Puzzeled about Sorting

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

I'm a litle bit puzzeled about sorting. The following code:

print join (', '
          , sort {$a <=> $b || $a cmp $b}
            (2, 0, 1, 99, 'aaa', 'AAA', undef, 'ZZZ', 'zzz'))
          , "\n";
[download]

produces

__OUTPUT__
, 0, AAA, ZZZ, aaa, zzz, 1, 2, 99
[download]

• Why is undef the first item?
• Why is 0 not within the other numbers?
Thanks for any hints!

pelagic

Comment on Puzzeled about Sorting Select or Download Code

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Re: Puzzeled about Sorting by Zaxo (Archbishop) on Jun 10, 2004 at 11:23 UTC
In numeric context given by the first comparison, undef, 0, and all the alphabetic strings evaluate to numeric zero. Being equal in numeric context, the comparison evaluates to 0 and the dictionary sort order kicks in. In string context undef is the empty string, which sorts first, then '0', then the alphabetic quantities in lexicographic order. Here's an example worth working out in your head before you try it: `$ perl -e'print join( ", ", sort {$a<=>$b \|\| $a cmp $b} qw/7 000 07 00 007 0/, "0 but true"), $/'` After Compline, Zaxo	[reply] [d/l]
Re: Puzzeled about Sorting by dave_the_m (Monsignor) on Jun 10, 2004 at 11:31 UTC
First you do a numeric sort. In this case, undef and the strings take on the value zero in a numeric context. Thus, 0, undef, 'aaa', 'AAA', 'ZZZ' and 'zzz' all compare equally and come before the >0 numbers. Then for all the ones above that are numerically equivalent to 0, you do a string comparison; now in string cxontext, undef is treated as the empty string "" and 0 is treated as the one character string "0". Thus they get sorted as undef, 0, AAA, ZZZ, aaa, zzz. Running with warnings enabled might be illuminating at this point. Dave.	[reply]
Re: Puzzeled about Sorting ('natural') by tye (Sage) on Jun 10, 2004 at 16:48 UTC
There are some fairly simple ways to get the numbers sorted together, found by searching for natural sort. - tye	[reply]
Re: Puzzeled about Sorting by pelagic (Priest) on Jun 10, 2004 at 11:59 UTC
Thanks a lot guys for that lesson! pelagic	[reply]
Re: Puzzeled about Sorting by Anonymous Monk on Jun 12, 2004 at 04:55 UTC
Ok, think about what your sort routine does: `sort {$a <=> $b \|\| $a cmp $b}` [download] It means that if $a and $b are equal numerically, then it will do a string comparison on them.	[reply] [d/l]