Re^11: NaN output

The problem (for me) is that there are no IVs involved in your construction

C:\>perl -MDevel::Peek -le "Dump(2);"
SV = IV(0x1daf1b8) at 0x1daf1bc
  REFCNT = 1
  FLAGS = (PADTMP,IOK,READONLY,pIOK)
  IV = 2

C:\>perl -MDevel::Peek -le "Dump(1025);"
SV = IV(0x53ee70) at 0x53ee74
  REFCNT = 1
  FLAGS = (PADTMP,IOK,READONLY,pIOK)
  IV = 1025
[download]

Or, alternatively, I could have assigned the values 2 and 1025 to two IV's.

The interpreter constructs a compile-time constant from the expression and stores it as an NV.

Yes, nothing remarkable about that - most perl users (you and I included) are aware of this and generally comfortable with it.
However, it implies to me that in perl, there are no distinct realms of ints and floats. And your remark that "NaN is a purely floating point concept" (which is correct) suddenly seemed a bit blurry wrt perl - because, in so many ways, perl doesn't draw a line between floats and ints - yet it does have NaNs.

Anyway ... I should have tried harder to explain the triviality or, better still, just made a mental note of it and said nothing.

I sure am glad I didn't create a demo where the upgrade from IV to NV occurred because of integer overflow ... then manipulate the NV to create a NaN ... then claim to have shown that a NaN *can* arise from integer overflow. Something like:

C:\>perl -le "$x= ~0; $x += 2; $x **= 35; $x /= $x; print $x;"
-1.#IND
[download]

;-))

(Complete and utter sophistry, of course.)

Cheers,
Rob

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Re^12: NaN output by BrowserUk (Patriarch) on Mar 07, 2014 at 00:38 UTC
Hm. $x starts undefined `[0] Perl> use Devel::Peek;; [0] Perl> Dump $x;; SV = NULL(0x0) at 0x3c6d400 REFCNT = 1 FLAGS = ()` [download] 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` [download] 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` [download] 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` [download] 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.** 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] [d/l] [select]
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
Re^16: NaN output by BrowserUk (Patriarch) on Mar 07, 2014 at 10:50 UTC
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