Max of 23 and 27 is 23?

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

Replies are listed 'Best First'.
Re: Max of 23 and 27 is 23? by ikegami (Patriarch) on Jul 02, 2015 at 19:57 UTC
`$#a` returns a magical value in lvalue contexts. (It used to always returns a magical value, but this was optimized.) `$ perl -MDevel::Peek -e'@a=qw( a b c ); sub { Dump($_[0]) }->( $#a );' SV = PVMG(0x1d36630) at 0x1cfcfd0 REFCNT = 1 FLAGS = (GMG,SMG) IV = 0 NV = 0 PV = 0 MAGIC = 0x1cf6ab0 MG_VIRTUAL = &PL_vtbl_arylen MG_TYPE = PERL_MAGIC_arylen(#) MG_OBJ = 0x1d07d10` [download] The behaviour you observe is a missing `SvGETMAGIC(sv)` to handle such scalars. `use strict; use warnings; use List::Util qw( max ); use Variable::Magic qw( cast wizard ); # Make $x a magical variable that always returns 5. cast(my($x), wizard( get => sub { ${ $_[0] } = 5 }, ) ); $x = 3; if ($ARGV[0]) { no warnings 'void'; 0+$x } print('max($x, 4)=', max($x, 4), "\n"); print('$x=', $x, "\n");` [download] `$ perl a.pl 0 max($x, 4)=4 $x=5 $ perl a.pl 1 max($x, 4)=5 $x=5` [download]	[reply] [d/l] [select]
Re^2: Max of 23 and 27 is 23? (XS--) by tye (Sage) on Jul 02, 2015 at 21:18 UTC
The behaviour you observe is a missing SvGETMAGIC(sv) to handle such scalars. Which is pretty much to be expected with the vast majority of XS code. Which is one reason why I almost never use List::Util routines. Must use XS for "speed!". Correctness, well, that doesn't matter quite as much. - tye	[reply]
Re^3: Max of 23 and 27 is 23? (XS--) by Anonymous Monk on Jul 09, 2015 at 21:28 UTC
> Which is one reason why I almost never use List::Util routines. Must use XS for "speed!". Correctness, well, that doesn't matter quite as much. The authors care about correctness. However, they cannot fix bugs that they are not informed of.	[reply]
Re: Max of 23 and 27 is 23? by roboticus (Chancellor) on Jul 02, 2015 at 17:39 UTC
luxgladius: It depends on how `max` is defined. FYI: min and max aren't built-in perl functions, so I'd start out with finding where it's coming from, and review the docs/source. ...roboticus When your only tool is a hammer, all problems look like your thumb.	[reply] [d/l]
Re^2: Max of 23 and 27 is 23? by luxgladius (Initiate) on Jul 02, 2015 at 18:00 UTC
Sorry, it's List::Util 'max'.	[reply]
Re: Max of 23 and 27 is 23? by toolic (Bishop) on Jul 02, 2015 at 17:55 UTC
Does it work outside the debugger? List::Util max? `use warnings; use strict; use List::Util qw(max); print max(27,23), "\n"; __END__ 27` [download] See also: the Basic debugging checklist	[reply] [d/l]
Re^2: Max of 23 and 27 is 23? by luxgladius (Initiate) on Jul 02, 2015 at 18:02 UTC
Yes, it's List::Util max, and no, it was causing a bug where my list was being shortened in a map when I continued it to keep growing.	[reply]
Re^3: Max of 23 and 27 is 23? by Anonymous Monk on Jul 02, 2015 at 18:10 UTC
So ... how (code please) was the "list being shortened in a map when [you] continued it to keep growing"?	[reply]
Re: Max of 23 and 27 is 23? by BrowserUk (Patriarch) on Jul 02, 2015 at 17:38 UTC
Where is max() from? 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. I'm with torvalds on this Agile (and TDD) debunked I told'em LLVM was the way to go. But did they listen!	[reply]
Re: Max of 23 and 27 is 23? by Anonymous Monk on Jul 02, 2015 at 18:07 UTC
Nothing odd here (perl 5.16.2 x86_64-linux, List::Util 1.35) ... `DB<2> use List::Util 'max' DB<3> p max( 1 , 4 ) 4 DB<4> @p = ( 1 .. 5 ) DB<5> @q = ( 1 .. 11 ) DB<6> p $#p , $#q 410 DB<7> p max( $#p , $#q ) 10 DB<8> p max( 0+$#p , $#q ) 10` [download]	[reply] [d/l]
Re: Max of 23 and 27 is 23? by Laurent_R (Canon) on Jul 02, 2015 at 19:56 UTC
Hi, I tried to reproduce exactly your scenario, but I do not get any weird result: `DB<1> @L = 0..27; # use @L rather than @l for more clarity (avoid m +istaking 1 and l) DB<2> p $#L; 27 DB<3> @F = 0..23; DB<4> p $#F; 23 DB<5> use List::Util 'max'; DB<6> p max($#L, $#F); 27` [download] Perhaps you should show how @l and @F are populating, although I fail to see what it could be at this point. My Perl version: `perl 5, version 14, subversion 4 (v5.14.4)` [download]	[reply] [d/l] [select]