Problem with BOOST_CHECK_CLOSE_FRACTION

Question

I'm using the Boost::Test library, and I am trying to check if an actual percent value is close to the expected value:

BOOST_CHECK_CLOSE_FRACTION(
    items[i].ExpectedPercent,
    items[i].ActualCount / totalCount,
    0.05);

For some reason this check fails even when the values are close enough:

difference between items[i].ExpectedPercent{0.40000000000000002}
   and items[i].ActualCount / totalReturned{0.42999999999999999}
                                    exceeds 0.050000000000000003

Is this a problem with Boost or a problem with how I am using Boost?

Answer 1

After some testing, it turns out that the documentation for BOOST_CHECK_CLOSE_FRACTION is incorrect. The tolerance should be specified as a fraction of the expected value.

So, TFAE:

BOOST_CHECK(abs(x - y) < (min(x, y) * 0.1));
BOOST_CHECK_CLOSE(x, y, 10);
BOOST_CHECK_CLOSE_FRACTION(x, y, 0.1);

Answer 2

It is a problem with how you are using boost.

The last argument is a percent tolerance, not an absolute variance value. 5% of 0.4 is 0.02.

Answer 3

Obviously BOOST_CHECK_CLOSE and BOOST_CHECK_CLOSE_FRACTION won't work if you check if something is near 0. Then you can use:

BOOST_CHECK(abs(x - y) < accurancy);