Re: GMP ceil/floor/trunc & round-offs

[Kevin Ryde]

Hans Aberg <address@hidden> writes:
>
>   (floating, precision) -> floating
> where the precision argument specoifies the precision of the function
> return.

mpf_set_prec does that, more or less.  Or mpf_set, since it uses the
precision of its target when copying.  I'd doubt the value of having a
specific rounding direction (ceil/floor/trunc), since all the other
mpf functions understand only trunc (as it happens).  Exact rounding
is left to mpfr.

> Note that there are special round-off techniques favoured by some,
> like that the number exactly in the middle in the round-off process is
> rounded up/down depending on what the next higher binary digit is. (I am
> myself somewhat sceptical about those round-off techniques, and I merely
> note that they do exist.)

Yes, round to even or whatever it's called.  That'd be an idea for
mpfr if anyone's keen.

>      int_ceil, int_floor, int_trunc: rational -> integer

mpz_set_q does the trunc version.  A direct call to mpz_fdiv_q or
mpz_cdiv_q can be used for the others.

>   frac_ceil, frac_floor, frac_trunc: rational -> rational

I guess that'd be mpq_ceil etc.  That sort of thing is among the many
operations not directly provided for mpq.  Use of mpq_numref and
mpq_denref is recommended.