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From |
Matthew Baker <matthew.baker@hunter.cuny.edu> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Mata void functions and pointers |

Date |
Thu, 17 May 2012 21:06:32 -0400 |

Brendan, Jeff, and Brian -- Thanks very much for all your helpful suggestions. I'm glad I asked! Best, Matt On Thu, May 17, 2012 at 6:41 PM, Jeff Pitblado, StataCorp LP <jpitblado@stata.com> wrote: > Matthew Baker <matthew.baker@hunter.cuny.edu> is working with pointers in > Mata. Originally he wrote: > >> I have a problem in which I am trying to populate the elements of >> a pointer with the results of a function. The problem is, the function >> is void and returns its results through an argument, which makes >> getting results into a pointer nontransparent. As a simple example, >> consider the following: >> (example omitted) > > Brendan Halpin <brendan.halpin@ul.ie> replied with a suggestion for Matthew's > specific, but it did not address Mathew's general need, so Mathew gave us more > information: > >> I should have been more clear! I would certainly prefer to use a >> vector to collect the elements, but the objects I'm collecting are of >> unknown dimension which is not known in advance, so perhaps I should >> have used an example like: >> >> /* Begin example */ >> clear all >> mata >> void example(A,B,Ab) Ab=J(A+round(3*runiform(1,1)),B,1) >> P=J(3,1,NULL) >> for (i=1;i<=3;i++) { >> example(i,i,Ab=.) >> Ab >> P[i]=&Ab >> } >> >> /* display results */ >> >> for (i=1;i<=3;i++) *P[i] >> end >> /* end example */ >> >> I have thought about trying to readjust the dimensions of the vector >> every time it is a little different, but this seems kind of clumsy and >> I was hoping for a better alternative. > > Mathew can preallocate each element of the P vector with an address to it's > own copy of a default Mata object, then pass that object to his 'example()' > function. Here is how that would work with Mathew's example above: > > ***** BEGIN: > clear all > mata: > void example(A,B,Ab) Ab=J(A+round(3*runiform(1,1)),B,1) > P=J(3,1,NULL) > for (i=1;i<=3;i++) { > P[i] = &J(0,0,.) // PREALLOCATION > example(i,i,*P[i]) > *P[i] > } > > /* display results */ > > for (i=1;i<=3;i++) *P[i] > end > ***** END: > > --Jeff > jpitblado@stata.com > * > * For searches and help try: > * http://www.stata.com/help.cgi?search > * http://www.stata.com/support/statalist/faq > * http://www.ats.ucla.edu/stat/stata/ -- Dr. Matthew J. Baker Department of Economics Hunter College and the Graduate Center, CUNY * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**Re: st: Mata void functions and pointers***From:*jpitblado@stata.com (Jeff Pitblado, StataCorp LP)

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