Re: [bug-gawk] atan2(y,x) return value for y = 0 and x = -0

[Hermann Peifer]

Here a few more observations (as already posted to c.l.a)

# The input value -0.0 (rather than -0) makes a difference
$ awk -M 'BEGIN{ print atan2(-.0,-.0) }'
-3.14159

# Using the mpfr extension with input value -0
$ awk -l mpfr 'BEGIN{ print mpfr_atan2(0,-0) }'
0.0000000000000000E-1

# The trick with -0.0 doesn't help here
$ awk -l mpfr 'BEGIN{ print mpfr_atan2(0,-.0) }'
0.0000000000000000E-1

Hermann

On 2014-01-18 15:05, Janis Papanagnou wrote:
> Complementary to Herman's view I think that -0 should be handled as +0;
> so I'd qualify the first sample below as "Why is the result pi here?", and
> the second sample as "Works as expected". Either way, I agree that this
> looks like a bug.
>
> Janis
>
>  > Date: Sat, 18 Jan 2014 14:56:19 +0100
>  > From: address@hidden
>  > To: address@hidden
>  > CC: address@hidden
>  > Subject: atan2(y,x) return value for y = 0 and x = -0
>  >
>  > Hi,
>  >
>  > This might be a feature or a bug, but I thought it would be worth
> reporting.
>  >
>  > # Works as expected
>  > $ awk 'BEGIN{ pi = atan2(0,-0) ; printf "%.16f\n", pi }'
>  > 3.1415926535897931
>  >
>  > # Why is the result 0 here ?
>  > $ awk -M 'BEGIN{ pi = atan2(0,-0) ; printf "%.16f\n", pi }'
>  > 0.0000000000000000
>  >
>  > # Works again as expected
>  > $ awk -M 'BEGIN{ pi = atan2(0,-0.1) ; printf "%.16f\n", pi }'
>  > 3.1415926535897931
>  >
>  > # I used gawk from git on Mac OS X, 10.9.1
>  > $ awk -V | head -1
>  > GNU Awk 4.1.60, API: 1.0 (GNU MPFR 3.1.1-p2, GNU MP 5.1.2)
>  >
>  > Hermann