Re: st: Sum of products by sub-groups in mata.

[Amadou DIALLO <stata.diallo@gmail.com>]

Dear Nick,
Sorry I was stuck in a meeting.
Many thanks for your answer. However, I find it difficult to do very
basic tasks in mata (as doing a simple egen sum = sum(x) in stata).
Following your example for instance, and doing minor changes, to
populate another matrix with the number of occurrences, I do the
following:

mata: y = ("frog", "frog", "toad", "newt")

mata: allz = .
mata: x = (1, 4, 2,3)
mata: z = (., ., .,.)
mata: sum= sum(x :* (y :== "frog"))
mata: z=sum:*(y :== "frog")
mata: z
mata: z1 = select(z, z[.,.]:>0)
mata: z1
mata: allz = z1
mata: allz

mata: sum= sum(x :* (y :== "toad"))
mata: z=sum:*(y :== "toad")
mata: z
mata: z2 = select(z, z[.,.]:>0)
mata: z2
mata: allz = allz , z2
mata: allz

mata: sum= sum(x :* (y :== "newt"))
mata: z=sum:*(y :== "newt")
mata: z
mata: z3 = select(z, z[.,.]:>0)
mata: z3
mata: allz = allz , z3
mata: allz

mata: sum= sum(x :* (y :== "blabla"))
mata: z=sum:*(y :== "blabla")
mata: z
mata: z4 = select(z, z[.,.]:>0)
mata: z4
mata: allz = allz , z4
mata: allz

This is not very elegant, I believe. I think mata definitely needs the
same sort of "if" qualifiers, as in Stata.

Many thanks for your usual support.

2014/1/20, Nick Cox <njcoxstata@gmail.com>:
> Mata has an -if- but nothing quite like Stata's -if- qualifier. But
> neither is in general a good solution to selective summation.
>
> : y = ("frog", "toad", "newt")
>
> : x = (1, 2,3)
>
> : sum(x :* (y :== "frog"))
>   1
>
> See also -select()- and -panelsetup()-.
>
> Nick
> njcoxstata@gmail.com
>
>
> On 20 January 2014 16:47, Amadou DIALLO <stata.diallo@gmail.com> wrote:
>> Hi,
>> I want to compute in mata total by subgroup (the equivalent of stata's
>> bys group: egen total = total(expense) ).How to do that?
>>
>> Also, is it possible to use "if" condition with mata? SOmething like:
>>
>>    mata:
>>         sum = 0
>>         for (i=1; i<= maxcountry; i++) {
>>            sum = sum + sum(expense) if country == i
>>         }
>>     sum
>>    end
>>
>> --
>> Amadou B. DIALLO, PhD.
>> Senior Economist, AfDB.
>> a.b.diallo@afdb.org
>> +21671101789
>> *
>> *   For searches and help try:
>> *   http://www.stata.com/help.cgi?search
>> *   http://www.stata.com/support/faqs/resources/statalist-faq/
>> *   http://www.ats.ucla.edu/stat/stata/
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
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>


-- 
Amadou B. DIALLO, PhD.
Senior Economist, AfDB.
a.b.diallo@afdb.org
+21671101789
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/faqs/resources/statalist-faq/
*   http://www.ats.ucla.edu/stat/stata/