And if you can’t, you can use integers and work in pennies/cents.
AstonJ
bbense
I understand the issues about Float and rounding. What I want to understand is how Decimal is used to solve those issues, since changing the precision only delays the onset of when rounding errors occurs.
What exactly about Decimal makes it appropriate for use as Money? Is is the fact that the significand is base10 rather than binary? Is it that you set the precision appropriate to the scale
of money that you are dealing with? Why do you trust Decimal, but not Float used within it’s
reasonable range?
I can make Decimal work correctly with >, but if that breaks the entire purpose of Decimal, then there is no point in submitting the patch.
0
bbense
NobbZ
As far as I can tell, Decimal does give you fixed point numbers oposed to floating point numbers.
A fixed point number will always have the same precision, regardless how often you do something with it. Lets say we have a fixed and a float of 1. The fixed has a precission of 3 digits after the point. So you can multiply the fixed by a quadrillion and add a 0.1 and you will have a quadrillion and a tenth. If you do the same with the float of 1, you will have only a quadrillion (or even less due to how floats deal with values they can represent exactly)
Also for fixed (a + b) + c == a + (b + c) does hold, while it does not for floats.
1
Qqwy
TypeCheck Core Team
Decimals (most implementations including the one in the Elkixir library) are created from two Bignums. A bignum is an integer of arbitrary size; internal logic ensures that when the number becomes larger than a small fixed-size ‘fixnum’ can handle, it is stored in multiple separate memory locations. Of course, using bignums is not always straightforward. In the case of JS, for instance, one would need a special library that does it. However, Erlang/Elixir has built-in support for them; all integers you create are secretly convertes to and from bignums whenever necessary.
A Decimal is just (mantissa * (10 ^ exponent)) where mantissa and exponent are both bignums.
A side note: Using an arbitrary integer subdivision for money works fine, until you want to convert from one valuta to the next.
1
bbense
I think you’re correct, rounding only occurs when Decimal is converted to another type. My mistake was thinking that it was just an extended floating point representation.
I looked at mostly the README and not the source code and got side tracked by all the references to rounding errors and floating point.
1