On Mon, May 24, 2010 at 12:49 PM, Weizhong Dai <weizhong....@gmail.com>wrote:
> Hi guys, I met a problem.
> When I tried this script below:
>
> //--------------------------------------------------------------------
> sub print_instruction {
> my ($disk, $start, $end) = @_;
> print "Move disk #$disk from $start to $end.\n";
> }
>
> sub hanoi {
> my ($n, $start, $end, $extra, $m) = @_;
> if ($n == 1) {
> $m->(1, $start, $end);
> } else {
> hanoi($n-1, $start, $extra, $end);
> $m->($n, $start, $end);
> hanoi($n-1, $extra, $end, $start);
> }
> }
>
> &hanoi(3, 'A', 'C', 'B', \&print_instruction);
> //-------------------------------------------------------------------------
>
> I got a error : Undefined subroutine &main:: called at ....
> this is an example in <High Order Perl>, so it should not be wrong, I
> just don't known what is the problem.
> Plz help me.
>
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>
>
No idea why Perl would cry about &main, my knowledge of Perl internals is
not that good yet. But I would bet that the problem lies in the following:
sub hanoi expects to receive 5 parameters, in side the sub you are calling
is once again but now with only 4 parameters.
hanoi($n-1, $start, $extra, $end);
$m->($n, $start, $end);
hanoi($n-1, $extra, $end, $start);
My guess is that Perl when you are pulling in the parameters using the my (
... ) = @_; line at the beginning of the sub places a ref to main at the end
for your convenience, thus the call to $m-> will be interpreted as a call to
&main which in turn upsets Perl.
I would try to hand in $m in the two recursive calls inside the hanoi sub.
hanoi($n-1, $start, $extra, $end, $m);
$m->($n, $start, $end);
hanoi($n-1, $extra, $end, $start, $m);
That should do the trick...
Don't trust the written word, in the end it was a human doing the writing so
mistakes are inevitable. ;-)
Regards,
Rob
