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Re^2: $array[ 'Infinity' ]

by quester (Vicar)
on Dec 17, 2007 at 00:25 UTC ( #657350=note: print w/replies, xml ) Need Help??

in reply to Re: $array[ 'Infinity' ]
in thread $array[ 'Infinity' ]

Using perl -MO=Deparse may shed some light on some of this weirdness, although I have no idea why on Earth anyone would implement 'inf' the way it works on my box.
perl -MO=Deparse -e 'print $n[inf]'
print $n[9**9**9];
as does
perl -MO=Deparse -e 'print $n["inf"]'


On the other hand,

perl -MO=Deparse -e 'print $n[-inf]'
print $n[-'inf'];


perl -MO=Deparse -e 'print "", inf==0 ? "a" : "b", 'inf'==0 ? "c" : "d +", -inf==0 ? "e" : "f"'
print '', 'inf' == 0 ? 'a' : 'b', 'inf' == 0 ? 'c' : 'd', -'inf' == 0 +? 'e' : 'f';

As nearly as I can tell at the moment, inf is only equivalent 9**9**9 if it is used by itself as a subscript; it is taken as a bareword in a simple assignment, so

perl -MO=Deparse -e '$n[inf], $n[1+inf], $n[-inf] = inf'
$n[9**9**9], $n[1 + 'inf'], $n[-'inf'] = 'inf';


My perl -v shows,

This is perl, v5.8.8 built for i486-linux-gnu-thread-multi


Does anyone know why? - quester

Replies are listed 'Best First'.
Re^3: $array[ 'Infinity' ]
by educated_foo (Vicar) on Dec 17, 2007 at 01:36 UTC
    That is terrifyingly weird. But the 9**9**9 is just an artifact of B::Deparse, as can be seen here. I'm not sure why it does that.

    Then again, it makes a kind of twisted sense that $a[inf] means "last element of @a", and (completely by accident) $a[-inf] means "last element counting from the end."

Re^3: $array[ 'Infinity' ]
by ww (Archbishop) on Dec 17, 2007 at 02:01 UTC
    Oddly (or maybe not so oddly) I get different output on a w2k box from the first cited by quester:
    perl -MO=Deparse -e "print $n[inf]" print $n[0]; -e syntax OK
    perl -v This is perl, v5.8.8 built for MSWin32-x86-multi-thread (with 33 registered patches, see perl -V for more detail) ... Binary build 819 [267479] provided by ActiveState http://www.ActiveSta Built Aug 29 2006 12:42:41
      It's been established that ActiveState's build doesn't support infinity.
Re: $array[ 'Infinity' ]
by benizi (Hermit) on Dec 19, 2007 at 23:55 UTC

    As to the "why", X = 9**9**9 == 9**(9**9) = 9**387420489. Nine to that power is a number aproximately 1.2 billion bits long. (log(9)/log(2) * (9**9)). So, for all intents and purposes, it's infinity. Since it's odd, -X will be negative infinity So, it avoids the (possible?) non-portability of a raw 'inf' or '-inf'.

    Update: Duh, Ben. Re: strikeout. Plus it's specifically negated. I was thinking (-9)**9**9)

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