Why does PHP not provide an Epsilon constant for floating point comparisons?

Question

After seeing many PHP questions about comparing the equality of floats where the answer is to simply choose an arbitrary value for Epsilon and then do if( abs($a-$b) < 0.000001 ).

The trouble is that Epsilon is typically much smaller than the values people tend to choose [2.22e-16 on my machine] and is actually quite simple to calculate:

$macheps = (float) 1.0;  
do {
    $macheps /= (float) 2.0;
} while( (float) (1.0 + ($macheps/2.0)) != 1.0 );
printf("Epsilon: %0.25f\n", $macheps);

C++ has std::numeric_limits<double>::epsilon(), Python has sys.float_info.epsilon, so why does PHP leave it up in the air?

Probably because different people will have different requirements for precision — Mark Baker, May 23 '14 at 19:46
"the answer is to simply choose an arbitrary value for Epsilon". This answer is simple, appealing, and dead wrong. Don't do that. — n. m. could be an AI, May 24 '14 at 10:08
`PHP_FLOAT_EPSILON` was added in PHP 7.2: _Smallest representable positive number x, so that x + 1.0 != 1.0_ — Salman A, Jan 17 '18 at 07:40

score 20 · Answer 1 · answered May 14 '18 at 08:30

20

I know this is an old question, but as of PHP 7.2 it is provided.

PHP_FLOAT_EPSILON (float)

Smallest representable positive number x, so that x + 1.0 != 1.0. Available as of PHP 7.2.0.

See http://php.net/manual/en/reserved.constants.php

answered May 14 '18 at 08:30

Luke Cousins

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score 3 · Accepted Answer · answered May 23 '14 at 21:53

C++'s std::numeric_limits<double>::epsilon() was never intended to be used in place of 0.000001 in a formula of the style abs($a-$b) < 0.000001. For instance, with most C++ compilation platforms, fabs(x - 2.5) < std::numeric_limits<double>::epsilon() is equivalent to x == 2.5, because std::numeric_limits<double>::epsilon() is a representation of the double definition near 1.

Some programmers may need to compare floating-point numbers up to some value, but that value has little reason to be related to the definition of the floating-point format near 1, so that's not a good reason to provide that constant in the language. Instead, the value should come either from requirements (“as small as needed”) or by inference (“the floating-point results can be 0.003 from the real result, so fabs(x - 2.5) < 0.003 will never be false if the real result can be 2.5”).

Why does PHP not provide an Epsilon constant for floating point comparisons?

2 Answers2