Re: st: Passing a subvector to a void function

[Sergiy Radyakin <serjradyakin@gmail.com>]

Joe, I am afraid that's not going to work. Mata passes structures like
matrices and vectors as pointers. In the help for pointers the
following is applicable (imho) to your case:

---quote1---
You cannot, however, point to the interior of objects.  That is, you
cannot code p = &X[2,3] and get a pointer that is equivalent to X[2,3]
in the sense that if you later coded *p=2, you would see the change
reflected in X[2,3].
--- end of quote 1 ---

--- quote 2 ---
The expression is evaluated and the result of the expression stored in
a temporary variable. That is why subsequently coding *p=2 does not
change X[2,3].  All *p=2 does is change the value of the temporary
variable.
-- end quote ---

So in your case the subvector is extracted correctly, stored in a temp
var, passed to the function, modified, but then disposed of. Instead I
suggest you pass enough information to the procedure being called to
be able to restrict the vector to a certain subvector itself and
receive a full vector as an argument.

Hope this helps. Note that since you are translating a Fortran code,
it might be easier just to create a Fortran plugin. In that case you
would only need to access some Stata objects, but keep the main logic.

Best, Sergiy

On Mon, Jul 29, 2013 at 9:31 AM, Joe Canner <jcanner1@jhmi.edu> wrote:
> Sergiy,
>
> Thanks for looking into this.  Sorry, I should have simplified the example before posting.  The issue is not whether the subroutine will modify the original vector, the issue is whether you can pass a sub-vector and have it come back modified.  To use your suggested code to illustrate:
>
> clear all
> mata
> void arrsum(real rowvector A, real k) {
> real scalar S, i
>   S=0
>   for(i=1;i<=cols(A);i++) {
>     S=S+A[1,i]
>   }
>   A[k]=S
> }
>
> A=1,2,3,4
> A
> arrsum(A[|2\.|],1)
> A
> end
>
> So, instead of passing the entire vector A, I am passing a subvector starting with the 2nd element of A.  What I expect (and want) is for arrsum to put the sum of elements 2-4 into element 2 (since it arrsum is putting the sum of elements 1-3 of the subvector into element 1 of the subvector).  However, if you run this code you will find that A is unmodified upon return.
>
> Regards,
> Joe
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Sergiy Radyakin
> Sent: Monday, July 29, 2013 2:56 AM
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: Passing a subvector to a void function
>
> Joe, you have too many irrelevant arguments in the procedure you are discussing, they complicate understanding. As to the subject you ask, consider the following example: it computes the sum of the array elements, then replaces the specified element of the array with the computed sum. The modified array is returned to the caller.
> Sergiy
>
> clear all
> mata
> void arrsum(real rowvector A, real k) {
>   S=0
>   for(i=1;i<=cols(A);i++) {
>     S=S+A[1,i]
>   }
>   A[k]=S
> }
>
> A=1,2,3,4
> A
> arrsum(A,1)
> A
> end
>
> On Fri, Jul 26, 2013 at 12:12 PM, Joe Canner <jcanner1@jhmi.edu> wrote:
>> A question for all you Mata programmers:
>>
>> I am converting a Fortran 90 program to Mata and the Fortran program passes subarrays as arguments to subroutines like this:
>>
>>         call idtang ( ndp, xd, yd, nt, iwk(jwipt), nl, iwk(jwipl),
>> iwk(jwiwl), iwk(jwiwp), wk )
>>
>> where iwk is an array and iwk(jwipt) means "pass a subarray of iwk starting at element jwipt".  The subroutine modifies this subarray and passes it back.
>>
>> I converted this to Mata as follows:
>>
>>         idtang ( ndp, xd, yd, nt, iwk[|jwipt\.|], nl, iwk[|jwipl\.|],
>> iwk[|jwiwl\.|], iwk[|jwiwp\.|], wk )
>>
>> but iwk is empty upon return.
>>
>> Is there a way to do this that will return the modified subvector and put it back in iwk at position jwipt?
>>
>> I got it to work by creating a new vector ipt, setting it to iwk[|jwipt\.|] and passing it to the function, but I was hoping to avoid having to make so many substantive changes to the original code.
>>
>> Thanks for your help!
>>
>> Joe Canner
>> Johns Hopkins University School of Medicine
>>
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