not-a-number's

[Mischa Baars]

On 01/16/2013 12:07 PM, Andrew Haley wrote:
> On 01/16/2013 09:27 AM, Mischa Baars wrote:
>> On 01/16/2013 10:06 AM, Andreas Schwab wrote:
>>> Mischa Baars <mjbaars1977@gmail.com> writes:
>>>
>>>> This means that the first 'if' statement should have been terminated when
>>> There is no such thing as a "terminated statement".  The first condition
>>> evaluates to true.
>> Whatever you want, although personally I think it is very unlikely that
>> '2' is the correct value to return :)
> Let's go through this line by line.  If there's anything you disagree
> with, speak up.  But if you disagree that GCC has implemented C
> correctly according to the standard, you must quote the relevant
> sections of the document and explain your belief, not in terms of what
> you think it should do, but in terms of what the standard actually
> says.
>
> Comparisons with NaN don't terminate a statement, and they don't
> return a NaN.  They return a boolean.
>
> Here's what Standard C, F.8.3 Relational operators, says:
>
>   x != x → false	            The statement x != x is true if x is a NaN.
>
>   x == x → true              The statement x == x is false if x is a NaN.
And indeed apparently the answer then is '2'. However, I don't think 
this is correct. If that means that there is an error in the C 
specification, then there probably is an error in the specification.
>
> So:
>
> {
>      double r = -NAN;
>
> //  if (!isnan(y) && !isnan(x))
> //  {
>          double a = absd(x);
>          double b = absd(y);
>
>          if (b != a)
>
> If one of a or b is a NaN,  the comparison is unordered, so we take this branch.
>
>          {
>              if (b < a)
>
> If one of a or b is a NaN, the comparison is unordered, so it returns
> false.  We'll take the else branch.
>
>              {
>                  if (b != 0)                             // octant 1, 4, 5 and 8 (open set)
>                  {
>                      r = 0;
>                  }
>                  else                                    // x-axis
>                  {
>                      r = 1;
>                  }
>              }
>              else
>              {
>                  if (a != 0)                             // octant 2, 3, 6 and 7 (open set)
>
> If a is a NaN, the comparison is unordered, so != returns true.  So we
> take this branch and return 2.
>
>                  {
>                      r = 2;                              // not-a-number's ???
>                  }
>                  else                                    // y-axis
>                  {
>                      r = 3;
>                  }
>              }
>          }
>          else                                            // between axes
>          {
>              r = 4;
>          }
> //  }
>
> Andrew.