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Laziness in a more consistent way

by tilly (Archbishop)
on Nov 03, 2001 at 02:50 UTC ( #122941=note: print w/ replies, xml ) Need Help??


in reply to Re: Let's get lazy
in thread Let's get lazy

Here is a more general solution to the problem. In this solution my iterators all indicate that they are done by returning an empty list, and when called next will restart. I didn't use i_grep, but I included it to show how you would do it.

Note that despite the length, the code is more straightforward than the original recursive code. And if this was part of a longer program, you would be reusing the bulk of this code.

use strict; my $iter = i_map( sub {print "@_\n"}, comb_iter( list_iter(1..2), list_iter('a'..'c'), list_iter(3..5) ) ); 1 while $iter->(); ################################################################### # The program proper ends here. These are utility functions that # # you could reuse # ################################################################### # Takes a list of iterators that are "restartable" # Returns a restartable iterator that iterates over all combinations # of outputs of the input iterators, creating a flat list of combinati +ons # of the inputs. (The output only makes sense in array context.) sub comb_iter { if (0 == @_) { return sub {()}; # Stupid case needed for generality. } elsif (1 == @_) { return shift; } else { my $outer_iter = shift; my $inner_iter = comb_iter(@_); my @last_outer; return sub { if (@last_outer) { my @ret = $inner_iter->(); if (@ret) { return (@last_outer, @ret); } else { @last_outer = $outer_iter->(); if (@last_outer) { return (@last_outer, $inner_iter->()); } else { return (); } } } else { @last_outer = $outer_iter->(); return (@last_outer, $inner_iter->()); } }; } } # Takes a function and an iterator, returns an iterator that uses that # function to filter the output. sub i_grep { my ($filter, $iter) = @_; my @last_ret = qw(just an initialization value); sub { while (@last_ret) { @last_ret = $iter->(); if ($filter->(@last_ret)) { return wantarray ? @last_ret : $last_ret[0]; } } return (); }; } # Takes a function and an iterator, returns an iterator that applies t +hat # function to the returns of the iterator. sub i_map { my ($filter, $iter) = @_; sub { my @ret = $iter->(); return @ret ? $filter->(@ret) : (); }; } # Takes a list and turns it into an iterator over that list sub list_iter { my @vals = @_; my $i = 0; sub { if ($i < @vals) { return $vals[$i++]; } else { $i = 0; return (); } }; }
Note that the specific problem in the original question can now be solved as the author wanted using i_grep, or you can produce more efficient iterator as follows:
my $genome_iter = i_map( sub {join '', @_}, join_iter( map { comb_iter( map { list_iter(qw(a c g n t)); } 1..$_ ) } 2..3 ) ); while (my $string = $genome_iter->()) { print "$string\n"; } # Takes a list of iterators, and returns an iterator that iterates # over each in turn sub join_iter { my @iter = @_; my $i = 0; return sub { while ($i < @iter) { my @ret = $iter[$i]->(); if (@ret) { return wantarray ? @ret : $ret[0]; } else { $i++; } } $i = 0; return (); }; }
Alternately if you want to turn the output into a list you can just create an easy method:
# Takes an iterator and returns a list of results sub iter2list { my $iter = shift; my @out; while (my @ret = $iter->()) { push @out, @ret; } return @out; }
Note that most of the length here is because I am having to build my iterator interface from scratch. That is a lot of work! And some of the code looks more complex because what we are used to seeing in a few nested loops our minds balk at when you see it as a similar number of nested calls.


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