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st: RE: RE: RE: query regarding mata


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: RE: RE: query regarding mata
Date   Sun, 11 Apr 2010 19:14:00 +0100

Well, in this version v appears to be undefined. Perhaps you intended to
initialise v=J(1,5,0). 

Nick 
n.j.cox@durham.ac.uk 

Abhimanyu Arora

Thanks for your reply
Yes, indeed. Here is the code, when I tried to use the do-while command.
But, the problem with this is that it gives,"error 3201, vector
required"
which is surprising.
Kind regards
Abhimanyu

mata

numeric vector fpt(real scalar x,real scalar y)
{

lastv=J(1,5,0)

do{

v[1,1]=0.5775+ln(exp(x+0.9*lastv[2])+exp(y+0.9*lastv[1]))
v[1,2]=0.5775+ln(exp(2*x+0.9*lastv[3])+exp(y+0.9*lastv[1]))
v[1,3]=0.5775+ln(exp(3*x+0.9*lastv[4])+exp(y+0.9*lastv[1]))
v[1,4]=0.5775+ln(exp(4*x+0.9*lastv[5])+exp(y+0.9*lastv[1]))
v[1,5]=0.5775+ln(exp(5*x+0.9*lastv[5])+exp(y+0.9*lastv[1]))

lastv=v

}while (mreldif(v,lastv)<10^(-6))
return(v)
}


fpt(2,3)


end

Nick Cox

This doesn't look complete. 

-mreldif(,)- will surely complain if -lastv- is not defined when it is
called. 

Is there more code upstream of this? 

Conversely, my guess is that -lastv- has been defined previously, but is
of different dimensions: hence the conformability message. 

You might try some initialisation of -lastv- to 1,1,1,1,1 before
entering the loop. 

Nick 
n.j.cox@durham.ac.uk 

Abhimanyu Arora

I am trying to use mata to find the fixed point of a contraction
mapping.
Basically I have 5 non-linear equations in 5 variables (all explicitly
expressed in terms of the others). Since I wasn't successful with
'optimize', I tried to use the following idea-start with a vector of
zeros
and put the elements of vector of unknowns into the RHS of the equations
till convergence is achieved (updated value 'almost equal' to previous
one).


Following is my code and output:



clear mata

. mata
------------------------------------------------- mata (type end to
exit)
------------------------------------------------------------------------
----
--------------------------
: 
: real vector fpt(real scalar x,real scalar y) \\x and y are the
parameters
of the equations 
> {
> v=0,0,0,0,0
> 
> while (mreldif(v,lastv)<10^(-6)) {
> 
> 
> lastv=v
> 
> v[1,1]=0.5775+ln(exp(x+0.9*v[1,2])+exp(y+0.9*v[1,1]))
> v[1,2]=0.5775+ln(exp(2*x+0.9*v[1,3])+exp(y+0.9*v[1,1]))
> v[1,3]=0.5775+ln(exp(3*x+0.9*v[1,4])+exp(y+0.9*v[1,1]))
> v[1,4]=0.5775+ln(exp(4*x+0.9*v[1,5])+exp(y+0.9*v[1,1]))
> v[1,5]=0.5775+ln(exp(5*x+0.9*v[1,5])+exp(y+0.9*v[1,1]))
> 
> 
> }
> return(lastv)
> }
note: variable lastv may be used before set

: 
: 
: fpt(2,3)
               mreldif():  3200  conformability error
                   fpt():     -  function returned error
                 <istmt>:     -  function returned error
r(3200);

: 
: 
: end
------------------------------------------------------------------------
----
------------------------------------------------------------------------
----
------------------------


The error code indicates that there is some operation that is not
compatible
with the matrices involved. It would be nice if you could point out the
mistake.


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