[docs] [issue14245] float rounding examples in FAQ are outdated
Martin v. Löwis
report at bugs.python.org
Sat Mar 10 15:51:00 CET 2012
Martin v. Löwis <martin at v.loewis.de> added the comment:
> I think it would be useful to mention explicitly that Python simply uses
> the native floating-point implementation in hardware and thus behaves
> very similarly to other languages which do this, for instance C or Java.
> This should clear up a lot of the behaviour for people who know other
> programming languages. "how the underlying platform handles
> floating-point" says something very similar, but the reader needs to
> understand what the "underlying platform" exactly is.
Well, people may just be as confused about the term "native
implementation" as they are seemingly confused about "underlying
platform".
>
> I have to admit, that I'm completely lost here --- why would a vastly
> inaccurate number (with more than half of digits wrong) be ever stored?
> If "1.2" is converted to a float (a C double in current implementation),
> it has 15.96 decimal digits of precision.
Since it has this number of decimal digits of precision, the text says
"about 16", rather than "exactly 17", which would mislead people into
thinking that they get 17 digits of precision.
I don't understand your confusion. The numbers are stored in binary,
and it's *not* the case that half of the digits are wrong. In binary,
all 53 bits of mantissa are correct. When displaying this number in
decimal (which is not the way in which it is stored), then you many more
decimal digits to make the decimal representation correct.
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