[macksd]

When you're doing floating point arithmetic on a computer, it will approximate and round certain values in ways that don't match the way humans do it when they're, e.g. doing accounting.

So you need to run a massive physics simulation really fast? Yes, floats are great.

You need to calculate taxes on a massive corporation's fiscal year? Bad idea.

Some libraries advertise "arbitrary precision", many computer systems have a "decimal" type intended for currency, etc. and then they won't make all the same mistakes, but as the OP said you still need to control rounding rules and make sure they match the law.

[Dylan16807]

> You need to calculate taxes on a massive corporation's fiscal year? Bad idea.

That depends on whether the hundred-billion-dollar corporation cares about being off by a dollar.

And by "off" I mean "different from how humans round", not necessarily further away from an infinite-precision calculation. In fact at "massive corporation" level I would guess that binary floating point is more accurate than a typical fractional penny system.

[hiatus]

> That depends on whether the hundred-billion-dollar corporation cares about being off by a dollar.

How many hundred billion dollar corporations are private? Public companies would care a great deal about accounting accuracy.

[Dylan16807]

Is it worse for a hundred billion corp to be off by a dollar than for a hundred million corp to be off by a third of a penny?

[chasd00]

it's not so much how much it is off but that it's off at all. If the numbers don't add up then they don't add up. If there's any kind of difference then it has to be found and accounted for and it becomes a needle in a haystack search to account for the difference. Think about trying to find $0.05 spread across hundreds of thousands of transactions due to rounding issues.

[Kranar]

I'm going to let you in on a secret...

Every single publicly listed company, every single one of them, is off when it comes to calculating their taxes by way more than just a dollar. And I don't mean clever accounting tricks or tax avoidance schemes, I just mean in terms of actual mistakes being made.

[macksd]

If they could just pay the dollar and never have to worry about it again, sure. But the point is for them to have confidence that the math is unimpeachable and identical to whatever auditor or tax official would compute at every step of the way so you don't just have to guess at correctness with some waving of hands.