Comparing Floating-Point Numbers (PHP Cookbook)

2.3. Comparing Floating-Point Numbers

2.3.1. Problem

You want to check whether two floating-point numbers are equal.

2.3.2. Solution

Use a small delta value, and check if the numbers are equal within that delta:

$delta = 0.00001;

$a = 1.00000001;
$b = 1.00000000;

if (abs($a - $b) < $delta) { /* $a and $b are equal */ }

2.3.3. Discussion

Floating-point numbers are represented in binary form with only a finite number of bits for the mantissa and the exponent. You get overflows when you exceed those bits. As a result, sometimes PHP (and other languages, too) don't believe two equal numbers are actually equal because they may differ toward the very end.

To avoid this problem, instead of checking if $a == $b, make sure the first number is within a very small amount ($delta) of the second one. The size of your delta should be the smallest amount of difference you care about between two numbers. Then use abs( ) to get the absolute value of the difference.

2.3.4. See Also

Recipe 2.4 for information on rounding floating-point numbers; documentation on floating-point numbers in PHP at http://www.php.net/language.types.float.