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andreasdotorg
andreasdotorg
andreasdotorg
@andreasdotorg

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andreasdotorg

@andreasdotorg

I'm a hacker, pretty much in the old school sense of the word. But I do know IT security too.

Joined April 2008

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    1. andreasdotorg‏ @andreasdotorg Oct 23
      Replying to @jxxf @mfeathers @Felienne

      Not at all, no.

      1 reply 0 retweets 0 likes
    2. John Feminella ⏣‏ @jxxf Oct 23
      Replying to @andreasdotorg @mfeathers @Felienne

      How much do you owe on a $1 transaction with 4.9% sales tax? Under this scheme, you'd represent this as 100¢ × 104.9%, which gets you a value that is close to but not equal to 104.9.pic.twitter.com/6yTcN8s7Zm

      2 replies 0 retweets 1 like
    3. andreasdotorg‏ @andreasdotorg Oct 23
      Replying to @jxxf @mfeathers @Felienne

      You check your local applicable law to see what rounding mode you need to apply, and apply that to get back to full cents.

      1 reply 0 retweets 0 likes
    4. andreasdotorg‏ @andreasdotorg Oct 23
      Replying to @andreasdotorg @jxxf and

      Note that actually knowing the fractional part (up to a certain precision) allows you to apply the correct rounding mode, unlike what happens when calculating using integers, where truncation always happens.

      1 reply 0 retweets 0 likes
    5. John Feminella ⏣‏ @jxxf Oct 23
      Replying to @andreasdotorg @mfeathers @Felienne

      In my experience, "use floating-point, manage the bookkeeping yourself, and pray you don't get unlucky with error propagation" is much more prone to errors than "use a decimal type".

      1 reply 0 retweets 1 like
    6. andreasdotorg‏ @andreasdotorg Oct 23
      Replying to @jxxf @mfeathers @Felienne

      Until you hit an integer overflow, and things go belly up in pretty dramatic ways.

      1 reply 0 retweets 0 likes
    7. andreasdotorg‏ @andreasdotorg Oct 23
      Replying to @andreasdotorg @jxxf and

      But my entire point here is that using floats is perfectly fine for money, if you follow a small number of rules, whereas using regexes for HTML parsing is bound to fail due to HTML being a recursive language, and regexes recognizing regular languages.

      2 replies 0 retweets 2 likes
    8. John Feminella ⏣‏ @jxxf Oct 23
      Replying to @andreasdotorg @mfeathers @Felienne

      I think that's fair. My point is that it's not necessarily obvious you're getting burned by FPEP until you're in deep, whereas it's usually clear if a regex isn't returning what you think it should. A lot of F500s have lost a lot of money to very silly FP errors!

      1 reply 0 retweets 1 like
    9. andreasdotorg‏ @andreasdotorg Oct 23
      Replying to @jxxf @mfeathers @Felienne

      I've never heard of any such case. Even if you use floats incorrectly, the accumulated error tends to be small.

      1 reply 0 retweets 0 likes
    10. John Feminella ⏣‏ @jxxf Oct 23
      Replying to @andreasdotorg @mfeathers @Felienne

      In financial calculations, it's common to iterate over a large number of periods (e.g., mortgages, bond payments, and so on) or a large number of transactions (e.g., all the payroll checks). Adding or subtracting iteratively easily leads to catastrophic cancellations with FPs.

      2 replies 0 retweets 0 likes
      andreasdotorg‏ @andreasdotorg Oct 23
      Replying to @jxxf @mfeathers @Felienne

      For such calculations, when you store back to an integer in every step, you accumulate a much bigger error in total. I don't see how floats would hurt you there in a worse way.

      6:00 AM - 23 Oct 2018
      1 reply 0 retweets 0 likes
        1. New conversation
        2. John Feminella ⏣‏ @jxxf Oct 23
          Replying to @andreasdotorg

          The alternative is not "use integers"; it's "use the right tool for the job, which is usually your language's Money or BigDecimal type".

          1 reply 0 retweets 2 likes
        3. andreasdotorg‏ @andreasdotorg Oct 23
          Replying to @jxxf

          If your language has such a thing, this is sane advice.

          1 reply 0 retweets 1 like
        4. 1 more reply

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