[R] R C API resize matrice

[Morgan Morgan]

Hi Jiedei,

Thank you for your reply.

To give you a bit more context, I wrote a function that find all the
positions (index) of all small matrices inside a larger matrix. At the
beginning I pre-allocate let's say a 100 by 2 matrix. However a lot of
values might remain empty in this matrix of postions so I have to resize it
down to keep only the relevant value. Does it make sense?

Please let me know what you think and if there is a safer way to do it?

Please let me know if you want more information or have any questions.

Best regards
Morgan

On Sat, 15 Jun 2019 00:15 King Jiefei, <szwjf08 using gmail.com> wrote:

> Hi,
>
> I don't think there is a native R API to do what you want here, but if the
> matrix is only used by you and not be exported to the other user, you can
> hack R data structure to achieve that goal.
>
> Because there is not too much context of your question, I will assume the
> whole point of resizing a matrix is to avoid the overhead of memory
> allocation, not to represent the same matrix with different dimension since
> your 'new' matrix has a different number of elements.
>
> Roughly speaking, a matrix in R is nothing but a vector with a dim
> attribute, you can verify it by R code:
> ```
> > A=matrix(1:6,2,3)
> > A
>      [,1] [,2] [,3]
> [1,]    1    3    5
> [2,]    2    4    6
> > attributes(A)
> $dim
> [1] 2 3
>
> > attributes(A)=NULL
> > A
> [1] 1 2 3 4 5 6
> ```
> Therefore, in order to resize the matrix, you need to change the dim
> attribute( to a smaller size). Unfortunately, R does its best to prevent
> you from doing such dangerous operation( and you should know this is* not
> correct!*), you have to go to the C level to hack R internal data
> structure. Let's say you want to resize the matrix A to a 2-by-2 matrix,
> here is what you need to do:
>
> C code:
> The code sets the second value of the dim attribute to 2.
> ```
> // [[Rcpp::export]]
> void I_know_it_is_not_correct(SEXP x,SEXP attrName) {
> INTEGER(Rf_getAttrib(x, attrName))[1]=2;
> }
> ```
>
> R code:
> ```
> > a=matrix(1:6,2,3)
> > I_know_it_is_not_correct(a,as.symbol("dim"))
> > a
>      [,1] [,2]
> [1,]    1    3
> [2,]    2    4
> > attributes(a)
> $dim
> [1] 2 2
> ```
>
> You get what you want. Please use it with your caution.
>
> Best,
> Jiefei
>
>
> On Fri, Jun 14, 2019 at 2:41 PM Morgan Morgan <morgan.emailbox using gmail.com>
> wrote:
>
>> Hi,
>>
>> Is there a way to resize a matrix defined as follows:
>>
>> SEXP a = PROTECT(allocMatrix(INTSXP, 10, 2));
>> int *pa  = INTEGER(a)
>>
>> To row = 5 and col = 1 or do I have to allocate a second matrix "b" with
>> pointer *pb and do a "for" loop to transfer the value of a to b?
>>
>> Thank you
>> Best regards
>> Morgan
>>
>>         [[alternative HTML version deleted]]
>>
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