Floating Point - Cant Add

[gregambrose]

Floating point arithmetic seems very broken (in PHP 4.4.0). Try the following

<?php

$a 	= "86.85";
$b 	= "347.71";
$c	= "260.86";

$tot = $a + $c;

$diff = $tot - $b;

print($diff);

?>

Why isn't this returning zero? Do I have to round() everything, even after an addition?

[toplay]

You should not put numbers, especially floats, in quotes. PHP will assign it initially as a string.

Anyway, floating numbers can go on after the decimal point, so the calculation value is a low number, that it's best to use round. Use type casting to tell PHP what you want. Example:

$a = (float) 86.85;
$b = (float) 347.71;
$c = (float) 260.86;

$tot = (float) $a + $c;

$diff = (float) round($tot - $b, 2);

echo $diff; // displays zero

EDIT:

You know PHP and other languages store floating point numbers in binary form with only limited numbers. You should not compare two floating numbers with an equal comparison because they don't always calculate to exact values. To compare two floats you can check if one number is within a very small amount of the second number. Example:

$diff = 0.00001;
$nbr1 = 5.00000001;
$nbr2 = 5.0;
if (abs($nbr1 - $nbr2) < $diff) {
   echo 'Consider the numbers as equal';  // displays this message
} else {
   echo 'The numbers are not equal';
}

[gregambrose]

Thanks for the reply.

I have used doubles for money, which now seems like a mistake. In Java doubles work fine, so I (wrongly) thought they would be ok in php. I know you should expect problems with division, but just adding a few numbers shouldn't be a problem I would have thought.

So, I need to round() every time I touch a double, just to be sure, or throw them away and use large integers, and hold money in cents.

[Weedpacket]

Hold the money in cents (this is how accounting and banking software does it); they're not that much larger unless you're talking about billions of dollars. Floating-point binary can't store one-tenth or one-hundredth precisely, because their binary expansions are nonterminating (the same way that one seventh is nonterminating when written in decimal).

PHP uses whatever floating-point operations are provided by your computer's processor (via C's standard library). Of course, it's known that Java's implementation of floating point is broken. Predictable, but broken. See, for example, http://www.cs.berkeley.edu/~wkahan/JAVAhurt.pdf

[gregambrose]

What a very interesting article. I've not finished it yet, but I get the idea. I really should be using integers for money, to avoid all the problems (if rockets can crash through misuse of floats, in navgation. I will feel safer using integers). Anyway, thanks for your comments.