double value subtraction without precision

Question

int base = 12;

double number = 12.2112;

double c = number - base;
//// c=0.211199999999999983   

this is c++ code, How could I get the outcome: c= 0.2112,

Answer 1

0.2112 cannot be exactly represented in Floating Point Notation: https://en.wikipedia.org/wiki/Floating_point#Accuracy_problems

If it's extremely important to you that 0.2112 be represented exactly, then the usual solution looks a bit like this:

int base = 120000;
int number = 122112;
int c = number - base;

cout << "Value of c: " << (c / 10000.0) << endl;

Know, however, that what we're doing here is implementing fixed-point numbers. If you need fixed-point numbers in your implementation, it may be worthwhile to research and implement a full fixed-point class that does everything you're trying to accomplish here.