`!mpq` data type ================ The `!mpq` data type can store rational numbers whose denominator and numerator have arbitrary size. Rational numbers are converted in canonical form on input (meaning that the denominator and the numerator have no common factors) and all the operators will return a number in canonical form. PostgreSQL integer types (`!int16`, `!int32`, `!int64`), `!numeric` and `mpz` can be converted to `!mpq` without loss of precision and without surprise. Floating point types (`!float4`, `!float8`) are converted without loss as well... but with some surprise, as many fractions with finite decimal expansion have no finite expansion in binary. .. code-block:: psql =# select 10.1::numeric::mpq as "numeric", -# 10.1::float4::mpq as "single", -# 10.1::float8::mpq as "double"; numeric | single | double ---------+----------------+---------------------------------- 101/10 | 5295309/524288 | 5685794529555251/562949953421312 `!mpq` values can be converted to integer types (both PostgreSQL's and `!mpz`): the result will be truncated. Conversion to `!float4` and `!float8` will round the values to the precision allowed by the types (in case of overflow the value will be *Infinity*). Conversion to `!numeric` will perform a rounding to the precision set for the target type. .. code-block:: psql =# select mpq('4/3')::integer as "integer", -# mpq('4/3')::float4 as "single", -# mpq('4/3')::decimal(10,3) as "decimal"; integer | single | decimal ---------+---------+--------- 1 | 1.33333 | 1.333 `!mpq` values can be compared using the regular PostgreSQL comparison operators. Indexes on `!mpq` columns can be created using the *btree* or the *hash* method. `!mpq` textual input/output --------------------------- .. function:: mpq(text) mpq(text, base) Convert a textual representation into an `!mpq` number. The form :samp:`{text}::mpq` is equivalent to :samp:`mpq({text})`. The string can be an integer like ``41`` or a fraction like ``41/152``. The fraction will be converted in canonical form, so common factors between denominator and numerator will be removed. The numerator and optional denominator are parsed the same as in `mpz`. White space is allowed in the string, and is simply ignored. The base can vary from 2 to 62, or if base is 0 then the leading characters are used: ``0x`` or ``0X`` for hex, ``0b`` or ``0B`` for binary, ``0`` for octal, or decimal otherwise. Note that this is done separately for the numerator and denominator, so for instance ``0xEF/100`` is 239/100, whereas ``0xEF/0x100`` is 239/256. .. note:: The maximum base accepted by GMP 4.1 is 36, not 62. .. function:: text(q) text(q, base) Convert the `!mpq` *q* into a string. The form :samp:`{q}::text` is equivalent to :samp:`text({q})`. The string will be of the form :samp:`{num}/{den}`, or if the denominator is 1 then just :samp:`{num}`. *base* may vary from 2 to 62 or from -2 to -36. For base in the range 2..36, digits and lower-case letters are used; for -2..-36, digits and upper-case letters are used; for 37..62, digits, upper-case letters, and lower-case letters (in that significance order) are used. If *base* is not specified, 10 is assumed. .. note:: The maximum base accepted by GMP 4.1 is 36, not 62. `!mpq` conversions ------------------ .. function:: mpq(num, den) Return an `!mpq` from its numerator and denominator. .. note:: The function signature accepts `!mpz` values. PostgreSQL integers are implicitly converted to `!mpz` so invoking the function as ``mpq(30,17)`` will work as expected. However if the numbers become too big for an `int8` they will be interpreted by PostgreSQL as `!numeric` and, because the cast from `!numeric` to `!mpz` is not implicit, the call will fail. Forcing a cast to `!mpz` (*e.g.* ``mpq(30::mpz,17::mpz)``) will work for numbers of every size. .. function:: num(q) den(q) Return the numerator or the denominator of *q* as `!mpz`. Arithmetic Operators and Functions ---------------------------------- All the arithmetic operators and functions return their their output in canonical form. .. table:: Arithmetic operators =========== =============================== =========================== =========== Operator Description Example Return =========== =============================== =========================== =========== `!-` Unary minus `!- '4/3'::mpq` -4/3 `!+` Unary plus `!+ '4/3'::mpq` 4/3 `!+` Addition `!'2/3'::mpq + '5/6'::mpq` 3/2 `!-` Subtraction `!'1/3'::mpq - '5/6'::mpq` -1/2 `!*` Multiplication `!'2/3'::mpq * '5/6'::mpq` 5/9 `!/` Division `!'2/3'::mpq / '5/6'::mpq` 4/5 `!<<` Multiplication by :math:`2^n` `!'2/3'::mpq << 3` 16/3 `!>>` Division by :math:`2^n` `!'2/3'::mpq >> 3` 1/12 =========== =============================== =========================== =========== .. function:: abs(q) Return the absolute value of *q*. .. function:: inv(q) Return 1/*q*. .. function:: limit_den(q, max_den=1000000) Return the closest rational to *q* with denominator at most *max_den*. The function is useful for finding rational approximations to a given floating-point number: .. code-block:: psql =# select limit_den(pi(), 10); 22/7 or for recovering a rational number that's represented as a float: .. code-block:: psql =# select mpq(cos(pi()/3)); 4503599627370497/9007199254740992 =# select limit_den(cos(pi()/3)); 1/2 =# select limit_den(10.1::float4); 101/10 This function is not part of the GMP library: it is ported instead `from the Python library`__. .. __: https://docs.python.org/library/fractions.html#fractions.Fraction.limit_denominator Aggregation functions --------------------- .. function:: sum(q) Return the sum of *q* across all input values. .. function:: prod(q) Return the product of *q* across all input values. .. function:: max(q) Return the maximum value of *q* across all input values. .. function:: min(q) Return the minimum value of *q* across all input values.