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Comment by amluto | Hacker News Reader

If someone sells you 12345.55 EUR vs USD at a rate of 1.12345, how many EUR do you think you end up with? Do you think all market participants even agree? What if the rate is 1.123456?

For added fun, you can introduce division. Some systems will allow you to sell 12345.55 USD to buy EUR at a rate of 1.12345.

The article’s “no lost data” tenet is not really viable when this sort of division is involved. Are you going to track your account balance is a rational number with an absolutely immense denominator forever?

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> If someone sells you 12345.55 EUR vs USD at a rate of 1.12345, how many EUR do you think you end up with?

1. If someone sells me 12345.55 EUR, I hope to end up with 12345.55 EUR.

2. That's the point though. They will sell you a certain amount of EUR at a certain dollar price. This, in turn, implies a rate (which might well have more than 5 digits behind the decimal). This is ideally close to the quoted rate, sure. But what counts is the actual EUR amount and the actual USD amount, not what rate was quoted or with how many digits.

Of course market participants agree?

Exchanges have calculation rules for every type of mark and payment and will always specify rounding.

You store the sums on either end, the currencies, the exchange rate and the final sum? No one has .0000145 cents in their account. Rounding occurs in the real world.

> You store the sums on either end, the currencies, the exchange rate and the final sum?

There is a remarkable amount of disagreement as to whether one should do one’s back office work based on the price or based on the quantity of the counter currency.

> No one has .0000145 cents in their account. Rounding occurs in the real world.

Indeed. But you either need to convince all parties to agree to round the same way or you need to accept small errors

My experience in consumer banking says that every instrument specifies the precision of the calculation, how and when rounding happens, and slew of little details.

So, yes, everyone has to understand how all their partners are doing rounding and summing.

In certain areas of the institutional finance world, everyone seems to accept that everyone's math is allowed to differ by a few cents, and they tally up the errors and move on with their lives.

I would, however, by quite surprised if my personal bank account did this.

When the bank owes you money, they round down.

When you owe the bank money, they round up.

Unless you work for the bank, in which case your annual salary is divided by 12 and rounded up for a monthly figure.

The only one who has to “agree” is the exchange. And it should be covered by the TOS

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