# [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)
> 	{
> 			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
>
> [1] http://www.ginac.de/tutorial/Applying-a-function-on-subexpressions.html#Applying-a-function-on-subexpressions
>
> Best regards,
> 	Alexei
>
> _______________________________________________
> GiNaC-list mailing list
> GiNaC-list at ginac.de
> https://www.cebix.net/mailman/listinfo/ginac-list
>
>

```