[obeattie]

I'm the Head of Engineering at Monzo. We don't use floating point at all in our core banking systems, which are all written in Go by our own engineering team :)

[runevault]

So is all the math just in cents and then you display it with the decimal if a user needs to see it?

[tclancy]

That is typically how you handle it (in my limited understanding). If you're going to make an error, let it be in the presentation layer and not the actual math.

[oblio]

Warning: I don't program applications for finance. However... I'm not sure that representing things in cents works. Mostly because of exchange rates. These can have quite a bit of decimals and go beyond cent-precision.

My money's on BigIntegers: https://msdn.microsoft.com/en-us/library/system.numerics.big..., using some sort of subdivision of cents as the base unit (1/1000ths of cents?)

I'm probably horribly wrong and I hope someone who actually knows what he's saying corrects me :)

[53475]

> My money's on BigIntegers: https://msdn.microsoft.com/en-us/library/system.numerics.big..., using some sort of subdivision of cents as the base unit (1/1000ths of cents?)

I was thinking something along these lines too. Would love to know what they used and the justifications behind their decision.

[runevault]

I now wonder if that is what c# does under the hood for currency type which is a rather safe way of dealing with a limited number of decimal places for money. Hm.

[obeattie]

Yep, that's exactly how it works.

[53475]

Any chance you could go into detail on this regarding data types? How do you handle exchange rates & such that can be to more precision than 100ths of a $ or other currency for instance?

[dharma1]

I don't know how it works under the hood but when I use Monzo for Euro payments the conversion is shown in cents - they probably round it and pocket the difference. But the actual rate is very good, interbank rates