equation help

-----Original Message-----
From: r-help-bounces at r-project.org 
[mailto:r-help-bounces at r-project.org] On Behalf Of Brendan Morse
Sent: Monday, April 13, 2009 10:54 AM
To: r-help at r-project.org
Subject: [R] equation help

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


Here is the equation



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]))

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. 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