Re: [Boost] [test] strange behaviour when comparing floating point numbers

From: Matthijs Möhlmann (matthijs_at_[hidden])
Date: 2020-04-21 20:15:46

On 21/04/2020 22:02, Edward Diener via Boost-users wrote:
> On 4/21/2020 3:12 PM, Matthijs Möhlmann via Boost-users wrote:
>> Hi,
>>
>> I am trying to compare two floating point numbers but am encountering
>> somewhat
>> strange behaviour.
>>
>> The minimal compilable example:
>> #define BOOST_TEST_MODULE tests
>> #define BOOST_TEST_DYN_LINK
>> #include <boost/test/unit_test.hpp>
>>
>> BOOST_AUTO_TEST_SUITE(testsuite)
>>
>> BOOST_AUTO_TEST_CASE(test1)
>> {
>> Â Â  namespace tt = boost::test_tools;
>> Â Â  BOOST_TEST(0.1 == 0.2, tt::tolerance(0.01)); // will fail
>> Â Â  BOOST_TEST(0.1f == 0.2f, tt::tolerance(0.02)); // will fail too,
>> but different error message...
>> }
>>
>> BOOST_AUTO_TEST_SUITE_END()
>>
>> When compiling with:
>> clang++ (clang version 7.0.1-8 (tags/RELEASE_701/final))
>> flags: -o main -g -std=c++17 -stdlib=libc++
>> -lboost_unit_test_framework main.cpp
>> boost: 1.72.0
>>
>> I'll get the following output:
>> Running 1 test case...
>> main.cpp(10): error: in "testsuite/test1": check 0.1 == 0.2 has
>> failed [0.10000000000000001 != 0.20000000000000001]. Relative
>> difference exceeds tolerance [1 > 0.01]
>> main.cpp(11): error: in "testsuite/test1": check 0.1f == 0.2f has
>> failed [0.100000001 != 0.200000003]
>>
>> *** 2 failures are detected in the test module "tests"
>>
>> Why am I getting two different messages? I expected those to be the
>> same...
>
> My guess, without knowing how Boost Test works, is that the first
> checks two doubles while the second checks two floats and Boost Test
> puts out different messages for each type.
>
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Thanks you did put me on the right track!

When specifying tt::tolerance(0.01) it will do the check for type
double. I had to do: tt::tolerance(float(0.01)) for the float type.

I should've read the documentation a little bit better:
https://www.boost.org/doc/libs/1_72_0/libs/test/doc/html/boost_test/testing_tools/extended_comparison/floating_point.html#boost_test.testing_tools.extended_comparison.floating_point.type_of_the_tolerance

Regards, Matthijs