[erlang-questions] Any way to correct the round off errors?

[Witold Baryluk]

Dnia 2009-09-20, nie o godzinie 19:00 +0200, Masklinn pisze:
> On 20 Sep 2009, at 18:12 , Witold Baryluk wrote:
> > Dnia 2009-09-20, nie o godzinie 18:06 +0200, Masklinn pisze:
> >> On 20 Sep 2009, at 17:22 , Witold Baryluk wrote:
> >>> If you want something which calculates correctly in decimal number,
> >>> use
> >>> integers multiplied with some 10^k power. It is called fixed point
> >>> arithmetic.
> >> Or use a decimal floating-point library such as gmp.
> >
> > Sure. But considering that Erlang already have arbitrary precision
> > integers, writing few functions for add/sub/mul/div isn't very hard :)
> 
> Arbitrary precision integers are quite different beasts (and easier to  
> handle) than arbitrary precision floats.

I was talking about arbitrary precision integer arithmetic (or fixed
point arithmetic).

Linking to gmp is just additional requirement, which is unnecessary
given the fact that Erlang already have integer arithmetic.

Something like this will suffice:


-module(fpa).

add({X,F},{Y,F}) -> {X+Y,F}.
sub({X,F},{Y,F}) -> {X-Y,F}.
neg({X,F}) -> {-X,F}.
mul({X,F},{Y,F}) -> {(X*Y) div F, F}.
div({X,F},{Y,F}) -> {(X*F) div Y, F}.
fpa_to_float({X,F}) -> X / F.
float_to_fpa6(X) -> {trunc(1000000*X), 1000000}.



"Accurate" binary floating point arithmetic is completely different
story, and few order of magnitud harder, and still quite active research
area in numerical analysis.

-- 
Witold Baryluk
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