Beefy Boxes and Bandwidth Generously Provided by pair Networks
We don't bite newbies here... much
 
PerlMonks  

Re: Hamming Sequences and Lazy Lists

by Roy Johnson (Monsignor)
on Mar 17, 2005 at 16:40 UTC ( #440428=note: print w/ replies, xml ) Need Help??


in reply to Hamming Sequences and Lazy Lists

Here is a lazy-list example of merge, with one of its arguments being a finite list, and the other an infinite list. I may not get around to figuring out the Hamming generator, but I think the techniques I've employed here tell you how it can be done.

use strict; use warnings; # Merge takes two iterators, which can be called # with no arguments, in which case they iterate, returning # their next value; or with a string 'peek', which will # yield the next value without effectively shifting it. # Exhausted iterators return undef on all subsequent calls # # Merge itself is an iterator, returning the next element in # the merged series, plus a continuation coderef sub merge { my ($a, $b) = @_; my ($car_a, $car_b) = ($a->('peek'), $b->('peek')); # Base case: if one of them is empty, return the other defined $car_a or return ($b->(), sub {merge($a, $b)}); defined $car_b or return ($a->(), sub {merge($a, $b)}); # Pull off lesser (both if equal) first element(s) my $low_car; if ($car_a <= $car_b) { $low_car = $a->() } if ($car_b <= $car_a) { $low_car = $b->() } return ($low_car, sub {merge($a, $b)} ); } my $I1 = do { my @arr = (1..10); sub { if (@arr == 0) { return undef; } elsif (@_) { # should be peek, but any other arg works (undocu +mented) return $arr[0]; } else { return shift @arr; } } }; my $I2 = do { my $i = 2; sub { if (@_) { return $i; } else { my $i_was = $i; $i += 2; return $i_was; } } }; my $iterations_to_print = 30; for ( my ($elem, $cont) = merge($I1, $I2); $iterations_to_print-- > 0; ($elem, $cont) = $cont->() ) { print "<$elem>\n"; }

Caution: Contents may have been coded under pressure.


Comment on Re: Hamming Sequences and Lazy Lists
Download Code
Re^2: Hamming Sequences and Lazy Lists
by Limbic~Region (Chancellor) on Mar 17, 2005 at 16:55 UTC
    Roy Johnson,
    I don't think this is right. It is my understanding that the results should be the multiples of the factors merged minus duplicates. If you change $iterations_to_print = 23 and @arr = (2,3,5), where is 15 in the results for instance? See Re: Hamming Sequences and Lazy Lists for my understanding and implementation of the problem.

    Cheers - L~R

      I did not implement Hamming here; only merging of two list iterators. But I have just posted a lazy Hamming solution.

      Caution: Contents may have been coded under pressure.
        Roy Johnson,
        Ok - my mistake. In fact, I completely missed the boat on the entire problem. I was so focused on lazy evaluation that I misunderstood "how do you generate the series of numbers composed of a given list of prime factors, where each can be used an unlimited number of times?" To me, that any positive multiple of any factor was valid.

        Cheers - L~R

Log In?
Username:
Password:

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://440428]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others exploiting the Monastery: (8)
As of 2015-07-06 22:36 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    The top three priorities of my open tasks are (in descending order of likelihood to be worked on) ...









    Results (84 votes), past polls