Re: [pure-lang-users] ln

[John C. <co...@cc...>]

Albert Graef scripsit:

> Unfortunately, on my Linux system both c_log (1/inf) and c_log (1/-inf) 
> return -inf in Pure, even though Pure correctly displays 1/inf as 0.0 
> and 1/-inf as -0.0. So it seems that the glibc log() function at least 
> does *not* distinguish between positive and negative zero. Maybe that's 
> a non-conformance in glibc, I'll have to check POSIX to see whether it 
> has something to say about log(-0.0).

It does, see
<http://www.opengroup.org/onlinepubs/000095399/functions/log.html>.

    Upon successful completion, these functions shall return the natural
    logarithm of x.

    If x is ±0, a pole error shall occur and log(), logf(), and logl()
    shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.

    For finite values of x that are less than 0, or if x is -Inf, a
    domain error shall occur, and either a NaN (if supported), or an
    implementation-defined value shall be returned.

    If x is NaN, a NaN shall be returned.

    If x is 1, +0 shall be returned.

    If x is +Inf, x shall be returned. [Option End]

The talk of HUGE_VAL is to cover non-IEEE-754 systems that don't have Inf,
and have to use the largest representable value as a proxy for it.

-- 
Dream projects long deferred             John Cowan <co...@cc...>
usually bite the wax tadpole.            http://www.ccil.org/~cowan
        --James Lileks