perlquestion
live4tech
<p>Hello all! I have never completely understood recursion. I have some understanding of what happens when you have one simple recursive call in a subroutine like the following:</p>
<code>
sub factorial
{
factorial
my ($n) = @_;
return 1 if $n == 0;
return factorial($n-1) * $n;
}
</code>
<p>The sub factorial is called again and again until $n is equal to 0. Then the 1 is returned and all the past calls are then 'really' executed in reverse order, producing the factorial of the number $n.</p>
<p>OK. But I do not understand how recursion works when there is more than 1 recursive call in a subroutine, like the following:
</p>
<code>
sub hanoi
{
my ($n, $start, $end, $extra) = @_;
if ($n == 1)
{
print "Move disk #1 from $start to $end.\n";
}
else
{
hanoi($n-1, $start, $extra, $end);
print "Move disk #$n from $start to $end.\n";
hanoi($n-1, $extra, $end, $start);
}
}
</code>
<p>The output will be:</p>
<code>
Move disk #1 from A to C.
Move disk #2 from A to B.
Move disk #1 from C to B.
Move disk #3 from A to C.
Move disk #1 from B to A.
Move disk #2 from B to C.
Move disk #1 from A to C.
</code>
<p></p>
<p> Can anyone explain precisely why this is the output. If the first recursive call were called over and over untill $n==1 and then they 'fell' back and all the calls were executed in reverse order, the third line of output does not make sense. Is the next recursive line then executed?
A step-by-step explanation is what I am looking for.</p>
<p>Thank you!</p>