Using mpq_t for aggregating currencies with wildly varying ranges
donovanhide at gmail.com
Tue Apr 15 21:36:18 UTC 2014
thanks very much for this clever tip! Almost too clever for the
bit-twiddling part of my brain to deal with :-)
Unfortunately it does look like gmp is simplifying the fractions in some
cases, even though I'm not calling canonicalize() anywhere...
Should I be looking at mpfr instead, or can I hack this some other way?
Feels like there's a missing method on the rational type for this set of
On 15 April 2014 16:57, Zimmermann Paul <Paul.Zimmermann at inria.fr> wrote:
> Dear Donovan,
> > Date: Tue, 15 Apr 2014 16:28:32 +0100
> > From: Donovan Hide <donovanhide at gmail.com>
> > Hi,
> > I've been doing some work on making the Ripple Currency format
> > in MySQL by writing a parser and summation functions for this currency
> > format:
> > https://ripple.com/wiki/Currency_Format
> > The docs aren't great! The issue I've got is that I have an mpq_t
> > to which I'm adding various transaction amounts which have hugely
> > exponents, which works well and gives exact results. All denominators are
> > always powers of ten. However when it comes to outputting the value as a
> > decimal string, I've been doing something like:
> > mpq_class total=make_rational(bigendian_decode(args->args));
> > *length=gmp_sprintf(result,"%.Ff",mpf_class(total,100).get_mpf_t());
> > which suffers from floating point drift and isn't accurate. I've been
> > looking at how pgmp for postgres solve the same problem and it's a bit
> > messy, using integers rather than rationals:
> > Does anyone have any suggestions on how to format rationals as exact
> > decimals, with the proviso that the denominator is always a power of ten
> > and thus the number should be non-recurring?
> > Cheers,
> > Donovan.
> if the denominator q is 10^n, then you can get n by mpz_scan1 (q, 0).
> Then call mpz_get_str (NULL, 10, p) to output the numerator p in a
> string, and shift the decimal point by n places to the left.
> But beware that GMP might simplify the fraction p/q and then q might
> not be exactly a power of 10.
> Paul Zimmermann
More information about the gmp-discuss