[Rcpp-devel] how to reference a row of a matrix in C++ in Rcpp

[Serguei Sokol]

Le 20/03/2018 à 12:10, akshay kulkarni a écrit :
> dear members,
> 
>                               I came to know from stackoverflow that the following references a row in a matrix in C++:
> 
> M[2] references 2nd row of the Matrix.
SO is too big to check this assertion by ourself. Do you have a link?

> 
> 
> I am using Rcpp to write C++ code in R.
> 
> 
> However, I ended up with the following inconsistency:
> 
>  > M
>       x y z
> [1,] 1 1 1
> [2,] 2 2 2
> [3,] 3 3 3
> [4,] 4 4 4
> [5,] 5 5 5
> [6,] 6 6 6
>  > cppFunction('IntegerVector tccp3(IntegerMatrix M) { IntegerVector x = M[2]; return x;}')
Try

cppFunction('IntegerVector tccp3(IntegerMatrix M) { IntegerVector x = M(2,_); return x;}')

Best,
Serguei.


>  > tccp3(M)
> [1] 0 0 0
>  > cppFunction('IntegerVector tccp4(IntegerMatrix M) { IntegerVector x = M[1]; return x;}')
>  > tccp4(M)
> [1] 0 0
> 
> tccp3 should return (3,3,3) and tccp4 should return  (2,2,2). Can you please shed light on what is going on?
> 
> very many thanks for your time and effort....
> 
> Yours sincerely,
> AKSHAY M KULKARNI
> 
> 
> 
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