[Rcpp-devel] redimension help

[Silkworth,David J.]

Further to our personal exchange I have reviewed Romain's recent post in reply to Fatemeh Riahi
http://lists.r-forge.r-project.org/pipermail/rcpp-devel/2011-May/002256.html

I've attempted to re-write my simple example in the list of vectors format as shown by Romain.

src<-'
int s = 7;  // result of original oversize estimate before process runs
int c=3;  //known column count established from a list argument (variable to function)
int r = 4;  // number of rows that more complex process found necessary to fill
int i=0;
     Rcpp::List Lmv(c) ;  // list of matrix column vectors
     for(  i=0; i<c; i++)  {
         Rcpp::IntegerVector m(s) ;
// insertion of next three lines partially fills the matrix column vectors
// the same way as previous sample code
for( int j=0; j<r; j++){
m[j]= (i+1)*(j+1);
}
Lmv[i]=m;
}
// this code does not access the vector elements as I wish
//for(int j=0; j<r;j++)  { 
//for( i=0;i<r;i++)  {
//Lmv[i][j]= (i+1)*(j+1);} }

return Lmv;

'

 fun <- cxxfunction(signature(),src, plugin = "Rcpp")

I can create the partially filled matrix as a list of vectors.

But I don't know how to access the individual vector elements outside of the initialization loop.

Then, when I tried to perform the vector erasures I had more trouble:

Adding these lines before the return call does NOT work as I wanted either.
for(i=0; i<c; i++)  {
for(int e=s-1; e>r-1;e--)  {
 Lmv[i].erase(e); }  // redimension the vector
}

I'm trying, but still stuck.



-----Original Message-----
From: Silkworth,David J. 
Sent: Tuesday, June 07, 2011 3:42 PM
To: 'Douglas Bates'
Subject: RE: [Rcpp-devel] redimension help

My apologies, Doug.

I've tried to distill the issue to a "simple", but complete example.

src <- '
int s = 7;  // result of original oversize estimate before process runs
int c=3;  //known column count established from a list argument (variable to function)
Rcpp::IntegerVector v(s);
Rcpp::IntegerMatrix m(s,c);

int r = 4;  // number of rows that more complex process found necessary to fill
for(int x=1; x<r+1;x++)  { v[x-1]=x; }  // just partial fill as process would
for(int j=0; j<r;j++)  { for(int i=0;i<r;i++)  {m(i,j)= (i+1)*(j+1);} }

for(int e=s-1; e>r-1;e--)  { v.erase(e); }  // redimension the vector

Rcpp::List L=Rcpp::List::create(v,m);
return L;
'

 fun <- cxxfunction(signature(),src, plugin = "Rcpp")

The erase loop on the vector performs a redimension "clean-up" so to speak from my over-estimate of required dimension.  The estimate is made at run time, just that it is made before the matrix is dimensioned and the rest of the function has executed.  The reason that there is shrinkage is that there are duplicate entries that the process finds and adjusts its matrix fill operation for.

I cannot duplicate such an erase operation on the matrix.  But I think that there is probably a different approach that would work on both the vector and matrix, if I was only smart enough.

I needed the matrix, because its number of columns is only determined at run time, so I need a way to have indexed labels for the column vectors that it creates.



Douglas Bates replied:

I think we are stuck too in that we can't tell what you are trying to
do.  (Well, at least I can't.).

Why do you bother erasing elements beyond the size() of the vector?
I'm pretty sure that will have no effect.

Perhaps if you were a bit more explicit about what you are trying to
do we could help.  As it stands you are telling us that you have tried
to use a method that isn't defined and all we can say is, "Yep, it
isn't defined."