R devel - May 2009 - suggestion for extending ?as.factor

In R-2.10.0, the development version, function as.factor() uses 17 digit
precision for conversion of numeric values to character type. This
is very good for the consistency of the resulting factor, however,
i expect that people will complain about, for example, as.factor(0.3)
being
  [1] 0.29999999999999999
  Levels: 0.29999999999999999

I suggest to extend the "Warning" section of ?as.factor by the
following
paragraph.

If as.factor() is used for a numeric vector, then the numbers are
converted to character strings with 17 digit precision using their
machine representation. This guarantees that different numbers are
converted to different levels, but may produce unwanted results, if
the numbers are expected to have limited number of decimal positions.
For example, as.factor(c(0.1, 0.2, 0.3)) produces
  [1] 0.10000000000000001 0.20000000000000001 0.29999999999999999
  Levels: 0.10000000000000001 0.20000000000000001 0.29999999999999999
In order to avoid this, convert the numbers to a character vector
using formatC() or a similar function before using as.factor().

Petr.

>>>>> "PS" == Petr Savicky <savicky at cs.cas.cz>
>>>>>     on Sun, 3 May 2009 22:32:04 +0200 writes:
>>>>> "PS" == Petr Savicky <savicky at cs.cas.cz>
>>>>>     on Sun, 3 May 2009 22:32:04 +0200 writes:
    PS> In R-2.10.0, the development version, function as.factor() uses 17
digit
    PS> precision for conversion of numeric values to character type. This
    PS> is very good for the consistency of the resulting factor, however,
    PS> i expect that people will complain about, for example, as.factor(0.3)
    PS> being
    PS> [1] 0.29999999999999999
    PS> Levels: 0.29999999999999999

    PS> I suggest to extend the "Warning" section of ?as.factor by
the following
    PS> paragraph.

    PS> If as.factor() is used for a numeric vector, then the numbers are
    PS> converted to character strings with 17 digit precision using their
    PS> machine representation. This guarantees that different numbers are
    PS> converted to different levels, but may produce unwanted results, if
    PS> the numbers are expected to have limited number of decimal positions.
    PS> For example, as.factor(c(0.1, 0.2, 0.3)) produces
    PS> [1] 0.10000000000000001 0.20000000000000001 0.29999999999999999
    PS> Levels: 0.10000000000000001 0.20000000000000001 0.29999999999999999
    PS> In order to avoid this, convert the numbers to a character vector
    PS> using formatC() or a similar function before using as.factor().

    PS> Petr.

Thank you, Petr, for the good suggestion.

I have added a (shorter) paragraph, though to the 'Details' not the
'Warning' section, and also one to the 'Examples' :

## Converting (non-integer) numbers:
as.factor(c(0.1, 0.2, 0.3)) # maybe not what you'd expect, so rather use
factor(format(c(0.1, 0.2, 0.3)))

Martin Maechler, ETH Zurich