[Bug-gsl] GSL Interpolation

[Andreas Ipp]

Hello,

I have a recommendation for improving the spline interpolation 
documentation:

One should mention that x-values have to be given in increasing order... 
i.e. x[0] < x[1] < x[2] < ...

I just had the problem where I wanted to include a spline for a function 
that I get from my calculation as e.g.
x[0] = 10; y[0] = ...
x[1] = 9;
x[2] = 8;

etc. so that x[0] > x[1] > x[2]...
Since I was new to interpolation, at first I could not understand the 
error message:

gsl: interp.c:134: ERROR: interpolation error
Default GSL error handler invoked.
Aborted


Also, since we only had the library and not the source code installed at 
my work place, I could not check which line caused the error.
Finally, by trial and error, I realized that reverse order of x is not 
allowed for interpolation, but it would have been nice, if it had been 
mentioned somewhere in the manual (in other systems, it doesn't matter 
whether x is sorted upward or downward...)

Sincerely,
Andreas Ipp