[Rd] RFC: hexadecimal constants and decimal points

[Prof Brian Ripley]

On Sun, 17 Apr 2005, Jan T. Kim wrote:

> On Sun, Apr 17, 2005 at 12:38:10PM +0100, Prof Brian Ripley wrote:
>> These are some points stimulated by reading about C history (and
>> related in their implementation).
>>
>>
>> 1) On some platforms
>>
>>> as.integer("0xA")
>> [1] 10
>>
>> but not all (not on Solaris nor Windows).  We do not define what is
>> allowed, and rely on the OS's implementation of strtod (yes, not strtol).
>> It seems that glibc does allow hex: C99 mandates it but C89 seems not to
>> allow it.
>>
>> I think that was a mistake, and strtol should have been used.  Then C89
>> does mandate the handling of hex constants and also octal ones.  So
>> changing to strtol would change the meaning of as.integer("011").
>
> I think interpretation of a leading "0" as a prefix indicating an octal
> representation should indeed be avoided. People not familiar to C will
> have a hard time understanding and getting used to this concept, and
> in addition, it happens way too often that numeric data are provided left-
> padded with zeros.
>
>> Proposal: we handle this ourselves and define what values are acceptable,
>> namely for as.integer:
>>
>> [+|-][0-9]+
>> NA
>> 0[x|X][0-9A-fa-f]+
>
> It can be a somewhat mixed blessing if the string representation of numeric
> values contain information about their base, in the form of the 0x prefix
> in this case.
>
> The base argument (#3) of C's strtol function can be set to to a base
> explicitly or to 0, which gives the prefix-based "auto-selection"
> behaviour. On the R level, such a base argument (to as.integer) could be
> included and a default could be set.

A lot of this is internal, not at R level.

> Personally, I would be equally happy with the default being 0 (auto-select)
> or 10. Considering the perhaps limited spread of familiarity with C's
> "0x" idiom, I somewhat favour a consistent and "stubborn" decimal behaviour
> (base defaults to 10), though.

Some people already rely on it, and those who don't know about it are 
unliekly to ever enter what they think is an illegal value, surely?

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595