CODEWITHSUNDEEP
c++rrcpp
Ruben
Ruben

Reputation: 304

Two apparently similar functions give a different result. Why?

I am using Rcpp to integrate a piece of C++ code in R. I am implementing a function in C++ in two ways:

// [[Rcpp::export]]
double rhobiweight(double x,double k = 2.0){
  double rho = 1.0;
  if(abs(x)<k){
    rho = 1.0-pow((1.0-pow(x/k,2)),3);
  }
  return rho/Erho(k) ;
}

// [[Rcpp::export]]
double rhobiweight2(double x,double k = 2.0){
  double rho = 1.0-pow((1.0-pow(x/k,2)),3);
  if(abs(x)>k){
    rho = 1.0;
  }
  return rho/Erho(k) ;
}

If the x-value is between 2 and 3, I get different results of these functions. I can't figure out why.

> set.seed(1)
> x = 3*rnorm(10^5)
> c = x
>   c2 = x
>   for(i in 1:length(x)){
+     c[i] = rhobiweight(x[i])
+     c2[i] = rhobiweight2(x[i])
+   }
>   print(sum(c-c2))
[1] -18564.31

Upvotes: 3

Views: 111

Answers (1)

J&#233;r&#244;me
J&#233;r&#244;me

Reputation: 8066

The problem comes from your if statement inside the function. The negative of < is >=. so you should either replace < with <= in one function or > with >= in the other, depending on the expected behaviour.

Your problem happens for value between 2.0 and 2.9 inclusive because the abs(int) will always return 2 for this range.

abs(x) takes an int an return an int. Your double x is implicitly conversion to an int in your case.

Upvotes: 2

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