Re: st: do loops and mata

["Thomas, Anthony" <anthony_h_thomas@brown.edu>]

But regardless of which matrix you want to store, Mark is correct that

P = P[i,.]

will overwrite the original matrix P which is not good, so if indeed
the Pi matrices need to be stored, just use some new name like:

W = P[i, .]
M[i] = &W

Anthony

On Thu, Mar 13, 2014 at 11:59 AM, Thomas, Anthony
<anthony_h_thomas@brown.edu> wrote:
> It seemed like the "Pi" were the matrices Jonathan wanted to store
> (since he wanted them named P1- P39) and I thought "Pi" was an attempt
> to do P`i' as one would in Stata, but the principle is the same either
> way.
>
> Thanks,
>
> Anthony
>
> On Thu, Mar 13, 2014 at 11:38 AM, Schaffer, Mark E
> <M.E.Schaffer@hw.ac.uk> wrote:
>> 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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