CODEWITHSUNDEEP
phpmathfloating-point
apis17
apis17

Reputation: 2855

php calculate float

I have a weird math calculation here. I hope someone will explain.

$a = 1.85/100;
$b = 1.5/100;
$c = 1.1/100;
$d = 0.4/100;
$e = 0.4/100;
$f = 0.4/100;
$g = 0.4/100;

$h = $a + $b + $c + $d + $e + $f + $g;

echo $h*100 ."<br>";
$i = $h-$a;
$i = $i-$b;
$i = $i-$c;
$i = $i-$d;
$i = $i-$e;
$i = $i-$f;
$i = $i-$g;

echo $i;

The last $i value should be 0 but it returns 6.93889390391E-18.

Upvotes: 3

Views: 6609

Answers (3)

Eineki
Eineki

Reputation: 14959

There is nothing wrong going there, there are simply rounding approximation.

In this very cas you should multiply all your values for 1000 and do a division at the end of the calculation or, better, resort to precise calculation extension

Upvotes: 2

Mark Byers
Mark Byers

Reputation: 839264

You should read this article:

What Every Computer Scientist Should Know About Floating-Point Arithmetic

Floating point arithmetic is not exact. You should expect small errors. The answer is correct to within a small rounding error. If you need to check if a floating point number is zero it is best not to check that it is exactly equal to zero but instead to check if it is sufficiently close to zero.

If you really need exact arithmetic, don't use floating point types. In your example you could multiply all your numbers by 100 and use integer arithmetic to get an exact answer.

Upvotes: 6

Tor Valamo
Tor Valamo

Reputation: 33809

any idea to correct my equation to show final result 0.00 ?

Yeah, round($i, 2)

The "discrepancies" are usually so small, that rounding it to 2 decimals will almost always solve the problem.

Upvotes: 3

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