st: query regarding mata

["Abhimanyu Arora" <abhimanyu.arora@student.kuleuven.be>]

Dear statalist users

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.

Thank you very much

Abhimanyu Arora
PhD student
KU Leuven


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