Multiply when float will going wrong result

[ivan88217]

When I use float to be a multiplier, then it will be converted to int and got wrong answer

$amount = new Money(10000);
dd($amount->multiply(0.3)); // Money(0)

I change the type hint from int|string to int|string|float and it work functional again

from

public function multiply(int|string $multiplier, int $roundingMode = self::ROUND_HALF_UP): Money

to

public function multiply(int|string|float $multiplier, int $roundingMode = self::ROUND_HALF_UP): Money

[wjzijderveld]

The whole point of this library is to avoid using floats for money 😅 That's why it's using strings internally and has some helpers to allow the usage of integers (it's using strings to allow for numbers greater than PHP_INT_MAX).

> php -r 'echo var_dump(0.1+0.2);'
float(0.30000000000000004)

Also see https://www.php.net/manual/en/language.types.float.php#language.types.float and https://floating-point-gui.de

[wilcol]

The whole point of this library is to avoid using floats for money 😅 That's why it's using strings internally and has some helpers to allow the usage of integers (it's using strings to allow for numbers greater than PHP_INT_MAX).

> php -r 'echo var_dump(0.1+0.2);'
float(0.30000000000000004)

Also see https://www.php.net/manual/en/language.types.float.php#language.types.float and https://floating-point-gui.de

I'm also having the same issue as the OP. If the reason is to avoid floats, why is there a roundingMode parameter present in the multiply function? (int * int = int) But more importantly; why would multiplication with a float not be a valid usecase to cover off in this library? In my case it's being used for VAT calculations. As a quick/dirty fix I'm now casting floats to strings, but it would help to get guidance if the multiply (and divide) function(s) should be used with integers only. The manual (https://www.moneyphp.org/en/stable/features/operation.html#multiplication-division) is also not explicit on this. Happy to hear the thoughts on this from the team.

Thanks for supporting this great library!

[wjzijderveld]

The problem isn't decimals, it's floats, as they they aren't precise and can result in expected results. $amount->multiply("0.3"); works just fine, casting to string isn't a dirty fix, it's the approach that allows the library to handle the calculations while keeping precision.

btw, disclaimer; I'm not part of this library, I've just been using it since Matthias first published it 12,5 years ago.

[wilcol]

Thanks @wjzijderveld for your inputs and thoughts. I understand the unexpected results rounding and truncation can give, that's why I use this library myself as well :) I'm looking at this from a 'clean code' and naming perspective. In my view, it's inconsitent to only accept int | string. Either use string only (and document its possible values), or accept all posible types string | int | float | etc... This makes the method signature explicit without any need to guess... and also keeps the method backwards compatible since no deprecation notices were given... Anyway, interesting food for thought!

[ivan88217]

Thanks @wjzijderveld for your points. Although I know avoid to use float is right, but I'm just confusing about different behavior from upgrade to new version. It broke my legacy system when I upgrade laravel version from 6 to 9, and I can't find out the document for that.

I'm currently using it to calculate tax rates, so if it can't accept float, things will be a bit troublesome.

[frederikbosch]

No float, it has been explained before.