"Andras Erdei" <aerdei@gmail.com> wrote
after i failed to convince people that the current boost::rational is useless,
As has been previously stated. it is not the rational Concept that is useless but rather the limitations imposed by finite number representations of the value_type. I feel that boost::rational is being unfairly blamed for that. One could create various integer types that address the problem: eg An integer that is bounded , but in ops the result_type can hold any computed result accurately: int16 operator *(int8,int8) ; int16 operator +(int8,int8) ; int16 operator -(int8,int8) ; rational<int8> operator / (int8,int8) ; // ... :-) int8 quotient (int8,int8) ; mimicks inbuilt int operator / behaviour int32 operator+(int16,int16) ;// etc int64 operator* (int32,int32) // etc and using boost::numeric_converter to downcast, makes this a basis for a family of integers.
- did a boost::rational implementation (moved to www.ccg.hu/pub/src/old/rat.cpp) which does +, -, * and / _at the same speed_ as int +, -, * and / -- if rational computation (without hardware support) has the same speed as integer, then why was floating point (which is much worse than rational) ever introduced in the first place? isn't this suspicious?
IMO rational is useful for storing Exactly a limited range of numbers, float for storing a larger range approximately. Therefore I disagree with a rounding policy on rationals. The roles are different. Rationals can be compared exactly whereas this is not very useful with floats, which is a major advantage for rationals It is possible to cast from rational to float, it is unwise to make a cast from float to rational, because the rational gives a precision guarantee that the float doesnt.
- did a test with boost::rational, executing +, -, * and / with 10000 random numbers; + and - gave the correct result only in 44 cases, and * and / in 79 cases -- does it sound useable?
Except that rational numbers used in practise are not entirely random. I would guess that 1/2 is most common, 1/3 etc.
sorry, but feeling frustrated :O)
FWIW your protestations have pointed out potential errors in some of my code, which is a good thing:-) regards Andy Little