BigRat is intended to represent (exact) rational numbers
of practically unlimited range; it is currently based on the
implementation in the GMP multiple precision library. This code forms
the interface between CoCoALib and the big integer/rational library
upon which it relies. It seems most unlikely that GMP will be
displaced from its position as the foremost library of this type; as a
consequence the class
BigRat may eventually be replaced by GMP's
own C++ interface.
The usual arithmetic operations are available with standard C++ syntax
but generally these incur run-time overhead since results are returned
through temporaries which are created and destroyed silently by the
compiler. Thus if the variables
c are each of
a = b+c; is a valid C++ statement for placing
the sum of
a, but the sum is first computed
into a hidden temporary which is then copied to
a, and then
finally the temporary is destroyed. As a general principle, the type
BigRat is provided for convenience of representing rational
values rather than for rapid computation.
There is an important exception to the natural syntax:
^ does not
denote exponentiation; you must use the function
We have chosen not to define
operator^ to perform exponentiation
because it is too easy to write misleading code: for instance,
a*b^2 is interpreted by the compiler as
(a*b)^2. There is no
way to make the C++ compiler use the expected interpretation.
Arithmetic may also be performed between a
BigRat and a machine
integer or a
BigInt. The result is always of type
(even if the value turns out to be an integer). Do remember, though,
that operations between two machine integers are handled directly by
C++, and problems of overflow can occur.
It is important not to confuse values of type
BigRat with values of type
RingElem which happen to belong to the ring
RingQQ. The distinction
is analogous to that between values of type
BigInt and value of type
RingElem which happen to belong to the ring
RingZZ. In summary, the
operations available for
RingElem are those applicable to elements of
any ordered commutative ring, whereas the range of operations on
values is wider (since we have explicit knowledge of the type).
A value of type
BigRat may be created from:
BigRat(its value is copied)
BigRat(n,d)a pair of integers (machine integers or
BigInts) specifying numerator and denominator in that order
stris a string of the form
Nis the decimal representation of the numerator and
Dthat of the denominator
BigRat(mpq_value)copy a GMP rational (of type
mpq_t) into a
BigRat; helps interfacing between CoCoALib and code using GMP directly.
BigRat(str) accept an optional arg
BigRat::AlreadyReduced which asserts that the value is already reduced
(i.e. positive denominator, and numerator and denominator are coprime).
Use this feature only if you are absolutely certain that there is no
common factor between the given numerator and denominator.
See Bugs section for why there is no ctor from a single integer, and
also for why
BigRat(0) is accepted by the compiler (but crashes at run-time).
NOTE: similar to operations on
BigInt -- see
/=-- definitions as expected; LHS must be of type
>=-- comparison (using the normal arithmetic ordering) -- see also the
++a) use these if you can
a++) avoid these if you can, as they create temporaries
IsZero(q)-- true iff
IsOne(q)-- true iff
IsMinusOne(q)-- true iff
IsOneNum(q)-- true iff
IsOneDen(q)-- true iff
sign(q)-- gives -1 (machine integer) to mean
qis negative, 0 (machine integer) to mean
qis zero, +1 (machine integer) to mean
power(a, b)-- returns
ato the power
b(result is always a
cmp(a,b)-- returns an
a < b, or
a == b, or
a > b.
CmpAbs(a,b)-- equivalent to
abs(q)-- gives the absolute value of
floor(q)-- returns a
BigIntfor the greatest integer
ceil(q)-- returns a
BigIntfor the least integer
round(q)-- returns a
BigIntwhich is the nearest to
q(halves round the same way as in
num(q)-- returns a
BigIntwhich is the numerator of
den(q)-- returns a positive
BigIntwhich is the denominator of
log(q)-- returns a double whose value is (approx) the natural logarithm of
FloorLog2(q) -- same as ``FloorLogBase(q,2)
FloorLog10(q) -- same as ``FloorLogBase(q,10)
FloorLogBase(q,base)-- returns largest integer
power(base,k) <= abs(q)
mantissa(q)-- returns a
doublebetween 0.5 and 1 (excluded)
mpqref(n)-- this gives a (const) reference to the
mpq_tvalue inside a
BigRatobject. You should use this accessor very sparingly!
Nothing very clever. Conversion from a string was a bit tedious.
Note that the ctor call
BigRat(0) actually calls the ctor from a string.
Unfortunately, this a C++ "feature". It will result in a run-time error.
I have replaced the bodies of the
BigRat ctors which take two integers
as arguments by a call to the common body
BigRat::myAssign. This does
mean that some wasteful temporaries are created when either of the
arguments is a machine integer. Time will tell whether this waste is
This code is probably not exception safe; I do not know what the
functions do when there is insufficient memory to proceed. Making the
code exception safe could well be non-trivial: I suspect a sort of
auto_ptr to an
mpq_t value might be needed.
BigRat ctors from a single (machine) integer because too often
I made the mistake of writing something like
BigRat(1/2) instead of
BigRat ctor from string also accept numbers with decimal points?
BigRat("3.14159")? We'll wait and see whether there is demand for this
before implementing; note that GMP does not offer this capability.
BigRat: to avoid confusion with
RingQQand its name in CoCoA system