[GiNaC-list] how to drop terms in expression with small coefficients?
Tomasz Rudny
trudi at wp.pl
Wed Sep 7 09:32:34 CEST 2011
Thank you very much!
You brought me 1 step closer to my PhD ;-)
Regards,
Tomasz.
PS. I have been using GiNaC for just 1 week, but I have already become
its supporter.
> Hello,
>
> On Sat, Sep 03, 2011 at 10:22:18PM +1000, Tomasz Rudny wrote:
>
>> I have the following problem - I have a polynomial P(x) and before I
>> proceed with some calculations on it, I want to make sure it does
>> not contain terms like for example:
>>
>> 10e-12 * x
>>
>> (in my case x is from (0, 1) so this is safe).
>>
>> I want to somehow get rid of such small terms.
>> In another words for polynomial P(x) = 2.3 * x ^ 2 + 10e-12 * x + 3.76
>>
>> I want to obtain polynomial Q(x) = 2.3 * x ^ 2 + 3.76
> #include<iostream>
> #include<string>
> #include<ginac/ginac.h>
> using namespace GiNaC;
>
> class drop_small_coeff : public map_function
> {
> const numeric small_val;
> public:
> ex operator()(const ex& e)
> {
> if (is_a<add>(e))
> return e.map(*this);
> numeric c = e.integer_content();
> if (c< small_val)
> return 0;
> else
> return e;
> }
> drop_small_coeff(const numeric& small_val_) : small_val(small_val_)
> { }
> };
>
> int main(int argc, char** argv)
> {
> parser p;
> ex e = p(std::string("x^3 + 10*x^2 + 1/1000*x + 1/10"));
> drop_small_coeff d(numeric("1/100"));
> std::cout<< e<< " => "<< d(e)<< std::endl;
> return 0;
> }
>
> By the way, the documentation [1] contains a similar example (look for
> map_rem_quad).
>
> [1] http://www.ginac.de/tutorial/Applying-a-function-on-subexpressions.html#Applying-a-function-on-subexpressions
>
> Best regards,
> Alexei
>
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>
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