Re^12: NaN output

Hm. $x starts undefined

[0] Perl> use Devel::Peek;;
[0] Perl> Dump $x;;
SV = NULL(0x0) at 0x3c6d400
  REFCNT = 1
  FLAGS = ()
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You set $x to the maximum unsigned integer value; it becomes an IV:

[0] Perl> $x = ~0;;
[0] Perl> Dump $x;;
SV = IV(0x3c6d3f8) at 0x3c6d400
  REFCNT = 1
  FLAGS = (IOK,pIOK,IsUV)
  UV = 18446744073709551615
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You add 2 to it, it overflows the IV, so perl converts it to an NV (NB: No NaN arises from the overflow of the IV.)

[0] Perl> $x += 2;;
[0] Perl> Dump $x;;
SV = PVNV(0x3cb85e8) at 0x3c6d400
  REFCNT = 1
  FLAGS = (NOK,pNOK)
  IV = -1
  NV = 1.84467440737096e+019
  PV = 0
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Then, you raise that NV to the power 35; and the NV overflows; resulting in an NV with the floating point value of positive infinity (1.#INF) (NB:the "result to large" error):

[0] Perl> $x **=35;;
[Result too large] Perl> Dump $x;;
SV = PVNV(0x3cb85e8) at 0x3c6d400
  REFCNT = 1
  FLAGS = (NOK,pNOK)
  IV = -1
  NV = 1.#INF
  PV = 0
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You then divide the NV 1.#INF, by itself, and you produce an indeterminate floating point value (1.#IND):

[0] Perl> $x /= $x;;
[0] Perl> Dump $x;;
SV = PVNV(0x3cb85e8) at 0x3c6d400
  REFCNT = 1
  FLAGS = (NOK,pNOK)
  IV = -1
  NV = -1.#IND
  PV = 0
[download]

The (special form of) NaN was produced by operating upon (erroneous) floating point (NV) values; not integer (IV) overflow.

NaN is a purely floating point concept, and cannot arise as a result of integer overflow.

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Re^13: NaN output by syphilis (Archbishop) on Mar 07, 2014 at 01:56 UTC
Nothing there that I didn't already know, though the phrase "(erroneous) floating point (NV) values" did catch my eye. I'm wondering why "(erroneous)" is in there. I've not previously encountered the notion that an infinity is an erroneous floating point value. (Or, if I have encountered it, I've not paid due attention to it.) Did I misconstrue ? Cheers, Rob	[reply]
Re^14: NaN output by BrowserUk (Patriarch) on Mar 07, 2014 at 02:06 UTC
the phrase "(erroneous) floating point (NV) values" did catch my eye. I'm wondering why "(erroneous)" is in there. 1.#INF means 'greater than the floating point representation can hold". Ie. Floating point overflow. See If your operation would generate a larger positive number than could be stored in a double, the operation will return 1.#INF on Windows or inf on Linux. Similarly your code will return -1.#INF or -inf if the result would be a negative number too large to store in a double. Ie. #INF is a 'floating point exception'. 'Exception is variously defined as: anomaly, irregularity, deviation, special case, departure, inconsistency, quirk, peculiarity, abnormality, oddity;. Ie. A value beyond the capabilities of the binary format to represent. Ie. an erroneousness value. With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply]
Re^15: NaN output by syphilis (Archbishop) on Mar 07, 2014 at 03:36 UTC
Ie. A value beyond the capabilities of the binary format to represent. Ie. an erroneousness value. Hmmm ... "erroneous" is not a term that I would normally use here. However, it could be handy for some leverage at some stage, so I'll keep it in mind. Interestingly, by my reading of Wikipedia, underflow (when the result of a calculation is a smaller number than the computer can actually store in memory) is also an exception in IEEE 754. So it, too, pertains to "a value beyond the capabilities of the binary format to represent". Maybe we'll see you referring to an "erroneous zero" at some time in the future ;-) Cheers, Rob	[reply]
Re^16: NaN output by BrowserUk (Patriarch) on Mar 07, 2014 at 10:50 UTC
Re^17: NaN output by syphilis (Archbishop) on Mar 07, 2014 at 10:55 UTC


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