How to control double precision computing to avoid rounding errors in C++ linux x86_64?

Question

In a C++ code on linux x86_64, I need to double precision computing (+ or -).

26.100000000000001 - 26 + 0.10000000000000001

I got:

0.20000000000000143

I want to get 0.2.

here, the display format is not import, the computing results will be used for some if-else branch conditions. So, I only want the if-else conditions compare the 4 digits after the decimal digit.

It seems a rounding error ?

How to restrict the computing precision to 4 digits after decimal point ?

I do not want to call any functions in order to avoid overhead.

I do not want to use stringstream due to transformation overhead.

Any better ideas ?

thanks

Answer 1

The computing precision is fine. It's the display precision you're trying to control. You can do that with setprecision() if using <iostream> or a formatter like "%.4f" if using <stdio.h>.

You are already calling functions since you are displaying the result as decimal!

P.S. 0.1 cannot be exactly represented by a float, double, or any binary-mantissa format. It factors into (1/2) * (1/5), and decimal 1/5 is an infinitely-repeating digit sequence in binary.

P.P.S. Go look at GMP. It might be your best hope.

Answer 2

If you just want to print it it, you can use printf("%10.4lf"). You can alter the precision to anything you want, of course.

Answer 3

If you are only interested in equality up to four decimal places, multiply everything by 10,000 and use integer arithmetic. (You might need to round after multiplying by 10,000.)