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Re: st: mata programming - input type


From   Tomáš Houška <xbender@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: mata programming - input type
Date   Thu, 21 Mar 2013 21:10:22 +0100

Hi Nick,

no problem. I got your point about carrying over Stata macros into
Mata from the previous post. I found that information useful, even
though I could not implement it to solve my current problem :-)

The example that you have posted works great - thank you for that!
(maybe one pointer for future readers about a small typo - second line
inside the Mata environment should read    for(i = 1; i <=
length(ids); i++) {

Thank you!
Tomas


2013/3/21 Nick Cox <njcoxstata@gmail.com>:
> You're right. I eat my words: that kind of interactive use within a
> loop does not work. I've evidently never tried that, or forgotten
> trying it.
>
> I was thinking that you were asserting generally that you can't call
> up Mata within a loop, which is wrong, because many programs do it.
>
> But not in that way.
>
> So what I suggest is that you turn it round and write something like this
>
> levelsof time_period, local(period_id)
> mata
> ids = tokens(st_local("period_id"))
> for(i = 1; i <= length(tokens); i++) {
>         EPSILON = st_matrix("EPSILON_M" + ids[i])
>         PRICE   = st_matrix("PRICE_M" + ids[i])
>         ...
> }
> end
>
> Nick
>
> On Thu, Mar 21, 2013 at 6:53 PM, Tomáš Houška <xbender@gmail.com> wrote:
>> Hello Nick,
>>
>> thank you for your help. I have been surprised by your answer as I
>> have met several discussions on Statalist talking about how Mata
>> cannot be processed within Stata loop. I have tried your proposal but
>> I am running into a problem comparable to the problem described in the
>> above mentioned discussions.
>>
>> Even in a very trivial example, in which I just want to list the macro
>> carried over from Stata to Mata:
>>
>> levelsof time_period, local(period_id)
>>
>> foreach period of local period_id {
>>
>>         mata
>>         mata_period = st_local("period")
>>         mata_period
>>         end
>> }
>>
>> ... I get an error "-Break-  r(1); " and the values of mata_period are
>> not shown. I expect this to be a comparable problem as e.g.
>> http://www.stata.com/statalist/archive/2007-07/msg00834.html
>> A solution to that would be establishing a mata function (taking
>> "time_period" as an argument) that I will call within the stata loop,
>> Or am I missing something?
>>
>> Thank you ,
>> Tomas
>>
>>
>>
>>
>> 2013/3/21 Nick Cox <njcoxstata@gmail.com>:
>>> It's just not true that Mata cannot be processed within a Stata loop,
>>> but that does not seem to be the problem here.
>>>
>>> The lack of macros in Mata poses challenges that are more evident than
>>> real to many programmers who know a lot of Stata. The question was
>>> addressed in
>>>
>>> SJ-11-2 pr0052  . . . . Stata tip 100: Mata and the case of the missing macros
>>>         . . . . . . . . . . . . . . . . . . . . . . . . W. Gould and N. J. Cox
>>>         Q2/11   SJ 11(2):323--324                                (no commands)
>>>         tip showing how to do the equivalent of Stata's macro
>>>         substitution in Mata
>>>
>>> and the gnomic answer is to solve the problem by avoiding it.
>>>
>>> In the Tip above the main contribution of the junior author (meaning,
>>> younger by just about 2 months) was to add a Perry Mason-type title to
>>> the work of the senior author.
>>>
>>> One good method is to pass macros once to Mata and thereafter to use
>>> Mata's string operators and functions.
>>>
>>> Try this:
>>>
>>> levelsof time_period, local(period_id)
>>>
>>> foreach period of local period_id {
>>>        mata
>>>        //import STATA matrices
>>>        period = st_local("period")
>>>        EPSILON = st_matrix("EPSILON_M" + period)
>>>        PRICE   = st_matrix("PRICE_M" + period)
>>>        ...
>>>        end
>>> }
>>>
>>> So,
>>>
>>> 1. -period- in Mata is just a string scalar containing the same same
>>> contents as a Stata local macro.
>>>
>>> 2. Adding that string scalar to given text is quite enough technique
>>> for what you want.
>>>
>>> 3. More complicated manipulations just need e.g. string functions in Mata.
>>>
>>> Nick
>>>
>>> On Wed, Mar 20, 2013 at 10:16 PM, Tomáš Houška <xbender@gmail.com> wrote:
>>>
>>>> I am trying to write this loop (here I give an example in "stata
>>>> language", but I am aware that this will never work as mata cannot be
>>>> processed within Stata loop):
>>>>
>>>> Example:
>>>>
>>>> levelsof time_period, local(period_id)
>>>>
>>>> foreach period of local period_id {
>>>> mata
>>>>         //import STATA matrices
>>>>         EPSILON = st_matrix("EPSILON_M`period'")
>>>>         PRICE   = st_matrix("PRICE_M`period'")
>>>>         SHARE   = st_matrix("SHARE_M`period'")
>>>>
>>>>         BETA = EPSILON :* ( (SHARE *(1:/PRICE)') )
>>>>
>>>>        st_matrix("BETA`period'",BETA)
>>>>
>>>> end
>>>> }
>>>>
>>>> Which means I have several matrices, each marked by period number and
>>>> I am trying to perform some operations on them (here I give a
>>>> simplified example of my code). I would like to create a mata function
>>>> that would take "period_id" as an input (period_id is always in
>>>> integer) and process the mata code, resulting in creating new matrix
>>>> BETA`period_id'.
>>>>
>>>> So far I have been able to come up with this:
>>>> Example 2:
>>>>
>>>> mata
>>>> mata clear
>>>> void mc_estim(string scalar period) {
>>>>         //import STATA matrices
>>>>         EPSILON = st_matrix("EPSILON_M`period'")
>>>>         PRICE   = st_matrix("PRICE_M`period'")
>>>>         SHARE   = st_matrix("SHARE_M`period'")
>>>>
>>>>         BETA = EPSILON :* ( (SHARE *(1:/PRICE)') )
>>>>
>>>>         st_matrix("BETA`period'",BETA)
>>>>         }
>>>> end
>>>> * run mata function with a selected input
>>>> mata mc_estim(534)
>>>>
>>>> But I am not sure how to use the "input" in the mata function code (I
>>>> expect that the notation is different from Statas local macros....) I
>>>> would appreciate any help on this.
>
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