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Re^3: cannot follow hanoi subroutine

by graq (Curate)
on Nov 05, 2007 at 09:03 UTC ( #648974=note: print w/ replies, xml ) Need Help??


in reply to Re^2: cannot follow hanoi subroutine
in thread cannot follow hanoi subroutine

First all, apologies for ruining some nice code. I have added a print statement and 4 comment points (A .. D).

#!/usr/bin/perl -w use strict; use warnings; my %piles = (A => [reverse 1..3], B => [], C => []); dumpPiles (); hanoi (scalar @{$piles{A}}, keys %piles); #A sub hanoi { my ($n, $start, $end, $extra, $recursion) = @_; print "Hanoi: Disk $n\n"; if ($n == 1) { #B report ($start, $end, $recursion); } else { hanoi($n-1, $start, $extra, $end); #C report ($start, $end, $recursion); hanoi($n-1, $extra, $end, $start); #D } } sub report { my ($start, $end) = @_; my $disk = pop @{$piles{$start}}; print "Moved disk $disk from $start to $end.\n"; push @{$piles{$end}}, $disk; dumpPiles (); } sub dumpPiles { for my $pile (sort keys %piles) { print "$pile: @{$piles{$pile}}\n"; } }

The first hanoi call happens at point A, where the first argument (which disk to move) is equal the number of discs on pile A; namely '3'. We then move into the hanoi iterative method where the logical statement at B is false and we move to point C.

At point C we call hanoi again (note we have not called report yet, but the first argument has been decremented to '2'. This means we arrive at point B for the second time and the logical statement is still false.

We arrive at point C again, calling hanoi on disk number 1. Now the statement at B is true and the iterations begin to unravel.

Perhaps, together with the additional print statement in the code, this will help to explain things?

-=( Graq )=-


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