Hallo,
I'm not really shure if it's a bug, but I cann't find other reason for
the following error.
Please take a look at the following programm:
#include <iostream>
#include <gsl/gsl_bspline.h>
int main()
{
//allocate workspace for five cubic splines
gsl_bspline_workspace * w = gsl_bspline_alloc(4, 5);
//construct five knots {0, 1, 2, 3, 4}
gsl_bspline_knots_uniform(0, 4, w);
//allocate a vector to save the b-spline values in it
//the size of the vector is in this case 7
gsl_vector * values = gsl_vector_alloc(gsl_bspline_ncoeffs(w));
//evaluate the b-splines at the point x = 0
gsl_bspline_eval(0, values, w);
std::cout << "x = 0, values: ";
for (std::size_t i = 0; i < values->size; i++)
std::cout << gsl_vector_get(values, i) << " ";
std::cout << std::endl;
gsl_bspline_eval(1, values, w);
std::cout << "x = 1, values: ";
for (std::size_t i = 0; i < values->size; i++)
std::cout << gsl_vector_get(values, i) << " ";
std::cout << std::endl;
gsl_bspline_eval(2, values, w);
std::cout << "x = 2, values: ";
for (std::size_t i = 0; i < values->size; i++)
std::cout << gsl_vector_get(values, i) << " ";
std::cout << std::endl;
gsl_bspline_eval(3, values, w);
std::cout << "x = 3, values: ";
for (std::size_t i = 0; i < values->size; i++)
std::cout << gsl_vector_get(values, i) << " ";
std::cout << std::endl;
gsl_bspline_eval(4, values, w);
std::cout << "x = 4, values: ";
for (std::size_t i = 0; i < values->size; i++)
std::cout << gsl_vector_get(values, i) << " ";
std::cout << std::endl;
}
This programm calculates the values of the B-Splines Bi, i = -1, 0, ...
, 5, at the points 0, 1, 2, 3 and 4.
The output schould be like this:
x = 0, values: 0.166667 0.666667 0.166667 0 0 0 0
x = 1, values: 0 0.166667 0.666667 0.166667 0 0 0
x = 2, values: 0 0 0.166667 0.666667 0.166667 0 0
x = 3, values: 0 0 0 0.166667 0.666667 0.166667 0
x = 4, values: 0 0 0 0 0.166667 0.666667 0.166667
But the programm prints the following output:
x = 0, values: 1 0 0 0
0 0 0
x = 1, values: 0 0.25 0.583333 0.166667 0 0 0
x = 2, values: 0 0 0.166667 0.666667 0.166667 0 0
x = 3, values: 0 0 0 0.166667 0.583333 0.25 0
x = 4, values: 0 0 0 0
0 0 1
The same error I get, when I'm construct and evaluate b-splines over a
bigger interval. The values of the first and the last two knots don't
get evaluated properly. Does anybody know the reasons for such a behavior?
Best regards,
Iryna.
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