Re: st: mata programming - input type

[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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