[GiNaC-devel] Expression comparison

[Peter Grobarcik]

Hi,

thanks again for the quick reply, I looked at the things closely and
try to summarize
them for me:
1) it is OK to use '==' and '!=' operators with expressions.
2) be careful with < <= > >= since they are for internal usage and do
not give you
the comparison of the value of the expression.

Correct?

Cheers,

Peter

2008/9/29, Jens Vollinga <jensv at nikhef.nl>:
> Hi,
>
> Peter Grobarcik schrieb:
>> I have following test case which does not work as I expected:
>>
>>     void testNumComparison() {
>>     	Num v0 = sqrt(Num(2));
>>     	Num v1 = sqrt(Num(10));
>>     	stringstream strm;
>>     	strm << "v0("<<v0<<") < v1("<<v1<<") ?";
>>     	CPPUNIT_ASSERT_MESSAGE(strm.str(),v0<v1);
>>    }
>
>> Can somebody give me (or point me to) some info about expression
>> comparison?
>
> First, sqrt(Num(20)) is internally still a symbolic expression. Only if
> you write sqrt(Num(20.0)) or evalf(sqrt(Num(20)) the expression will
> consist of a numeric object that can be tested with <,>,!=,== in the
> expected way. GiNaC doesn't compute sqrt(20) to its value 4.47... as
> long as the numbers involved (20) are exact and are not of floating
> point type. evalf, of course, forces the evaluation.
>
> Second, ==,!= work (on symbolic expressions) very different from <,>.
> == and != can be evaluated immediately and this is what GiNaC does.
> Effectively, GiNaC will return true or false. <,> on the other hand test
> for the internal ordering of the expressions and determine the canonical
> ordering. This ordering is mainly for internal purposes of GiNaC and has
> nothing to do with the would-be value of the expression.
>
> Regards,
> Jens
>
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