c++ - Doubles rounding again -

in program there precisions (some positive integer, in cases supposed of form ) doubles, double * precision should become integer.

but know floating point numbers inaccurate, so, example 1.3029515 saved 1.3029514999999998..., , in program need write such floating point number file, want 1.3029515 written instead of 1.3029514999999998....

previously precision of form used in program, , i've reached wanted result piece of code below:

// have function doubles equality check inline bool samedoubles(const double& lhs, const double& rhs, const double& epsilon) {     return fabs(lhs - rhs) < epsilon; }  inline void rounddowndouble(double& value, const unsigned int& numberofdigitsinfraction = 6) {     assert(numberofdigitsinfraction <= 9);     double factor = pow(10.0, numberofdigitsinfraction);     double oldvalue = value;     value = (((int)(value * factor)) / factor);     // when, example, 1.45 stored 1.4499999999..., can wrong value, so, need check below     double diff = pow(10.0, 0.0 - numberofdigitsinfraction);     if(samedoubles(diff, fabs(oldvalue - value), 1e-9)) {         value += diff;     } }; 

but now, can't reach wanted results same technique, i've tried function below, have not succeeded:

// calculates logarithm of number given base double inline lognbase(double number, double base) {     return log(number)/log(base); }  // samedoubles function same in above case  inline void rounddowndouble(double& value, unsigned int precision = 1e+6) {     if(samedoubles(value, 0.0)) { value = 0; return; }     double oldvalue = value;     value = ((long int)(value * precision) / (double)precision);     // when, example, 1.45 stored 1.4499999999..., can wrong value, so, need check below     int pwr = (int)(lognbase((double)precision, 10.0));     long int coeff = precision / pow(10, pwr);     double diff = coeff * pow(10, -pwr);     if(samedoubles(diff, fabs(oldvalue - value),  diff / 10.0)) {         if(value > 0.0) {             value += diff;         } else {             value -= diff;         }     } } 

for 1.3029515 value , precision = 2000000 function returns incorrect 1.302951 value (expression (long int)(value * precision) becomes equal 2605902 instead of 2605903).

how can fix this? or maybe there smart way make rounding happen correctly?

you're doing rounding hard way. easy way instead:

double rounding = 0.5; if (value < 0.0) rounding = -0.5; value = ((long int)(value * precision + rounding) / (double)precision); 

now there's no need rest of code.