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Re: Finding missing numbers in a list

by Aristotle (Chancellor)
on Sep 03, 2004 at 15:50 UTC ( #388318=note: print w/replies, xml ) Need Help??


in reply to Finding missing numbers in a list

I'll assume the list is sorted. If not, either sort it or use List::Util's min/max functions to set up the min( @list ) .. max( @list ) range.

my @list = ( 41888, 41889, 41890, 41892, 41895 ); my @missing = do { my %missing; undef @missing{ $list[ 0 ] .. $list[ -1 ] }; delete @missing{ @list }; keys %missing; };

That's the simplest way; it takes some memory and I'm not sure about its efficiency, so use with very large lists is not necessarily advised.

Update: another option, assuming that the list really is sorted (can't rely on min/max here):

my @missing; { my $i = 0; for ( $list[ 0 ] .. $list[ -1 ] ) { ++$i, next if $_ == $list[ $i ]; push @missing, $_; } }

Update: and just because I had to, the same thing in functional lingo:

my @missing = do { my $i = 0; grep { $_ == $list[ $i ] ? ++$i && 0 : 1 } $list[ 0 ] .. $list[ -1 + ]; };

Makeshifts last the longest.

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