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

[Serguei Sokol]

Dear Akshay,

In previous messages, Dirk and I told you that this list was not
the right place for beginner questions. Please stop asking that kind
of questions here.

Best,
Serguei.

Le 21/03/2018 à 09:40, akshay kulkarni a écrit :
> dear serguei,
> 
>                           I have another question:
> 
> If M is a matrix, M.nrow() should return the number of rows...but I encountered the following inconsistency:
> 
> 
>  > M
>       x  y  z
> [1,] 1  0  0
> [2,] 4  3  0
> [3,] 3  1  1
> [4,] 2 90 87
> [5,] 9 76 23
> [6,] 5 23 13
>  > cppFunction('IntegerVector tccp5(IntegerMatrix M) { int x = M.nrow(); return x;}')
>  > tccp5(M)
> [1] 0 0 0 0 0 0
> 
> can you please explain what is happening( I am very new to Rcpp and C++)?
> 
> very many thanks for your time and effort...
> yours sincerely,
> AKSHAY M KULKARNI
> 
> 
> 
> ----------------------------------------------------------------------------------------------------------------------------------------------------------------
> *From:* Serguei Sokol <serguei.sokol at gmail.com>
> *Sent:* Tuesday, March 20, 2018 7:26 PM
> *To:* akshay kulkarni; Rcpp R
> *Subject:* Re: [Rcpp-devel] how to reference a row of a matrix in C++ in Rcpp
> 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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