Recipe 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.
|
