Compare floats in php

Question

I want to compare two floats in PHP, like in this sample code:

$a = 0.17;
$b = 1 - 0.83; //0.17
if($a == $b ){
 echo \'a and b are same\';
}
else {
 echo \'a         


        

Answer 1

If you write it just like that it will probably work, so I imagine you've simplified it for the question. (And keeping the question simple and concise is normally a very good thing.)

But in this case I imagine one result is a calculation and one result is a constant.

This violates a cardinal rule of floating point programming: Never do equality comparisons.

The reasons for this are a bit subtle¹ but what's important to remember is that they usually don't work (except, ironically, for integral values) and that the alternative is a fuzzy comparison along the lines of:

if abs(a - y) < epsilon

---

_{1. One of the major problems involves the way we write numbers in programs. We write them as decimal strings, and as a result most of the fractions we write do not have exact machine representations. They don't have exact finite forms because they repeat in binary. Every machine fraction is a rational number of the form x/2ⁿ. Now, the constants are decimal and every decimal constant is a rational number of the form x/(2ⁿ * 5^m). The 5^m numbers are odd, so there isn't a 2ⁿ factor for any of them. Only when m == 0 is there a finite representation in both the binary and decimal expansion of the fraction. So, 1.25 is exact because it's 5/(2²*5⁰) but 0.1 is not because it's 1/(2⁰*5¹). In fact, in the series 1.01 .. 1.99 only 3 of the numbers are exactly representable: 1.25, 1.50, and 1.75.}