Re: [gentoo-science] question about signbit

[Darren Dale <dd55@cornell.edu>]

Hi Marco,

On Saturday 22 October 2005 5:05 pm, Marco Matthies wrote:
> Darren Dale wrote:
> > On my system, SciPy's signbit function reports that the sign bit is not
> > set for any number, positive or negative. Could someone here help me
> > understand how to test the libc signbit function? I have to admit I have
> > no experience with C programming.
>
> Hi Darren,
>
> the signbit fuction is actually a macro (as the manpage says) defined in
> math.h that in turn calls the right inline function (for the type
> needed) which is defined in mathinline.h --- so as far as i can see,
> libc should not be involved, only header files. I have attached a small
> example below on how to use the function. Please note the use of
> -std=c99 (you may also use -std=gnu99) as the macro is only activated
> when in C99 mode and gcc's default mode is C89 ("ANSI C"). If you're
> interested in the differences between the two standards the wikipedia
> entry on c has some info:
> http://en.wikipedia.org/wiki/C_programming_language
>
>
> the program (save it under signbit_test.c):
> [cut]
> #include <math.h>
> #include <stdio.h>
>
> int main() {
>          printf("sign of  1.7 is %d\n", signbit(1.7));
>          printf("sign of -1.1 is %d\n", signbit(-1.1));
>          printf("sign of -0.0 is %d\n", signbit(-0.0));
>          printf("sign of  0.0 is %d\n", signbit(0.0));
>          return 0;
> }
> [/cut]
>
> compile with:
> gcc -Wall -std=c99 -lm signbit_test.c -o signbit_test
>
> run with:
> ./signbit_test
>
> should produce this output:
> sign of  1.7 is 0
> sign of -1.1 is -2147483648
> sign of -0.0 is -2147483648
> sign of  0.0 is 0
>
> This was run with gcc 3.4.4 on amd64, if you want to i can try on a x86
> install in qemu.

Here is another test:

#include <math.h>
#include <stdio.h>

int main() {
	printf("signbit(-1): %d\n", signbit(-1));
	printf("isnan(0.0/0): %d\n", isnan(0.0/0));
	printf("isinf(1.0/0): %d\n", isinf(1.0/0));
	return 0;
}

which yields:

signbit(-1): -2147483648
isnan(0.0/0): 1
isinf(1.0/0): 1

Do you know why signbit doesn't yield 1? I wonder if this might be the source 
of the problem in Scipy.

Thanks,
Darren
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