Hi Christophe
Thanks for the tip on inverting instead of division, I didn't know.
Interesting.
In my case size(x) = [1 10000] ... (circa) ... but it calculates quickly
(and only once).
My implementation of the first order Struve function is based on
approximation.
I was wondering if there's anybody who has made a general
implementation. (?)
Following http://mathworld.wolfram.com/StruveFunction.html, using the
gamma function ... maybe this would be better if coded in Fortran?
Best regards,
Claus
On 26-01-2015 11:21, Dang, Christophe wrote:
Hello,
De : Claus Futtrup
Envoyé : samedi 24 janvier 2015 19:31
z = 2/%pi - besselj(0,x) + (16/%pi - 5) .* (sin(x)./x) ..
+ (12 - 36/%pi) * ( (1-cos(x))./(x.^2) );
A division cost much more than several multiplications (I think something like
a 7 factor).
If you have a big amount of data (and thus a long calculation time),
you can reduce this calculation time,
but maybe at the expense of the precision if x take very high or very low
absolute values
(overflow or underflow of x^-2),
with the following trick (not tested):
denominator = x.^(-2);
pi_inv = 1/%pi;
z = 2*pi_inv - besselj(0,x) + ((16*pi_inv - 5) .* sin(x).*x ..
+ (12 - 36*pi_inv) * (1-cos(x))).*denominator;
It replaces 2*size(x) divisions by 1*size(x) division and 2*size(x)
multiplications,
which is usually less time consuming.
Forget it if size(x) is small or x reach very high or very low absolute values.
HTH
--
Christophe Dang Ngoc Chan
Mechanical calculation engineer
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