3 ms·

If you actually want to do something useful and accurate and auditable and performant and space-efficient, because you have a lot of money values to bank with,

If you actually want to do something useful and accurate and auditable and performant and space-efficient, because you have a lot of money values to bank with, you really don't want too much abstraction. My experiences in various bits of banking include where juniors ignore advice NOT to store currency in floating point values and then come whining that arithmetic is broken, and tech dudes in a lab deciding that every single FX flow in an investment bank should have 2MB of (unshared) calendar hidden inside its abstraction which made some individual trades too big to load even for powerful machines... Fixed point calcs in integers are good.

Waterluvian 3y ago
Floating point feels like an incredibly grokkable concept that was just not taught well for a long time. Maybe too mathematically (of course). Or maybe that was just my experience. I feel like any dev team should pick up a copy of this for on-boarding: https://jvns.ca/blog/2023/06/23/new-zine--how-integers-and-floats-work/ https://jvns.ca/blog/2023/06/23/new-zine--how-integers-and-f...
- fidotron 3y ago
  Floating point being grokkable doesn’t make it any more suitable for this application. Floating point is inherently an approximation - your bank balance should not be an approximation.
  - DamonHD 3y ago
    More to the point binary fp absolutely is a bad approximation to, and does poor arithmetic on, common legal sub-1 decimal currency values, ie those that do not have an exact binary fp representation.
  - Spivak 3y ago
    > Floating point is inherently an approximation - your bank balance should not be an approximation. I think this is a perfect example of bad floating point teaching. Floating point is not an approximation in any sense. If the numerical result of your calculation is representable in floating point you will get an exact answer always. And for results that aren't representable you decide exactly what should be done about that. It's like saying integers are an approximation because 5/2 == 2.
    - deleted 3y ago
      [deleted]
    - dahfizz 3y ago
      In programming, an `int` perfectly represents the integers between INT_MIN and INT_MAX. A `float` on the other hand, approximates the real numbers. It can perfectly represent exactly 0% of them. Having control over the rounding behavior is meaningless - floats cannot correctly represent any non-contrived calculation. Exact representation is important in financial systems.
      - fidotron 3y ago
        TBF, be careful here. The IEEE floats can represent a subset of integers in their range exactly. For example, 64 bit floats can represent the range of 32 bit ints accurately (and more). That said, it is bizarre to claim that if the result can be represented you get the exact result, when the core problem is that the result cannot be represented because the representation is an approximation.
        dahfizz 3y ago
        > The IEEE floats can represent a subset of integers in their range exactly. For example, 64 bit floats can represent the range of 32 bit ints accurately (and more). I know, I am being a little facetious. A double has a 52 bit mantissa, so it can exactly represent integers that need 52 or less bits. Still, as a percent, a float can represent 0% of the reals. There are an infinite amount of numbers it cannot represent, even if we give it lower and upper bounds. Whereas an int can represent 100% of the integers within a lower and upper bound.
      - Spivak 3y ago
        > floats cannot correctly represent any non-contrived calculation Like adjusting all your financial calculations to use microcents and partitoning instead of division to keep the result representable by integers? Neither can represent 1/3 even shifted. When you want to do exact calculations with floats, and you can, you just have to set yourself up so that the result is exactly representable, it's not as intractable as you make it seem. > A `float` on the other hand, approximates the real numbers Okay so that's not at all what they do, they represent subsets of the reals, just like how integers represent a subset of the reals. Even arbitrary precision libraries can only represent a subset of the rationals.
  - Waterluvian 3y ago
    Indeed. I'm just making a tangential comment. Definitely want to work in unsigned fractional cents.

Waterluvian 3y ago

Floating point feels like an incredibly grokkable concept that was just not taught well for a long time. Maybe too mathematically (of course). Or maybe that was just my experience. I feel like any dev team should pick up a copy of this for on-boarding: https://jvns.ca/blog/2023/06/23/new-zine--how-integers-and-floats-work/ https://jvns.ca/blog/2023/06/23/new-zine--how-integers-and-f...

fidotron 3y ago
Floating point being grokkable doesn’t make it any more suitable for this application. Floating point is inherently an approximation - your bank balance should not be an approximation.
- DamonHD 3y ago
  More to the point binary fp absolutely is a bad approximation to, and does poor arithmetic on, common legal sub-1 decimal currency values, ie those that do not have an exact binary fp representation.
- Spivak 3y ago
  > Floating point is inherently an approximation - your bank balance should not be an approximation. I think this is a perfect example of bad floating point teaching. Floating point is not an approximation in any sense. If the numerical result of your calculation is representable in floating point you will get an exact answer always. And for results that aren't representable you decide exactly what should be done about that. It's like saying integers are an approximation because 5/2 == 2.
  - deleted 3y ago
    [deleted]
  - dahfizz 3y ago
    In programming, an `int` perfectly represents the integers between INT_MIN and INT_MAX. A `float` on the other hand, approximates the real numbers. It can perfectly represent exactly 0% of them. Having control over the rounding behavior is meaningless - floats cannot correctly represent any non-contrived calculation. Exact representation is important in financial systems.
    - fidotron 3y ago
      TBF, be careful here. The IEEE floats can represent a subset of integers in their range exactly. For example, 64 bit floats can represent the range of 32 bit ints accurately (and more). That said, it is bizarre to claim that if the result can be represented you get the exact result, when the core problem is that the result cannot be represented because the representation is an approximation.
      - dahfizz 3y ago
        > The IEEE floats can represent a subset of integers in their range exactly. For example, 64 bit floats can represent the range of 32 bit ints accurately (and more). I know, I am being a little facetious. A double has a 52 bit mantissa, so it can exactly represent integers that need 52 or less bits. Still, as a percent, a float can represent 0% of the reals. There are an infinite amount of numbers it cannot represent, even if we give it lower and upper bounds. Whereas an int can represent 100% of the integers within a lower and upper bound.
    - Spivak 3y ago
      > floats cannot correctly represent any non-contrived calculation Like adjusting all your financial calculations to use microcents and partitoning instead of division to keep the result representable by integers? Neither can represent 1/3 even shifted. When you want to do exact calculations with floats, and you can, you just have to set yourself up so that the result is exactly representable, it's not as intractable as you make it seem. > A `float` on the other hand, approximates the real numbers Okay so that's not at all what they do, they represent subsets of the reals, just like how integers represent a subset of the reals. Even arbitrary precision libraries can only represent a subset of the rationals.
- Waterluvian 3y ago
  Indeed. I'm just making a tangential comment. Definitely want to work in unsigned fractional cents.