today I'm thinking about the "don't use floating point for money" advice I hear all the time. It obviously has a lot of truth to it.
But -- Excel/Google Sheets uses floating point for all of its calculations, people use spreadsheets for money calculations all the time, and it generally seems to work just fine -- the results get rounded for display.
So I'm trying to figure out if there's a more nuanced guideline than "never use floating point for money".
thinking about trying to define a "safe zone" and a "danger zone" for floating point. maybe something like:
safe zone:
* all integer values (like 1.0, 234.0) behave 100% exactly the way you'd expect, UNLESS (!!!) they're more than 2^52. You can check for equality, it's fine.
* adding up ~100 numbers and rounding the result to 4 decimal places or so is going to work fine, as long as the numbers are roughly the same size
@b0rk Subtracting large numbers close in value can put you into the danger zone:
>>> v = 10e9
>>> (v+1)/3 - v/3
0.33333301544189453
losing more than half your significant digits.
Julia Evans
Graeme Williams