Test that two floating point numbers do not differ by more than a fixed number of possibly representable values for the type.

Used in: Template parameter to crpcut::match<matcher>(...) in an ASSERT_PRED(pred, ...) check.

Verify that a and b do not differ by more than a fixed number of “Units of Least Precision”. See Cygnus information on comparing floating point numbers for an introduction to ULPS. The two parameters a and b must have the same floating point type.

It is, optionally, possible to decide whether infinity is a valid number (one ULPS higher than std::numeric_limits<T>::max()) by by setting the second parameter to the constructor to one of crpcut::include_inf or crpcut::exclude_inf. By default crpcut::exclude_inf is assumed.

	Caution
	ULPS diff only works for float and double, and is only tested on x86 and compatible architectures and is almost certain to fail horrendously on others.

Example: the test program

     
     #include <crpcut.hpp>
     #include <numeric>
     #include <limits>
     
     template <typename T>
     class moving_avg
     {
     public:
       moving_avg() : avg(T()), n(T()) {}
       moving_avg& operator+=(T t) { ++n; avg-= (avg - t)/n; return *this; }
       operator T() const { return avg; }
     private:
       T avg;
       T n;
     };
     
     static const unsigned count = 300;
     
     TEST(too_narrow)
     {
       moving_avg<float> mavg;
       float sum = 0.0;
       for (unsigned n = 0; n < count; ++n)
         {
           mavg+= 1.0f/3 + float(n);
           sum+= 1.0f/3 + float(n);
        }
       float avg = sum/count;
       ASSERT_PRED(crpcut::match<crpcut::ulps_diff>(2), float(mavg), avg);
     }
     
     TEST(close_enough)
     {
       moving_avg<float> mavg;
       float sum = 0.0;
       for (unsigned n = 0; n < count; ++n)
         {
           mavg+= 1.0f/3 + float(n);
           sum+= 1.0f/3 + float(n);
         }
       float avg = sum/count;
       ASSERT_PRED(crpcut::match<crpcut::ulps_diff>(10), float(mavg), avg);
     }
     
     int main(int argc, char *argv[])
     {
       return crpcut::run(argc, argv);
     }

      

fails one test:

     FAILED: too_narrow
     phase="running"  --------------------------------------------------------------
     /home/bjorn/devel/crpcut/doc-src/samples/ulps_diff.cpp:56
     ASSERT_PRED(crpcut::match<crpcut::ulps_diff>(2), float(mavg), avg)
       param1 = 149.833
       param2 = 149.833
     crpcut::match<crpcut::ulps_diff>(2) :
         Max allowed diff = 2 ULPS
         Actual diff = 5 ULPS
     
     -------------------------------------------------------------------------------
     ===============================================================================
     Total 2 test cases selected
     UNTESTED : 0
     PASSED   : 1
     FAILED   : 1

      

See also crpcut::abs_diff and crpcut::relative_diff for alternative floating point number matching methods.