[Rcpp-devel] how to reference a row of a matrix in C++ in Rcpp
akshay kulkarni
akshay_e4 at hotmail.com
Tue Mar 20 16:54:51 CET 2018
dear serguei,
Thanks...
Can I also write this:
M[1:i, n:m] as we write in R?
If not, are there any packages that do it for me?
what about RcppEigen?
Basically, I want to write the following R code:
{ p <- 1;
for(i in 550:nrow(x)){
y[p] <- f(x[1:i,])
p <- p+1;
return(y)
}
in C++ using Rcpp. what is the best way to do that?
x is an xts object containing OHLC historical stock prices of the stock x and f acts on the daily increments of x.
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 7:26:51 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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