Re: st: a question about equation solution

[Matthew Baker <matthew.baker@hunter.cuny.edu>]

The mystery is resolved by observing that you are in fact getting the
requested answers...given that you have restricted the search interval
to the (0,1) range. If I understand it correctly, the first argument
of mm_root is to contain a scalar to be replaced with the answer, the
second the function you are finding a root for, and here is the key -
the third and fourth arguments are the lower and upper search limits.
So, if you change, the line of code:
r=mm_root(x=.,&y(),0,1,1e-9,1000,v[i,2],v[i,3],v[i,4])
To
r=mm_root(x=.,&y(),-10,10,1e-9,1000,v[i,2],v[i,3],v[i,4])
(The only change is in the third and fourth arguments)
You should get the sought-after answer.

Hope that helps!

Matt Baker

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




On Tue, Aug 30, 2011 at 12:10 PM, dai_yunhao
<dai_yunhao@smail.hust.edu.cn> wrote:
> Hi, everybody,
>
> I don't know whether the mata program could solve the equation, and I run a
> program like this:
>
> clear all
> set obs 10
> g a=1
> g b=2
> g c=_n
> g x=.
> order x a b c
> mata
> v=J(1,1,.)
> st_view(v,., "x a b c")
> function y(x,a,b,c) {
> return (-c+a+b+x)
> }
> for (i=1;i<=rows(v);i++) {
> r=mm_root(x=.,&y(),0,1,1e-9,1000,v[i,2],v[i,3],v[i,4])
> v[i,1]=x
> }
> end
> l,noo
> gen d=c-a-b
>
> and the result is:
>
>     +---------------------+
>     | x   a   b    c    d |
>     |---------------------|
>  1. | 0   1   2    1   -2 |
>  2. | 0   1   2    2   -1 |
>  3. | 0   1   2    3    0 |
>  4. | 1   1   2    4    1 |
>  5. | 1   1   2    5    2 |
>     |---------------------|
>  6. | 1   1   2    6    3 |
>  7. | 1   1   2    7    4 |
>  8. | 1   1   2    8    5 |
>  9. | 1   1   2    9    6 |
>  10. | 1   1   2   10    7 |
>     +---------------------+
>
> Shouldn't the x be the solution of the equation x+a+b=c ,and the value of x
> equal to the value of d ?
>
>
>
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