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Re: st: question concerning translation from Matlab to Mata


From   Matthew J Baker <[email protected]>
To   [email protected]
Subject   Re: st: question concerning translation from Matlab to Mata
Date   Thu, 10 Mar 2011 22:39:18 -0500 (EST)

Another problem that might produce the conformability error is the use of (y==0). I'm guessing that this might be better coded as (y:==0).

MJB

Dr. Matthew J. Baker
Department of Economics
Hunter College and the Graduate Center, CUNY


---- Original message ----
>Date: Thu, 10 Mar 2011 18:09:53 -0500
>From: [email protected] (on behalf of "Brian P. Poi" <[email protected]>)
>Subject: Re: st: question concerning translation from Matlab to Mata  
>To: [email protected]
>
>
>
>On 3/10/2011 5:55 PM, DC wrote:
>> Hi All,
>>
>> I'm attempting to translate some code for an optimal stopping problem
>> that I had previously written in Matlab to Mata and I've encountered a
>> problem
>> in writing down a likelihood function.
>>
>> A simplified version in Matlab would be a multinomial logit with three
>> choices, choice 0 parameter normalized to zero, :
>> ...
>> [n k]=size(x);
>> D = 1+exp(x*b(1:k))+exp(x*b(k+1:2*k));
>> p_0 = 1./D;
>> p_1 = exp(x*b(1:k))./D;
>> p_2 = exp(x*b(k+1:2*k))./D;
>> lnlike=-sum( (y==0).*log(p_0) + (y==1).*log(p_1) +(y==2).*log(p_2));
>> ....
>> However is it not clear how to specify and estimate something like
>> this using Mata
>> I tried the following:
>>
>> void lmle(todo,b,y, x,lnf, S, H)
>> {
>> 	k = cols(x)
>> 	b1 = b[1::k,1]
>> 	b2 = b[k+1::2*k,1]
>> 	xb1 = x*b1'
>> 	xb2 = x*b2'
>> 	
>> 	d = 1 :+ (exp(xb1) + exp(xb1))
>> 	p_0 = 1 :/ d
>> 	p_1 = exp(xb1) :/ d
>> 	p_2 = exp(xb2) :/ d
>> 	lnf = (y==0) :* log(p_0) + (y==1) :* log(p_1)  + (y==2) :* log(p_2)	
>> }
>>
>> S = optimize_init()
>> optimize_init_evaluator(S,&lmle())
>> optimize_init_evaluatortype(S, "gf0")
>> optimize_init_argument(S,1,y)
>> optimize_init_argument(S,2,x)
>> optimize_init_params(S, J(1,cols(x),0))
>>
>> b = optimize(S)
>>
>
>
>One immediate problem is that -optimize- passes the parameters as a row 
>vector, not a column vector, so the lines
>
> > 	b1 = b[1::k,1]
> > 	b2 = b[k+1::2*k,1]
>
>should probably be
>
>  	b1 = b[1, 1::k]
>  	b2 = b[1, k+1::2*k]
>
>That may or may not be the only problem.
>
>    -- Brian Poi
>    -- [email protected]
>
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