Odd behavior comparing doubles, two PHP double values aren't equivalent

Question

I have two seemingly equal double values in PHP (at least when echoing them).

But when comparing them with double equals, for some reason, it evaluates to false. Are there any special considerations when performing this kind of comparison?

Answer 1

You shouldn't compare floating point numbers using the == operator.

See the big warning and explanation in the php manual

What will work is asserting that the two numbers are within a certain small distance of each other like this:

if(abs($a - $b) < 0.0001) {
    print("a is mostly equal to b");
}

The reason is because of rounding errors due to floating point arithmetic performed after the decimals are converted to binary, then converted back to decimal. These back and forth conversions cause the phenomenon where 0.1 + 0.2 does not equal 0.3.

Answer 2

float and double should never be compared for equality: there are precision errors that will make two numbers different even if they seem equal (when they are printed out, they are usually rounded).

Proper way to compare is using some DELTA constant:

define(DELTA, 0.00001); // Or whatever precision you require

if (abs($a-$b) < DELTA) {
  // ...
}

Also note that this is not PHP specific but also important in other languages (Java, C, ...)