comparing float/double values using == operator

Question

The code review tool I use complains with the below when I start comparing two float values using equality operator. What is the correct way and how to do it? Is there a helper function (commons-*) out there which I can reuse?

Description

Cannot compare floating-point values using the equals (==) operator

Explanation

Comparing floating-point values by using either the equality (==) or inequality (!=) operators is not always accurate because of rounding errors.

Recommendation

Compare the two float values to see if they are close in value.

float a;
float b;

if(a==b)
{
..
}

Answer 1

IBM has a recommendation for comparing two floats, using division rather than subtraction - this makes it easier to select an epsilon that works for all ranges of input.

if (abs(a/b - 1) < epsilon)

As for the value of epsilon, I would use 5.96e-08 as given in this Wikipedia table, or perhaps 2x that value.

Answer 2

It wants you to compare them to within the amount of accuracy you need. For example if you require that the first 4 decimal digits of your floats are equal, then you would use:

if(-0.00001 <= a-b && a-b <= 0.00001)
{
..
}

Or:

if(Math.abs(a-b) < 0.00001){ ... }

Where you add the desired precision to the difference of the two numbers and compare it to twice the desired precision.

Whatever you think is more readable. I prefer the first one myself as it clearly shows the precision you are allowing on both sides.

a = 5.43421 and b = 5.434205 will pass the comparison