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Re: useful macros

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On 05/30/2012 11:45 AM, Ben Bacarisse wrote:
> Malcolm McLean <malcolm.mclean5@btinternet.com> writes:
> 
>> בתאריך יום רביעי, 30 במאי 2012 04:26:11 UTC+1, מאת winndy:
>>> On 5月27日, 上午4时08分, Malcolm McLean <malcolm.mcle...@btinternet.com>
>>> wrote:
...
>>>> uniform() - generate a random number on 0 - 1.0 minus epsilon
...
>> I write these macros and put them at the top of pure ANSI source files.
>>
>> I didn't provide definitions because most regulars could easily write
>> these macros in under a minute.
> 
> The first is a little tricky.  If you insist on absolute uniformity and
> exactly the closed interval [0, 1-DBL_EPSILON], I'd have to work for
> more than a minute. ...

The wording is ambiguous; it could be interpreted as implying that the
macro takes an argument named 'epsilon'. If so, the result should
probably have the same type as 'epsilon', which would be the only reason
I can come up with why a macro would be more appropriate than a
function. The macro would have to use _Generic().

I suspect you're thinking about a function which can produce every
representable number between 0 and nextafter(1.0, 0.0), with a uniform
probability distribution. I agree, that would take a lot more than one
minute to write up. However, I'd guess he was thinking of something
along the lines of

#define uniform() (rand()/(1.0 + RAND_MAX))

which can produce only RAND_MAX+1 different values, but those values can
be uniformly distributed if RAND_MAX+1 is an integer power of FLT_RADIX
and RAND_MAX*DBL_EPSILON < 1.0, which is often the case.

Note: while 1 + DBL_EPSILON is, by definition, the same as
nextafter(1.0, 2.0), 1 - DBL_EPSILON need not be the same as
nextafter(1.0, 0.0), and usually won't be. The model for the
representation of floating point values described in 5.2.4.2.2 is not
mandatory; it's used solely for exposition of the requirements of that
section. However, if it's a sufficiently accurate description of the
actual representation of a double, then nextafter(1.0, 0.0) should be
1.0-DBL_EPSILON/FLT_RADIX.

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useful macros Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-05-26 13:08 -0700
  Re: useful macros winndy <jiaoyk@gmail.com> - 2012-05-29 20:26 -0700
    Re: useful macros Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-05-30 00:47 -0700
      Re: useful macros Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-05-30 16:45 +0100
        Re: useful macros James Kuyper <jameskuyper@verizon.net> - 2012-05-30 13:10 -0400
          Re: useful macros Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-05-30 21:54 +0100
          Re: useful macros "BartC" <bc@freeuk.com> - 2012-05-30 23:24 +0100
            Re: useful macros James Kuyper <jameskuyper@verizon.net> - 2012-05-30 18:55 -0400
              Re: useful macros "BartC" <bc@freeuk.com> - 2012-05-31 00:45 +0100
            Re: useful macros Keith Thompson <kst-u@mib.org> - 2012-05-30 17:53 -0700
              Re: useful macros "BartC" <bc@freeuk.com> - 2012-05-31 10:29 +0100

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