[Rcpp-devel] A strange question while using Rcpp

[张晔]

Thanks, Dirk.
I have upload my code to Github: https://github.com/ZhangYet/RcppTest/
And I made a mistake:

| res1 <- mydcov(x,y)/sqrt(mydcov(x,x)*mydcov(y,y))
|
| a <- mydcov(x,y)
| b <- mydcov(x,x)
| c <- mydcov(y,y)
| res2 <- a/sqrt(b*c)
|
| and res2 is different from res1.(res2 is the right answer.)

I checked my test result. res2 is not the right answer, but it's 
different from res.

于 2014/1/17 21:36, Dirk Eddelbuettel 写道:
> On 17 January 2014 at 20:51, 晔张 wrote:
> | Dear all,
> |
> | I have a strange question such that I can't conclude it in the title.
> | I have write an function:
> | //[[Rcpp::export]]
> | double mydcov(NumericVector x, NumericVector y)
>
> It would help us to see the function to have any guess at what may be wrong.
>   
> | This function is computing  ditance covariance. This function is OK. It give
> | the right answer.
> | I use this function to compute distance correltion.
> |
> | //[[Rcpp::export]]
> | double mydcor(NumericVector x, NumericVector y)
> | {
> |     return mydcov(x,y)/sqrt(mydcov(x,x)*mydcov(y,y));
> | }
> |
> | Then, I compile the source code using sourceCpp.
> | And then mydcor(x,y) give a wrong answer.
> |
> | Here comes the strange part:
> | In R, I run code as follow
> |
> | res1 <- mydcov(x,y)/sqrt(mydcov(x,x)*mydcov(y,y))
> |
> | a <- mydcov(x,y)
> | b <- mydcov(x,x)
> | c <- mydcov(y,y)
> | res2 <- a/sqrt(b*c)
> |
> | and res2 is different from res1.(res2 is the right answer.)
> |
> | And I have another test, rewriting the source code for mydcor():
> | version1:
> | //[[Rcpp::export]]
> | double mydcor(NumericVector x, NumericVector y)
> | {
> |     double a = mydcov(x,y)
> |     double b = mydcov(x,x);
> |     double c = mydcov(y,y);
> |     return (a/sqrt(b*c));
> | }
> |
> | version2:
> | //[[Rcpp::export]]
> | double mydcor(NumericVector x, NumericVector y)
> | {
> |     double a = mydcov(x,y)
> |     Rcout<<a<<"\n";
> |     double b = mydcov(x,x);
> |     Rcout<<b<<"\n";
> |     double c = mydcov(y,y);
> |     Rcout<<c<<"\n"
> |     return (a/sqrt(b*c));
> | }
> | This time, version2 make a right answer.
> | It's too weird. I havn't meet such situation before.
>
> I am not sure we are in a position to help you here.
>
> Regards, Dirk
>
> |
> | Happy Lunar New Year (the most important festival in China), and Thanks in
> | advance.
> |
> | Best regards
> | ZhangYet
> |
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