Re: [Bug?] Vector view

[Jeremy Theler]

AFAIK, doubles have approx 15-16 decimal digits of precision.
I think your assignment q_test[0]=1.12348; is actually

q_test[0] = 1.1234800000000000341771055900608189

I bet that if you expand 1.12348 as an actual base-2 IEEE floating-
point representation you'll see the point.

--
jeremy theler
www.seamplex.com


On Wed, 2020-03-25 at 20:19 +0000, Huynh, Hoai Nam wrote:
> <
> https://stackoverflow.com/posts/60794220/timeline
> >
> 
> Hello,
> 
> 
>    Would you please answer the question below regarding
> gsl_vector_view_array() ?
> 
> 
> Using the GSL (GNU Scientific Library), I'm trying to understand why
> gsl_vector_view_array() returns a slighly modified value after
> assignment.
> 
> In the code below, I declare a vector_view 'qview_test' which is
> linked to table q_test[0]=0.0 and display its value which is 0.0.
> Then, I change the value of q_test[0]=1.12348 and expecting the same
> value for qview_test, but it gets alterated to
> qview_test=1.1234800000000000341771055900608189.
> 
> How do you explain such a result ? How to replicate the result
> without GSL (for ex. with Matlab) ?
> 
> #include <iostream>
> #include <gsl/gsl_blas.h>
> using namespace std;
> 
> double q_test[1]={0.0};
> gsl_vector_view qview_test;
> 
> int nb_variable = 1;
> 
> int main()
> {
> 
>     qview_test=gsl_vector_view_array(q_test,nb_variable);
> 
>     cout.precision(35);
>     cout << "qview before: " <<
> gsl_vector_get(&qview_test.vector,0)<< endl;
> 
>     // Assign value
>     q_test[0]=1.12348;
> 
>     cout << "qview after: " << gsl_vector_get(&qview_test.vector,0)
> << endl;
> 
>     return 0;
> }
> 
> Thanks,
> 
> 
> Best regards,
> 
> Dr. Hoai Nam HUYNH
> 
> Postdoctoral fellow
> 
> 
> 
> The University of British Columbia
> 
> Department of Mechanical Engineering
> 
> Manufacturing Automation Laboratory
> 
> 2366 Main Mall (ICICS Room 091)
> 
> Vancouver BC V6T 1Z4 Canada
> 
> Phone: +1 604-827-4370
> 
> www.mal.mech.ubc.ca<http://www.mal.mech.ubc.ca/
> >
> 
> [UBC E-mail Signature]
> 
>