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Re: sorting a file - multilevel

by sgifford (Prior)
on Jun 14, 2008 at 02:15 UTC ( #692042=note: print w/replies, xml ) Need Help??

in reply to sorting a file - multilevel

As runrig mentioned, Unix sort(1) is a great tool for this, although the syntax sometimes requires a little trial and error.

To do this from Perl, read each line into some kind of data structure, then define your own sorting function that compares two of these data structures by looking at each of the fields, returning 1 if the first is greater, -1 if the second is greater, or going on to the next field if they are the same. The cmp and <=> ("spaceship") operators will help you with this, and they can be cascaded with the || "or" operator.

Here's a simple example (untested):

sub mysort { return $a->[0] <=> $b->[0] || $a->[1] <=> $b->[1] || $a->[9] cmp $b->[9] } my @list; while (<>) { chomp; push @list, [ split ]; } @list = sort mysort @list;

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[Corion]: hippo: If you have a Unicode-wise Perl then likely some zero-width characters in $x would work. Maybe $x = "\x{200b}" works.
[Corion]: Hmm - no, that outputs 1 for me on 5.14 - perl -wle "my $x = qq(\x{200b}); warn $x; warn length $x"
[hippo]: Smart - I'll give that a go. Thanks.
[hippo]: Ah
[Corion]: But maybe there is some other Unicode string that will be true but have a zero width
[hippo]: For explanation, I've seen this construct in someone else's code (no names, no pack drill) and couldn't think of a situation to trigger it.
[Corion]: You'll have to look somewhere esoteric for that. Maybe some tied variable or special dualvar can also trigger that. But it's certainly not a common occurrence
[Corion]: And on 5.20, the following also outputs no find:perl -wle 'for my $x ("\x{2000}".."\ x{1fffff}") { if( $x && ! length $x ) { warn qq(<$x>); warn length $x; die } }'
[Corion]: (this time on Unix)
[hippo]: Understood. I'll have to go through the code and see if it's doing anything fancy with ties, dual-vars or non-scalars. In the end, it's probably a bug though.

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