at 9:09 PM, Joe P King wrote:
>
>> Hi all, I appreciate the help this list has given me before. I have a
>> question which has been perplexing me. I have been working on doing a
>> Bayesian calculating inserting studies sequentially after using a
>> non-informative prior to get a meta-analysis type result. I created a
>> function using three iterations of this, my code is below. I insert
>> prior
>> mean and precision (I add precision manually because I want it to be
>> zero
>> but if I include zero as prior variance I get an error cant divide
>> by zero.
>> Now my question is instead of having the three iterations below, is
>> there
>> anyway to create a loop, the only problem is I want to have elements
>> from
>> the previous posterior to be the new prior and now I cant figure out
>> how to
>> do the code below in a loop.
>
> Perhaps if you set the problem up with vectors, it would become more
> obvious how to do it with indexing or loops:
>
> n <-  c(5, 10, 15)
> ybar <- c(12,12,14)  # and so on...
>
> -- 
> David
>> The data below is dummy data, I used a starting
>> mu of 1, and starting precision of 0.
>>
>>
>>
>> bayes.analysis.treat<-function(mu0,p0){
>>
>> n1 = 5
>>
>> n2 = 10
>>
>> n3 = 15
>>
>> ybar1 = 12
>>
>> ybar2 = 13
>>
>> ybar3 = 14
>>
>> sd1 = 2
>>
>> sd2 = 3
>>
>> sd3 = 4
>>
>> #posterior 1
>>
>> var1 = sd1^2 #sample variance
>>
>> p1 = n1/var1 #sample precision
>>
>> p1n = p0+p1
>>
>> mu1 = ((p0)/(p1n)*mu0)+((p1)/(p1n))*ybar1
>>
>> sigma1 = 1/sqrt(p1n)
>>
>> #posterior 2
>>
>> var2 = sd2^2 #sample variance
>>
>> p2 = n2/var2 #sample precision
>>
>> p2n = p1n+p2
>>
>> mu2 = ((p1n)/(p2n)*mu1)+((p2)/(p2n))*ybar2
>>
>> sigma2 = 1/sqrt(p2n)
>>
>> #posterior 3
>>
>> var3 = sd3^2 #sample variance
>>
>> p3 = n3/var3 #sample precision
>>
>> p3n = p2n+p3
>>
>> mu3 = ((p2n)/(p3n)*mu2)+((p3)/(p3n))*ybar3
>>
>> sigma3 = 1/sqrt(p3n)
>>
>> posterior<-cbind(rbind(mu1,mu2,mu3),rbind(sigma1,sigma2,sigma3))
>>
>> rownames(posterior)<-c("Posterior 1", "Posterior 2", "Posterior 3")
>>
>> colnames(posterior)<-c("Mu", "SD")
>>
>> return(posterior)}
>>
>>
>>
>> -------------------------------------------
>>
>> Joe King, M.A.
>>
>> Ph.D. Student
>>
>> University of Washington - Seattle
>>
>> 206-913-2912
>>
>> <mailto:jp at joepking.com> jp at joepking.com
>>
>> -------------------------------------------
>>
>> "Never throughout history has a man who lived a life of ease left a
>> name
>> worth remembering." --Theodore Roosevelt
>>
>>
>>
>>
>> 	[[alternative HTML version deleted]]
>>
>> ______________________________________________
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>> PLEASE do read the posting guide
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>
> David Winsemius, MD
> West Hartford, CT
>
> ______________________________________________
> 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.

David Winsemius, MD
West Hartford, CT