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Incrementing "Infinity" bug

by LanX (Canon)
on Mar 24, 2013 at 14:31 UTC ( #1025148=perlquestion: print w/ replies, xml ) Need Help??
LanX has asked for the wisdom of the Perl Monks concerning the following question:

OK another case where symmetry is broken

(continuing Re^3: Converting a list of numbers to use a range operator)

this works

DB<159> $a=inf => "inf" DB<160> --$a => "inf" DB<161> ++$a => "inf" DB<162> ++$a => "inf"

this doesn't

DB<156> $a=inf => "inf" DB<157> ++$a => "ing" DB<158> ++$a => "inh"

any clean way to avoid this?

I suppose any internal "numeric flag" or "dual var slot" is only set after (a clear) numeric operation...

Cheers Rolf

( addicted to the Perl Programming Language)

Update

Stupid workaround proving my idea

DB<174> $a=inf+0 => "inf" DB<175> ++$a => "inf" DB<176> ++$a => "inf"

IMHO still a bug!

tested in 5.10

could someone plz test in newer versions and reply?

perl -e '$a=inf; print ++$a,"\n"' ing

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Re: Incrementing "Infinity" bug
by Athanasius (Monsignor) on Mar 24, 2013 at 14:56 UTC
    0:52 >perl -E "$a = inf; print ++$a, qq[\n];" ing 0:52 >perl -E "$a = inf + 0; print ++$a, qq[\n];" 1.#INF 0:52 >perl -Mbigint -E "$a = inf; print ++$a, qq[\n];" inf 0:52 >perl -v This is perl 5, version 16, subversion 0 (v5.16.0) built for MSWin32-x +86-multi-thread-64int (with 1 registered patch, see perl -V for more detail)

    Hope that helps,

    Athanasius <°(((><contra mundum Iustus alius egestas vitae, eros Piratica,

Re: Incrementing "Infinity" bug (numification, perlnumber, magic increment decrement)
by Anonymous Monk on Mar 24, 2013 at 22:30 UTC

    This is not a bug

    Perl has magic increment operator, it works on strings

    Perl not have magic decrement operator, so string is turned into a number

    inf looks like a string so it is a string, not a number, use int to force numification

    $ perl -MData::Dump -e " $f = inf; dd $f; $f--; dd $f; " "inf" 1.#INF $ perl -MData::Dump -e " dd int inf " 1.#INF $ perl -MData::Dump -e " dd int Infinity " 1.#INF

    perlnumber, perldata, 2010 :) Infinity and Inf?

      It's a bug or at least a design flow.

      Creating a constant which returns a numified inf easily fixes this problem

      DB<100> use constant inf => inf+0 => 0 DB<101> $a=inf => "inf" DB<102> ++$a => "inf" DB<103> ++$a => "inf"

      So why doesn't Perl do this per default?

      do we really need to use hacks like 1e9999 to create inf?

      Cheers Rolf

      ( addicted to the Perl Programming Language)

        It's a bug or at least a design flow.

        How do you figure? I think it isn't

        So why doesn't Perl do this per default?

        You know why, tradition (backwards compatibility ). "barewords" have always been strings

        Also style, lowercase constants are unbecoming :p

        do we really need to use hacks like 1e9999 to create inf?

        We never needed that, use  0+'inf' or  int '-inf' ...

        But hey, one of the math modules could export ... or even  use feature 'infinity'; could happen though I don't see the benefit, but what do I know

        $ perl -le " for(1..4){ print for \int 'infinity', \int'-infinity' } +" SCALAR(0x99bc74) SCALAR(0x99bc64) SCALAR(0x99bc74) SCALAR(0x99bc64) SCALAR(0x99bc74) SCALAR(0x99bc64) SCALAR(0x99bc74) SCALAR(0x99bc64)

        update: nans, infs, and vomit, How to create nan/inf says bigint/bigrat export inf that stringifies to inf

        Also note Re^3: nans, infs, and vomit (underlying c-runtime and strtod, atof)

Re: Incrementing "Infinity" bug
by kcott (Abbot) on Mar 25, 2013 at 06:28 UTC

    G'day Rolf,

    "tested in 5.10

    could someone plz test in newer versions and reply?"

    Same result in 5.14.2:

    $ perl -e '$a=inf; print ++$a,"\n"' ing $ perl -v This is perl 5, version 14, subversion 2 (v5.14.2) built for darwin-th +read-multi-2level

    As you were probably aware, warnings alert you and strictures disallow it altogether:

    $ perl -Mwarnings -e '$a=inf; print ++$a,"\n"' Unquoted string "inf" may clash with future reserved word at -e line 1 +. ing $ perl -Mstrict -e '$a=inf; print ++$a,"\n"' Bareword "inf" not allowed while "strict subs" in use at -e line 1. Execution of -e aborted due to compilation errors.

    -- Ken

      Hi Ken!

      > As you were probably aware, warnings alert you and strictures disallow it altogether:

      Yes, I became aware after posting the first time.

      I expected inf and it's brothers▓ to be constants because of their magic behavior when being numified.

      Introducing constants╣ for inf, -inf and NaN (even all uppercase like INF) would help clarifying the situation.

      Cheers Rolf

      ( addicted to the Perl Programming Language)

      ╣) like I demonstrated here

      ▓) every string starting with inf (no matter if upper or lower case) numifies to inf:

      DB<106> $a="Infinity"+0 => "inf" DB<109> $a="Inftyddssssd"+0 => "inf" DB<110> $a="Inf"+0 => "inf" DB<111> $a="InF"+0 => "inf"

      UPDATE: line 109 with"Inftyddssssd" fails with activated warnings, anything else with lc($else) ~~ ['inf','infinity'] won't

      DB<123> use warnings; $a='InFiNiTy'+0 => "inf" DB<125> use warnings;$a="Inftyddssssd"+0 Argument "Inftyddssssd" isn't numeric in addition (+) at (eval 74)[mul +ti_perl5db.pl:644] line 2.

        I came across Inf (and friends) some years ago, decided they all looked somewhat flakey, and haven't used them since. Your posting has (somewhat) piqued my interest again. Here's some interesting results from Scalar::Util's looks_like_number() function.

        $ perl -E ' use Scalar::Util qw{looks_like_number}; my @infs = (inf, Inf, INF, infinity, Infinity, INFINITY, -inf, -Inf, -INF, -infinity, -Infinity, -INFINITY, Inftyddssssd, infighting, Infighting, -Inftyddssssd, -infighting, -Infighting); say "$_ : ", looks_like_number($_) for @infs; ' inf : 20 Inf : 20 INF : 20 infinity : 20 Infinity : 20 INFINITY : 20 -inf : 28 -inf : 28 -inf : 28 -inf : 28 -inf : 28 -inf : 28 Inftyddssssd : 0 infighting : 0 Infighting : 0 -Inftyddssssd : 0 -infighting : 0 -Infighting : 0

        I also noted that the documentation for this function links to perlapi - looks_like_number, which may be of interest ot you.

        -- Ken

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