Instead of using accel_find, consider using gsl_interp_bsearch.
The documentation in gsl-1.10/interp/bsearch.c for this
function is given below.
It is also simple to rewrite the accel_find function slightly,
as I did in C++ for O2scl at
http://o2scl.sourceforge.net/o2scl/html/search__vec_8h-source.html
Take care,
Andrew
/*
* Note that this means the relationship of 'x' to x_array[index]
* and x_array[index+1] depends on the result region, i.e. the
* behaviour at the boundaries may not correspond to what you
* expect. We have the following complete specification of the
* behaviour.
* Suppose the input is x_array[] = { x0, x1, ..., xN }
* if ( x == x0 ) then index == 0
* if ( x > x0 && x <= x1 ) then index == 0, and sim. for other
interior pts
* if ( x == xN ) then index == N-1
* if ( x > xN ) then index == N-1
* if ( x < x0 ) then index == 0
*/
----- Original Message ----
From: Daniel J Farrell <address@hidden>
To: address@hidden
Sent: Tuesday, October 23, 2007 8:26:49 AM
Subject: [Help-gsl] Question about interpolation
Hello,
I am using gsl_interp.h functions to look-up the index of a C array
which is the nearest to the value, x. However if the value of the x
is the maximum value of the array GSL doesn't find it correctly.
Example,
double array[5] = {1,2,3,4,5};
gsl_interp_accel *a = gsl_interp_accel_alloc();
unsigned index = gsl_interp_accel_find (a, array, 5, 5);
//index = 3
This is because (as the docs state, http://www.gnu.org/software/gsl/
manual/html_node/Index-Look_002dup-and-Acceleration.html) the value x
must fall into a bin which has the lower value LESS THAN EQUAL TO and
an upper value LESS THAN x.
How can I get a return index = 4?
Cheers,
Dan.
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