Differing output between C++ and NativeCall in Raku - c++

Question

I am trying to write a function for cumulative distribution function taken from here.
This is my cpp code:
#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;
double normalCDF( double x )
{
return 0.5 * ( 1.0 + erf( M_SQRT1_2 * x ) );
}
int main() {
cout << normalCDF(4.8) << endl;
cout << normalCDF(5.4) << endl;
cout << normalCDF(5.6) << endl;
cout << normalCDF(5.8) << endl;
cout << normalCDF(5.1) << endl;
cout << normalCDF(-37.5) << endl;
cout << normalCDF(-36.0) << endl;
cout << normalCDF(-37.6) << endl;
cout << normalCDF(-37.5) << endl;
return 0;
}
This is the output when compiled in linux with gcc 6.3.0
0.999999
1
1
1
1
0
0
0
0
I wanted to call same code from raku using NativeCall, so I modified code
test.cpp
extern "C" double normalCDF( double x )
{
return 0.5 * ( 1.0 + erf( M_SQRT1_2 * x ) );
}
created dynamic shared .so library and wrote nativecall code as:
use NativeCall;
sub normalCDF(num64) returns num64 is native('libtest.so') { * };
say normalCDF((4.8).Num);
say normalCDF((5.4).Num);
say normalCDF((5.6).Num);
say normalCDF((5.8).Num);
say normalCDF((5.1).Num);
say normalCDF((-37.5).Num);
say normalCDF((-36.0).Num);
say normalCDF((-37.6).Num);
say normalCDF((-37.5).Num);
The output is:
0.999999206671848
0.9999999666795515
0.9999999892824097
0.9999999966842541
0.9999998301732593
0
0
0
0
Why is the output of same algorithm differing, though the data containers are used as recommended.
System information:
Ubuntu 18.04 64bit with gcc 6.3.0
Rakudo is 2019.07.1 version.

Answer 1

You need to print the floating point numbers with increased precision. The following will give you the maximum precision:
#include <limits>
#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;
double normalCDF( double x )
{
return 0.5 * ( 1.0 + erf( M_SQRT1_2 * x ) );
}
int main() {
typedef std::numeric_limits< double > dmax;
cout.precision(dmax::max_digits10);
cout << normalCDF(4.8) << endl;
cout << normalCDF(5.4) << endl;
cout << normalCDF(5.6) << endl;
cout << normalCDF(5.8) << endl;
cout << normalCDF(5.1) << endl;
cout << normalCDF(-37.5) << endl;
cout << normalCDF(-36.0) << endl;
cout << normalCDF(-37.6) << endl;
cout << normalCDF(-37.5) << endl;
return 0;
}
Notice the #include <limits> and the first two lines in main, although line 2 is what matters.