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Limbic Region (3985)
Limbic Region
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http://gatcomb.org/joshua/perl.html
I love to code perl. If you want to hire me, see details on my homepage.
(email not shown publicly)
http://gatcomb.org/joshua/perl.html
I love to code perl. If you want to hire me, see details on my homepage.
Journal of Limbic Region (3985)
Thursday September 23, 2004
06:59 PM
Fascination With Iterators
After declaring I was taking a programming hiatus only a few days ago, I have written two pieces of code I am fairly proud of. The first was an efficient way to process every line in 1 file against every line in another file without slurping the files into memory.
The second was an iterator for x size subsets of y
#!/usr/bin/perl
use strict;
use warnings;
my $iter = combo( 3 , 1 .. 5 );
while ( my @combo = $iter->() ) {
print "@combo\n";
}
sub combo {
my $by = shift;
return sub { () } if ! $by || $by =~ /\D/ || @_ < $by;
my @list = @_;
my @position = (0 .. $by - 2, $by - 2);
my @stop = @list - $by .. $#list;
my $end_pos = $#position;
my $done = undef;
return sub {
return () if $done;
my $cur = $end_pos;
{
if ( ++$position[ $cur ] > $stop[ $cur ] ) {
$position[ --$cur ]++;
redo if $position[ $cur ] > $stop[ $cur ];
my $new_pos = $position[ $cur ];
@position[ $cur .. $end_pos ] = $new_pos .. $new_pos + $by;
}
}
$done = 1 if $position[0] == $stop[0];
return @list[ @position ];
}
}
__END__
1 2 3
1 2 4
1 2 5
1 3 4
1 3 5
1 4 5
2 3 4
2 3 5
2 4 5
3 4 5
Notice that the only looping construct in the entire thing is a bare block. It is very fast ;-)
The second was an iterator for x size subsets of y
#!/usr/bin/perl
use strict;
use warnings;
my $iter = combo( 3 , 1
while ( my @combo = $iter->() ) {
print "@combo\n";
}
sub combo {
my $by = shift;
return sub { () } if ! $by || $by =~
my @list = @_;
my @position = (0
my @stop = @list - $by
my $end_pos = $#position;
my $done = undef;
return sub {
return () if $done;
my $cur = $end_pos;
{
if ( ++$position[ $cur ] > $stop[ $cur ] ) {
$position[ --$cur ]++;
redo if $position[ $cur ] > $stop[ $cur ];
my $new_pos = $position[ $cur ];
@position[ $cur
}
}
$done = 1 if $position[0] == $stop[0];
return @list[ @position ];
}
}
__END__
1 2 3
1 2 4
1 2 5
1 3 4
1 3 5
1 4 5
2 3 4
2 3 5
2 4 5
3 4 5
Notice that the only looping construct in the entire thing is a bare block. It is very fast
Or is it a BUG?"
"Let's call it an accidental feature. :-)"
--Larry Wall in <6909@jpl-devvax.JPL.NASA.GOV>
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