good chemistry is complicated,and a little bit messy -LW PerlMonks

### Re: Recursively-generated Iterators

by Roy Johnson (Monsignor)
 on Jun 02, 2005 at 01:53 UTC ( #462720=note: print w/replies, xml ) Need Help??

Just another example of a conversion (using the boilerplate recipe). I'm posting mostly so it will be here to refer back to if I revisit the topic. It's interestingly different from the other examples, and the recipe still works well (although I get a curious Use of uninitialized value in array element warning if I don't turn those warnings off (which I do, below)).

What I'm generating is permutations, like Algorithm::Loops NextPermute.

```#!perl
use strict;
use warnings;

sub no_repeat_combos {
return [@_] unless @_ > 1;
# Find the first occurrence of each unique element
my %seen;
defined(\$seen{\$_[\$_]}) or \$seen{\$_[\$_]} = \$_ for 0..\$#_;
# For each unique element, stick it on the front of each
# of the no-repeat-combos of the rest
map {
my \$first_pos = \$_;
my @rest = @_[ grep {\$first_pos != \$_} 0..\$#_ ];
map [\$_[\$first_pos], @\$_], no_repeat_combos(@rest);
} (sort {\$a <=> \$b} values %seen);
}

sub nrc_iter {
# Base cases get assigned to an array, which the iterator shifts t
+hrough
my @base_case = ([@_]);
return sub{ shift @base_case } unless @_ > 1;
# Find the first occurrence of each unique element
my %seen;
defined(\$seen{\$_[\$_]}) or \$seen{\$_[\$_]} = \$_ for 0..\$#_;
# For each unique element, stick it on the front of each
# of the no-repeat-combos of the rest
my @arg_list = @_;
my @sub_iter =
map {
my \$first_pos = \$_;
my @rest = @_[ grep {\$first_pos != \$_} 0..\$#_ ];
sub {
my \$recurse = nrc_iter(@rest);
my \$set;
no warnings 'uninitialized';
sub { (\$set = \$recurse->()) ? [\$arg_list[\$first_pos], @\$set]
+ : () }
}
} sort {\$a <=> \$b} values %seen;
# Below here is boilerplate: if you've done the above steps right,
+ just plug
# this in, and it works. It returns the first iterator from the li
+st that
# returns anything.
# Grab and unwrap an iterator from the list
my \$iter = (shift @sub_iter)->();
return sub {
my \$rval;
\$iter = (shift @sub_iter)->()
until (\$rval = \$iter->() or @sub_iter == 0);
return \$rval;
}
}

for ([1], [1,1], [1,2], [qw(a b a)], [qw(a b b a)]) {
print "=== @\$_ ===\n";
my \$i = nrc_iter(@\$_);
print "  @\$_\n" while \$_ = \$i->();
}

Caution: Contents may have been coded under pressure.

Create A New User
Domain Nodelet?
Node Status?
node history
Node Type: note [id://462720]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others surveying the Monastery: (6)
As of 2022-12-09 14:00 GMT
Sections?
Information?
Find Nodes?
Leftovers?
Voting Booth?

No recent polls found

Notices?