RE: st: do loops and mata

["Schaffer, Mark E" <M.E.Schaffer@hw.ac.uk>]

Jonathan, Anthony,

I think there might be a typo or 2 in Anthony's code (e.g., "M[i] = &P") but I agree with Anthony that this is the way to go.

Below his code but with (I think) the typos fixed.  I also changed M to a column vector (easier to read).

--Mark

mata

P = st_data( ., ("ee", "eu" ,"ue", "uu") )

M = J(rows(P), 1, NULL)

 for ( i=1; i<=rows(P) ;i++) {

 Pi = P[i,.]
 xi = rowshape(Pi,2)
// store xi in M
 M[i] = &xi

 }

// look at M
M
// not useful - just a bunch of memory junk

// get 10th matrix
*M[10]

end

> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-
> statalist@hsphsun2.harvard.edu] On Behalf Of Thomas, Anthony
> Sent: 13 March 2014 15:18
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: do loops and mata
> 
> Hi Jonathan,
> 
> You could also try putting each matrix in an array in Mata. They
> wouldn't have a nice naming system like Stata matrices do, but then
> your code could be contained to Mata. Note that as Nick points out,
> this may be needlessly complicated, but if you're interested in a Mata
> only solution this is what I would do.
> 
> **************** CODE ****************
> 
> P = st_data( ., ("ee", "eu" ,"ue", "uu") ) P
> 
> // assuming P has 39 rows...?
> M = J(1, 39, NULL)
> 
>  for ( i=1; i<=rows(P) ;i++) {
> 
>  P= P[i,.]
>  P
> // store P in M
>  M[i] = &P
> 
>  xi=rowshape( Pi, 2)
>  xi
> 
>  }
> 
> ****************** END ****************
> 
> The matrix M holds pointers to the 39 smaller matrices P1 - P39. These
> pointers tell the computer where the 39 small matrices are stored in
> the system's memory. Then you can ask Mata to retrieve one of the
> matrices by doing the following:
> 
> // look at M
> M
> // not useful - just a bunch of memory junk
> 
> // get 34th matrix
> *M[34]
> 
> Others may have a better solution, but this is just what came to mind.
> Arrays are cool in general and come in handy in a variety of
> situations.
> 
> Anthony
> 
> 
> On Thu, Mar 13, 2014 at 10:36 AM,  <J.Wadsworth@lse.ac.uk> wrote:
> > Thanks again Nick. I'll persevere using the old command structure
> >
> > The reason I need the sequence of matrices is that my colleagues and I are
> writing a paper that involves decomposing the contribution of labour market
> transition probabilities to changes in the labour market stocks. After some
> algebra this essentially involves multiplying the sequence of transition
> probability matrices from t= 1 to t=t. So I need to refer specifically to each
> transition matrix at each period t and loop this because the calculation will be
> different at each t (involve a different set of matrices)
> >
> > Hope that makes things a little more transparent. Let me know if mata ever
> gets macros
> >
> > Jonathan
> >
> > -----Original Message-----
> > From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-
> statalist@hsphsun2.harvard.edu] On Behalf Of Nick Cox
> > Sent: 13 March 2014 14:10
> > To: statalist@hsphsun2.harvard.edu
> > Subject: Re: st: do loops and mata
> >
> > The short answer is that there is no exact Stata equivalent, because Mata as
> such does not have macros. But although you did not specify an exact Mata
> equivalent, I guess the longer answer would depend on what you want to do
> with these matrices. It could be that you need something deeper, e.g. pointers
> or structures.
> >
> > Indeed, although it's always fair just to ask about language details as a matter
> of curiosity, I remain very puzzled why you want to do this. I've never wanted to
> do anything like this in Mata: that means no more than it says, and can be put
> down to narrowness of my experience, but it's why I am puzzled.
> > Nick
> > njcoxstata@gmail.com
> >
> >
> > On 13 March 2014 13:48,  <J.Wadsworth@lse.ac.uk> wrote:
> >> Dear Nick
> >>
> >> Thanks for the quick response. The commands is indeed almost
> >> equivalent to vec (except I need to go from a 39X4 matrix to 39  2 by
> >> 2 matrices rather than a column vector ) but I still need a loop to
> >> extract the 39 separate matrices and label them differently
> >>
> >> I can do this in the old matrix sequence using the commands
> >>
> >> mkmat ee eu ue uu, matrix(P)
> >> matrix list P
> >>
> >> local i=1
> >> while `i'<=rowsof(P) {
> >> matrix P`i'= P[`i',1...]        /* loops to give 39 different 1 by 4 matrices of
> transition probabilities */
> >> local i=`i'+1
> >> }
> >>
> >> matrix list P1
> >> matrix list P39
> >>
> >> but I was interested in doing the mata equivalent
> >>
> >> thanks again
> >>
> >> Jonathan Wadsworth
> >>
> >>
> >> -----Original Message-----
> >> From: owner-statalist@hsphsun2.harvard.edu
> >> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nick Cox
> >> Sent: 13 March 2014 12:49
> >> To: statalist@hsphsun2.harvard.edu
> >> Subject: Re: st: do loops and mata
> >>
> >> This looks like a way of reinventing -stack-. Am I wrong?
> >>
> >> Nick
> >> njcoxstata@gmail.com
> >>
> >>
> >> On 13 March 2014 12:34,  <J.Wadsworth@lse.ac.uk> wrote:
> >>
> >>> As a novice to mata I'm trying to generate a set of 39 matrices using
> >>> a loop - essentially extracting each row of a larger matrix P
> >>> sequentially and then transforming the vector into a matrix using the
> >>> rowshape command
> >>>
> >>> I've got as far as this - which does indeed scroll through a matrix P
> >>> 39 times and extract a row at a time and writes to a mtrix Pi, but
> >>> then it overwrites the matrix Pi each time so that I'm left with just
> >>> one matrix called Pi at the end of the loop instead of 39 matrics
> >>> called P1--P39
> >>>
> >>> Does anyone know how to adapt the code below to generate and store 39
> different matrices named p1, p2, ..p39 ?
> >>
> >>> mata
> >>>
> >>>
> >>> P = st_data( ., ("ee", "eu" ,"ue", "uu") ) P
> >>>
> >>> for ( i=1; i<=rows(P) ;i++) {
> >>>
> >>> Pi= P[i,.]
> >>> Pi
> >>>
> >>> xi=rowshape( Pi, 2)
> >>> xi
> >>>
> >>> }
> >>>
> >>> end
> >>
> >> *
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