[Python-ideas] PEP 485: A Function for testing approximate equality

[Steven D'Aprano]

On Thu, Feb 05, 2015 at 09:53:49PM -0600, Ron Adam wrote:

> I prefer the weak version myself.  Because if you graph the result of True 
> values for rel_tolerance=1.  You get a graph where all like signed numbers 
> are close.   A tolerance of .5 gives a graph of the fifty percent of middle 
> like signed numbers.  And you can think of it as a percentage of the larger 
> value.  Which tends to be easier than thinking about percent increase.

I'm afraid I don't understand this description.


> Also this recommends using this method.
> 
>     http://c-faq.com/fp/fpequal.html

You stopped reading too soon:

[quote]
Doug Gwyn suggests using the following ``relative difference'' function. 
It returns the relative difference of two real numbers: 0.0 if they are 
exactly the same, otherwise the ratio of the difference to the larger of 
the two.
[end quote]

Anyone got a copy of Knuth handy and check the reference given above?

Knuth Sec. 4.2.2 pp. 217-8

I'd like to know what he has to say.



-- 
Steve