Question

In my previous question Comparing a double and int, without casting or conversion, I found out how the the difference between two doubles was tripping the comparison.

I came accross the method setprecision(), which will help display all the numbers after decimal.

So, the difference of 6.15 and 3.15 was found to be : 3.00000000000000044408920985006

Now, when it gets compared with 3, it returns a result saying it is greater than 3.

How do I force it to take only a limited number of digits?

When I used 6.1 and 3.1, the difference was : 2.99999999999999955591079014994

How should I make the precision so that we know that it is actually equal to 3, and not less than.

Answer 1

Hopefully you should be knowing that floating/double cannot be exactly represented in binary and truncation happens because of the recurring decimal. Your comparison with a float/double with an integer will always fail.

Even your using setprecision will not work because its a method of iomanip to set the precision of the display and not the value being stored.

The portable way of comparing doubles is not to use the ‘==’ operator but to do something like

bool Compare(double a,double b) {
    std::fabs(a - b) < std::numeric_limits<double>::epsilon();
}

You can use this to compare double with float and or integer. You can also write a similar compare function for float

bool Compare(float a,float b) {
    std::fabs(a - b) < std::numeric_limits<float>::epsilon();
}