[R] equation help

[David Winsemius]

On Apr 13, 2009, at 10:54 AM, Brendan Morse wrote:

> Hi everyone, I am having a bit of trouble correctly structuring an  
> equation in R.
>
> Here is the equation

It's really a series of assignments.
>
>
> Here is what I thought
>
> for(i in 1:numItem)for(x in 1:numCat)
>
>          Ptheta[,i,x]<-(exp(-1.702*a[i]*(theta-b[i,x+1]))

I am guessing that what you really want is:

for(i in 1:numItem){
          for(x in 1:numCat){
                    Ptheta[i,x]<-(exp(-1.702*a[i]*(theta-b[i,x+1]))
                    } }  #obviously untested

That should populate the i, x entries of a matrix with your  
calculations. The extra comma in your attempt was telling R that you  
wanted a three dimensional array. The other possibility as a  
construction would be to use expand.grid

>
>
> My problem is that I am not sure how to get it to read the equation  
> as having two indexing points (i and x). Right now it is trying to  
> read it as a matrix, but each value (i and x) are only vectors.

Each value is an integer by the time it gets to the innards of the loop.

> Does anyone have any suggestions on how to get R to recognize this?  
> As an example, numItem may = 20 and numCat may = 3 and I am trying  
> to evaluate this over all values of numCat for each value of numItem.
>
> Thanks,
> Brendan
-- 
David Winsemius, MD
Heritage Laboratories
West Hartford, CT