how to detect division by zero?

how to detect division by zero?

[Rouben Rostamian]

Hello Guile users!

How does one detect the result of a division by 0.0 in guile?

In guile 1.6.4 we have:

guile> (/ 1 0.)
+#.#

The usual tests are not informative:

guile> (number? (/ 1 0.))
#t

guile> (positive? (/ 1 0.))
#t


-- 
Rouben Rostamian


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Re: how to detect division by zero?

[Roland Orre]

On Tue, 2004-01-20 at 15:17, Rouben Rostamian wrote:
> How does one detect the result of a division by 0.0 in guile?

You could define the values of the not_a_numbers and test with those.
In guile 1.7 symbols for these values are already predefined, so you
could use the following definitions to be future compatible. However,
I just found that (/ 0.0 0.0) can not be tested neither in 1.6 or 1.7.

(define +inf.0 (/ 1.0 0.0))
(define -inf.0 (/ -1.0 0.0))
(define +nan.0 (/ 0.0 0.0))

guile-user> (= +inf.0 (/ 100.0 0.0))
#t
guile-user> (= -inf.0 (/ -100.0 0.0))
#t

Of some reason does the followoing return false
guile-user> (= +nan.0 (/ 0.0 0.0))

/Roland




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Re: how to detect division by zero?

[John W. Eaton]

On 20-Jan-2004, Roland Orre <orre@nada.kth.se> wrote:

| On Tue, 2004-01-20 at 15:17, Rouben Rostamian wrote:
| > How does one detect the result of a division by 0.0 in guile?
| 
| You could define the values of the not_a_numbers and test with those.
| In guile 1.7 symbols for these values are already predefined, so you
| could use the following definitions to be future compatible. However,
| I just found that (/ 0.0 0.0) can not be tested neither in 1.6 or 1.7.
| 
| (define +inf.0 (/ 1.0 0.0))
| (define -inf.0 (/ -1.0 0.0))
| (define +nan.0 (/ 0.0 0.0))
| 
| guile-user> (= +inf.0 (/ 100.0 0.0))
| #t
| guile-user> (= -inf.0 (/ -100.0 0.0))
| #t
| 
| Of some reason does the followoing return false
| guile-user> (= +nan.0 (/ 0.0 0.0))

NaN values never compare equal.  To check for them, you need isnan.  I
helped to write a patch for this some time ago.  Doesn't guile 1.7 (or
some newer than 1.6 version) have isinf, isnan, and other IEEE
floating point functions and values built in?

jwe


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Re: how to detect division by zero?

[Paul Jarc]

"John W. Eaton" <jwe@bevo.che.wisc.edu> wrote:
> NaN values never compare equal.  To check for them, you need isnan.

Does this work?
(define (nan? x) (not (= x x)))


paul


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Re: how to detect division by zero?

[Roland Orre]

On Tue, 2004-01-20 at 17:02, John W. Eaton wrote:
> On 20-Jan-2004, Roland Orre <orre@nada.kth.se> wrote:
> NaN values never compare equal.  To check for them, you need isnan.  I
> helped to write a patch for this some time ago.  Doesn't guile 1.7 (or
> some newer than 1.6 version) have isinf, isnan, and other IEEE
> floating point functions and values built in?

Yes, you are right. I had missed that. There are e.g. nan? and inf?

/Roland	



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Re: how to detect division by zero?

[Marius Vollmer]

Roland Orre <orre@nada.kth.se> writes:

> Of some reason does the followoing return false
> guile-user> (= +nan.0 (/ 0.0 0.0))

In addition to nan? you can also use 'eqv?' for comparison:

    guile> (eqv? +nan.0 (/ 0.0 0.0))
    #t

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Re: how to detect division by zero?

[Bernard Urban]

"Rouben Rostamian" <rostamian@umbc.edu> writes:

> Hello Guile users!
> 
> How does one detect the result of a division by 0.0 in guile?
> 
> In guile 1.6.4 we have:
> 
> guile> (/ 1 0.)
> +#.#
> 
> The usual tests are not informative:
> 
> guile> (number? (/ 1 0.))
> #t
> 
> guile> (positive? (/ 1 0.))
> #t
> 

I read recently on this list that
there is some explicit test available for infinity or nan in 1.7. 

I take the opportunity to pinpoint a bug in 1.6.4:
guile
guile> (version)
"1.6.4"
guile> (/ 0)
+#.#
guile> (/ 1 0) 
standard input:3:1: In procedure / in expression (/ 1 0):
standard input:3:1: Numerical overflow
ABORT: (numerical-overflow)

Type "(backtrace)" to get more information or "(debug)" to enter the debugger.
guile> 

In one case, the code in numbers.c for scm_divide computes the hardware
generated positive infinity, in the other an explicit call to
scm_num_overflow is made after testing the divisor value. 

-- 

Bernard Urban


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