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Timing of SET_VECTOR_ELT

5 messages · Duncan Murdoch, Terry Therneau, Simon Urbanek

Original
Terry Therneau
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Assume a C program invoked by .Call, that returns a list.

  Near the top of the program we allocate space for all the list elements. (It is my habit 
to use "xyz2" for the name of the R object and "xyz" for the pointer to its contents.)

     PROTECT(means2 = allocVector(REALSXP, nvar));
     means = REAL(means2);
     PROTECT(u2 = allocVector(REALSXP, nvar));
     u = REAL(u2);
     PROTECT(loglik2 = allocVector(REALSXP, 2));
     loglik = REAL(loglik2);

     PROTECT(rlist = mknamed(VECSXP, outnames));

Can I assign the individual elements into rlist using SET_VECTOR_ELT at this point, or do 
I need to wait until the end of the program, after I've filled in means[i], u[i], etc.?  I 
likely depends on whether I'm assigning a pointer or a copy.


Terry T.
Simon Urbanek
#
On Apr 1, 2013, at 1:10 PM, Terry Therneau wrote:

            

      
      
      
    

        
You're assigning a pointer, so it doesn't matter.

FWIW, you can avoid all the PROTECTion mess if you alloc+assign, e.g.

SEXP rlist = PROTECT(mknamed(VECSXP, outnames));
SEXP means = SET_VECTOR_ELT(rlist, 0, allocVector(REALSXP, nvar));
...

since you only need to protect the parent object.

Cheers,
Simon

  
  


  


      

    

    

      
      

        


  

        

      Duncan Murdoch
    

    

      
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On 01/04/2013 1:10 PM, Terry Therneau wrote:

            

      
      
      
    

        
You could assign them at this point, but only if you're sure the 
pointers won't change.   You might change the pointers if you reallocate 
a vector to lengthen it or shorten it, for example. Generally speaking 
this won't happen behind your back, but you might forget and do it in 
the future when you modify the function, so I'd say it's safer to wait 
until the last minute to put the pointers in the list.

Duncan Murdoch

  
  


  


      

    

    

      
      

        


  

        

      Terry Therneau
    

    

      
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On 04/01/2013 12:44 PM, Simon Urbanek wrote:

            

      
      
      
    

        
Neat trick.  I take it that SET_VECTOR_ELT returns a pointer to the object that was just 
created?  I haven't found a description of the function in any of the documents, only 
examples of its use, so this is a surprise to me.
    Lacking documentation, can I count on it in the future?

Terry T.

  
  


  


      

    

    

      
      

        


  

        

      Simon Urbanek
    

    

      
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On Apr 1, 2013, at 2:54 PM, Terry Therneau wrote:

        

            

      
      
      
    

        
Well, it's a pretty fundamental function, so if its behavior changed, the whole world would collapse ;) so if you can't rely on SET_VECTOR_ELT then I don't know what else you can rely on. Its return value is also used in R itself, so it's not an obscure use. It may look a bit scary as the upper-case may suggest it's a macro, but R-exts clarifies that it is a function so the above is ok (and it's frequently used with allocations in R itself).

Cheers,
Simon