PEP proposal for round(x,n) enhancement

Markus Schaber markus at schabi.de
Thu Sep 13 00:27:04 CEST 2001


Hi,

Christopher Smith <csmith at blakeschool.org> schrub:

> Markus writes:
>>The problem is that you can't store this information in a floating
>>point number. I just tried the following:
>>
>>>>> 0.2
>>0.20000000000000001
>>
>>This, I give the number with 1 significant digit, and end up having
>>displayed 17 "significant" digits.
> 
> Yes and no.  You told Python to show you in decimal format how '0.2'
> is
> represented as a binary number.  If you had said "print 0.2" you would
> have
> obtained 0.2 and if you had said "print 0.2000" you would have
> obtained the same
> thing (even though you wanted 4 significant digits).

And this is what I meant - the float doesn't store how many digits are 
significant, so when you round to n digits, you often get an 
approximation to the rounded number, and this way don't have any 
information about the accuracy afterwards.

>>So you may only round (or whatever we call it) to powers of two, but
>>not to powers of 10 (digits) without having rationals or decimals.
> 
> Yes, you are right in that there is no way to exactly represent some
> numbers
> exactly as floating point numbers but this has nothing to do with
> rounding a
> number (to the closest approximation that you can) to a specific
> digit. This is
> the task of the round() function.  My proposal only has to do with
> enhancing the
> syntax used by the round() function.

And enhancing the semantics :-)

markus
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