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Re^5: Recursion: the Towers of Hanoi problem

by abitkin (Monk)
on Oct 25, 2004 at 14:38 UTC ( #402234=note: print w/ replies, xml ) Need Help??

in reply to Re^4: Recursion: the Towers of Hanoi problem
in thread Recursion: The Towers of Hanoi problem

Strangely, using my $l=$_[3]-- and using a(@_[0,2,1,3]) etc. doesn't work at all. @_ is not localised ($_3 ends up very negative and the solution becomes deeply recursive), which makes me wonder if the above solution works. I haven't compared the output to the original solution's output.

from perlsub:
The array @_ is a local array, but its elements are aliases for the actual scalar parameters.

Basically, if you think about it, @_ has to be localized. Otherwise if you called a function inside a function (something I hope most of us do) your @_ array would be wiped out, leaving you with the parameters you sent into the child function you just called. The particular behavior your seeing is probably becuase the @_ array contains aliases to scalars.

Second, using 0 based discs is okay, it's easy enough for the user to add 1 in their head (or to start thinking like a cs person.)

Third, and this is where it gets interesting, when removing all the whitespace I can I get (for your solution):

sub a{if(my$l=pop){a(@_[0,2,1],$l-1);print"Move disc $l from $_[0] to +$_[2] ";a(@_[1,0,2],$l-1);}}a 'A'..'C',pop;
Which just happens to be the same amount of space as (my solution):
sub a{my$l=pop;a(@_[0,2,1],--$l)."Move disc $l from $_[0] to $_[2] ".a(@_[1,0,2],$l)if$l>0;}print a 'A'..'C',pop;
so it seems that both solutions are (atleast as far as this little experiment goes) equal. My goal is to squeeze everything that I have now down to 115 or 110 by Tuesday.

Oh and I have compared our outputs, and with the exception of the disc number (mine being one less than yours) they are exactly the same.

perlsub not perlvar

pop instead of shift to remove chars

EDIT3 (10/26/04):
Best I've done, modifing jasper's code, is 111 chars.

sub a{if(my$l=pop){a(@_[0,2,1],--$l);print"Move disc $l from $_[0] to +$_[2] ";a(@_[1,0,2],$l);}}a 'A'..'C',pop;

Kwyjibo. A big, dumb, balding North American ape. With no chin.

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