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

akshay kulkarni akshay_e4 at hotmail.com
Wed Mar 21 09:40:42 CET 2018

```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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