undefined value as an ARRAY

mgwmgw has asked for the wisdom of the Perl Monks concerning the following question:

I am writing some perl which is giving me an error that I do not understand.

# x is 2 to the 128th
my $x = Math::BigFloat->new(q(2.0));
$x->bpow(q(128));
# ...more stuff...
# $product starts as x!
  $product = $x->copy();
  $product->bfac();
[download]

The error I get is

Can't use an undefined value as an ARRAY reference at
    /System/Library/Perl/5.8.6/Math/BigInt/Calc.pm line 899 (#1)
    (F) A value used as either a hard reference or a symbolic referenc
+e must
    be a defined value.  This helps to delurk some insidious errors.
    
Uncaught exception from user code:
    Can't use an undefined value as an ARRAY reference at /System/Libr
+ary/Perl/5.8.6/Math/BigInt/Calc.pm line 899.
 at /System/Library/Perl/5.8.6/Math/BigInt/Calc.pm line 899
    Math::BigInt::Calc::_acmp('ARRAY(0x18690a0)', 'ARRAY(0x18690d0)') 
+called at /System/Library/Perl/5.8.6/Math/BigInt/Calc.pm line 1304
    Math::BigInt::Calc::_fac('Math::BigInt::Calc', 'ARRAY(0x1868fc8)')
+ called at /System/Library/Perl/5.8.6/Math/BigFloat.pm line 1789
    Math::BigFloat::bfac('Math::BigFloat=HASH(0x1868f5c)') called at r
+epeat.pl line 19
[download]

I have googled and not found this question asked before. Can anyone explain what is going wrong? Thanks.

Comment on undefined value as an ARRAY Select or Download Code

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Re: undefined value as an ARRAY by BrowserUk (Patriarch) on Aug 12, 2010 at 03:28 UTC
You're probably running out of memory! According to Wolfram/Alpha, 2**128! is 10^{10^{40.11273643527576}} and has 85070591730234615865843651857942052838 trailing zeros. As such, it would require more memory than will ever exist to calculate. 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. RIP an inspiration; A true Folk's Guy	[reply]
Re: undefined value as an ARRAY (Math::BigFloat) by toolic (Bishop) on Aug 12, 2010 at 02:07 UTC
I can't answer your question directly, but it seems as though you have either uncovered a bug or you have exceeded some not-so-obviously specified limit of the `bfac` function of Math::BigFloat. It seems the maximum value for this function is between 223 and 224. This does not generate the error: `use strict; use warnings; use diagnostics; use Math::BigFloat; my $x = Math::BigFloat->new(2**23); print "x=$x\n"; my $product = $x->copy(); $product->bfac(); print "p=$product\n";` [download] You could submit a bug report on CPAN. But, before doing so, you could use Super Search to see if anyone has encountered similar issues with this module. I tried Math::BigInt as well, but I get the same results.	[reply] [d/l] [select]
Re: undefined value as an ARRAY by graff (Chancellor) on Aug 12, 2010 at 02:38 UTC
Curiously, when I run the following command-line script on my mac (to replicate the essentials of your code), the "bfac" call apparently does not return, and the process runs in a seemingly infinite loop, chewing up cpu time: `perl -MMath::BigFloat -e '$\|++;$x=Math::BigFloat->new("2.0"); print "\ncalling bpow at ",scalar(localtime)."\n";$x->bpow("128"); print "calling copy at ",scalar(localtime)."\n";$p=$x->copy(); print "calling bfac at ",scalar(localtime)."\n";$p->bfac(); print "finished at ",scalar(localtime)."\n"' calling bpow at Wed Aug 11 21:44:28 2010 calling copy at Wed Aug 11 21:44:28 2010 calling bfac at Wed Aug 11 21:44:28 2010` [download] (still running an hour later... oh well, I'm just going to kill it now.) FWIW, I'm using the version that comes with "snow leopard" (osx 10.6.4): `This is perl, v5.10.0 built for darwin-thread-multi-2level` [download] When you ran your script, did it crash right away, or did it run for a long time before crashing? (Updated my command-line script to include the (rather important) first line of the command.)	[reply] [d/l] [select]
Re^2: undefined value as an ARRAY by oko1 (Deacon) on Aug 12, 2010 at 03:24 UTC
I suspect you'd have to wait a bit more than an hour. Given this: #!/usr/bin/perl -w use strict; use Benchmark qw/:all/; use Math::BigFloat; cmpthese( 1, { '210' => sub { my $x = Math::BigFloat->new(210); my $prod +uct = $x->copy(); $product->bfac(); }, '211' => sub { my $x = Math::BigFloat->new(211); my $prod +uct = $x->copy(); $product->bfac(); }, '212' => sub { my $x = Math::BigFloat->new(212); my $prod +uct = $x->copy(); $product->bfac(); }, '213' => sub { my $x = Math::BigFloat->new(213); my $prod +uct = $x->copy(); $product->bfac(); }, '214' => sub { my $x = Math::BigFloat->new(214); my $prod +uct = $x->copy(); $product->bfac(); } }); [download] I get run times like these: `s/iter 214 213 212 211 210 214 139 -- -78% -97% -99% -100% 213 31.3 345% -- -88% -97% -99% 212 3.88 3488% 706% -- -78% -95% 211 0.870 15902% 3493% 346% -- -76% 210 0.210 66195% 14786% 1748% 314% --` [download] With that kind of progression, 2**128 is, um, likely to take a while. :) -- "Language shapes the way we think, and determines what we can think about." -- B. L. Whorf	[reply] [d/l] [select]
Re^2: undefined value as an ARRAY by toolic (Bishop) on Aug 12, 2010 at 02:46 UTC
When you ran your script, did it crash right away, or did it run for a long time before crashing? Another data point: I get the error immediately. When I try smaller numbers, I do not get the error, but it does seem to take a while. `This is perl, v5.8.8 built for MSWin32-x86-multi-thread` [download]	[reply] [d/l]
Re^3: undefined value as an ARRAY by morgon (Priest) on Aug 12, 2010 at 02:56 UTC
If your script works for smaller numbers then it's probably a bug in either Math::BigFloat itself or in a library it uses . If it is an option for you try it on another system or with a newer version of Perl.	[reply]