c++ - modf returns 1 as the fractional: -

i have static method, receives double , "cuts" fractional tail leaving 2 digits after dot. works almost time. have noticed when receives 2.3 turns 2.29. not happen 0.3, 1.3, 3.3, 4.3 , 102.3. code multiplies number 100 uses modf divides integer value 100 , returns it. here code catches 1 specific number , prints out:

static double dround(double number) {      bool c = false;     if (number == 2.3)         c = true;       int factor = pow(10, 2);     number *= factor;       if (c) {         cout << " number *= factor : " << number << endl;         //number = 230;// when not marked comment code works well.     }       double returnval;     if (c){         cout << " fractional : " << modf(number, &returnval) << endl;         cout << " integer : " <<returnval << endl;     }       modf(number, &returnval);     return returnval / factor; } 

it prints out:

number *= factor : 230

fractional : 1

integer : 229

does know why happening , how can fix this? thank you, , have great weekend.

remember floating point number cannot represent decimal numbers exactly. 2.3 * 100 gives 229.99999999999997. modf returns 229 , 0.9999999999999716.

however, cout's format display floating point numbers 6 decimal places default. 0.9999999999999716 shown 1.

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you use (roughly) upper error limit value represents in floating point avoid 2.3 error:

#include <cmath> #include <limits> static double dround(double d) {    double inf = copysign(std::numeric_limits<double>::infinity(), d);    double thenumberafter = nextafter(d, inf);    double epsilon = thenumberafter - d;     int factor = 100;    d *= factor;    epsilon *= factor/2;    d += epsilon;     double returnval;    modf(number, &returnval);    return returnval / factor; } 

result: http://www.ideone.com/ywmua